 Section 3 of the Outline of Science, Volume 1. The Outline of Science, Volume 1 by J. Arthur Thompson. Section 3. Part 2. The Story of Evolution. Introductory, the beginning of the earth, making a home for life, the first living creatures. 1. The evolution idea is a master key that opens many doors. It is a luminous interpretation of the world, throwing the light of the past upon the present. Everything is seen to be an antiquity, with a history behind it, a natural history, which enables us to understand, in some measure, how it has come to be as it is. We cannot say more than understand in some measure, for while the fact of evolution is certain, we are only beginning to discern the factors that have been at work. The evolution idea is very old, going back to some of the Greek philosophers, but it is only in modern times that it has become an essential part of our mental equipment. It is now an everyday intellectual tool. It was applied to the origin of the solar system and to the making of the earth before it was applied to plants and animals. It was extended from these to man himself. It spread to language, to folkways, to institutions. Within recent years, the evolution idea has been applied to the chemical elements, for it appears that uranium may change into radium, that radium may produce helium, and that lead is the final stable result when the changes of uranium are complete. Thus, all the elements may be the outcome of an inorganic evolution. Not less important is the extension of the evolution idea to the world within as well as to the world without, for alongside of the evolution of bodies and brains is the evolution of feelings and emotions, ideas and imagination. Organic evolution means that the present is a child of the past and the parent of the future. It is not a power or a principle. It is a process, a process of becoming. It means that the present-day animals and plants and all the subtle interrelations between them have arisen in a natural, noble way from a preceding state of affairs on the whole somewhat simpler. And that, again, from forms and interrelations simpler still, and so on backwards and backwards for millions of years till we lose all clues in the thick mist that hangs over life's beginnings. Our solar system was once represented by a nebula of some sort and we may speak of the evolution of the sun and the planets, but since it has been the same material throughout that has changed in its distribution and forms, it might be clear to use some word like genesis. Similarly, our human institutions were once very different from what they are now and we may speak of the evolution of government or of cities. But man works with a purpose, with ideas and ideals in some measure controlling his actions and guiding his achievements so that it is probably clearer to keep the good old word history for all processes of social becoming in which man has been a conscious agent. Now, between the genesis of the solar system and the history of civilization, there comes the vast process of organic evolution. The word development should be kept for the becoming of the individual, the chick out of the egg, for instance. Organic evolution is a continuous natural process of racial change by successive steps in a definite direction, whereby distinctively new individualities arise, take root and flourish, sometimes alongside of and sometimes sooner or later in place of the originative stock. Our domesticated breeds of pigeons and poultry are the results of evolutionary change whose origins are still with us, in the rock dove and the jungle fowl, but in most cases in wild nature the ancestral stocks of present-day forms are long since extinct and in many cases they are unknown. Evolution is a long process of coming and going, appearing and disappearing, a long drawn-out, sublime process like a great piece of music. Two. The beginning of the earth. When we speak the language of science we cannot say in the beginning, for we do not know of and cannot think of any condition of things that did not arise from something that went before. But we may qualify the phrase and legitimately inquire into the beginning of the earth within the solar system. If the result of this inquiry is to trace the sun and the planets back to a nebula, we reach only a relative beginning. The nebula has to be accounted for. And even before matter there may have been a prematerial world. If we say, as was said long ago, in the beginning was mind, we may be expressing or trying to express a great truth, but we have gone beyond science. The Nebula Hypothesis One of the grandest pictures that the scientific mind has ever thrown upon the screen is that of the Nebula Hypothesis. According to Laplace's famous form of this theory, 1796, the solar system was once a gigantic glowing mass spinning slowly and uniformly around its center. As the incandescent world cloud of gas cooled and its speed of rotation increased, the shrinking mass gave off a separate whirling ring which broke up and gathered together again as the first and most distant planet. The main mass gave off another ring and another till all the planets, including the earth, were formed. The central mass persisted as the sun. Laplace spoke of his theory which Kant had anticipated 41 years before with scientific caution. Conjectures which I present with all the distrust which everything, not the result of observation or of calculation, ought to inspire. Subsequent research justified his distrust for it has been shown that the original Nebula need not have been hot and need not have been gaseous. Moreover, there are great difficulties in Laplace's theory of the separation of successive rings from the main mass and of the condensation of a whirling gaseous ring into a planet. So it has come about that the picture of a hot gaseous Nebula revolving as a unit body has given place to other pictures. Thus Sir Norman Lockyer pointed out, 1890, that the earth is gathering to itself millions of meteorites every day. This has been going on for millions of years. In distant ages the accretion may have been vastly more rapid and voluminous and so the earth has grown. Now the meteoritic contributions are undoubted but they require a center to attract them and the difficulty is to account for the beginning of a collecting center or planetary nucleus. Moreover, meteorites are sporadic and erratic, scattered hither and thither rather than collecting into unit bodies. As Professor Chamberlain says, quote, meteorites have rather the characteristics of the wreckage of some earlier organization than of the parentage of our planetary system, end quote. Several other theories have been propounded to account for the origin of the earth but the one that has found most favor in the eyes of authority is that of Chamberlain and Moulton. According to this theory a great nebula mass condensed to form the sun from which under the attraction of passing stars planet after planet the earth included was heaved of in the form of knotted spiral nebulae like many of those now observed in the heavens. Of great importance were the knots for they served as collecting centers drawing flying matter into their clutches. Whatever part of the primitive bold escaped and scattered was drawn out into independent orbits around the sun forming the planetissimals which behave like minute planets. These planetissimals formed the food on which the knots subsequently fed. The growth of the earth. It has been calculated that the newborn earth the earth knot of Chamberlain's theory had a diameter of about 5,500 miles but it grew by drawing planetissimals into itself until it had a diameter of over 8,100 miles at the end of its growing period. Since then it has shrunk by periodic shrinkages which have meant the buckling up of successive series of mountains and it has now a diameter of 7,918 miles but during the shrinking the earth became more varied. A sort of slow boiling of the internally hot earth often forced molten matter through the cold outer crust and there came about a gradual assortment of lighter materials nearer the surface and heavier materials deeper down. The continents are built of the lighter materials such as granites while the beds of the great oceans are made of the heavier materials such as basalts. In limited areas land has often become sea and sea has often given place to land but the probability is that the distinction of the areas corresponding to the great continents and oceans goes back to a very early stage. The lithosphere is the more or less stable crust of the earth which may have been to begin with about 50 miles in thickness. It seems that the young earth had no atmosphere but ages passed before water began to accumulate on its surface before in other words there was any hydrosphere. The water came from the earth itself to begin with and it was long before there was any rain dissolving out saline matter from the exposed rocks and making the sea salt. The weathering of the high grounds of the ancient crust by air and water furnished the material which formed the sandstones and mudstones and other sedimentary rocks which are set to amount to a thickness of over 50 miles in all. Three. Making a home for life. It is interesting to inquire how the callous, rough and tumble conditions of the outdoor world in early days were replaced by others that allowed of the germination and growth of that tender plant we call life. There are very tough living creatures and the average organism is ill suited for violence. Most living creatures are adapted to mild temperatures and gentle reactions. Hence the fundamental importance of the early atmosphere heavy with plantesimal dust in blanketing the earth against intensities of radiance from without, as Chamberlain says, and inequalities of radiance from within. This was the first preparation for life but it was an atmosphere without free oxygen. Not less important was the appearance of pools and lakelets of lakes and seas. Perhaps the early waters covered the earth and water was the second preparation for life. Water that can dissolve a larger variety of substances in greater concentration than any other liquid. Water that in summer does not readily evaporate altogether from a pond nor in winter freeze throughout its whole extent. Water that is such a mobile vehicle and such a subtle cleaver of substances. Water that forms over 80% of living matter itself. Of great significance was the abundance of carbon, hydrogen and oxygen in the form of carbonic acid and water in the atmosphere of the cooling earth. For these three wonderful elements have a unique ensemble of properties ready to enter into reactions and relations making great diversity and complexity possible favouring the formation of the plastic and permeable materials that build up living creatures. We must not pursue the idea but it is clear that the stones and mortar of the inanimate world are such that they build a friendly home for life. Origin of living creatures upon the earth During the early chapters of the earth's history no living creature that we can imagine could possibly have lived there. The temperature was too high there was neither atmosphere nor surface water. Therefore it follows that at some uncertain but inconceivably distant date living creatures appeared upon the earth. No one knows how but it is interesting to consider possibilities. From ancient times it has been a favourite answer that the dust of the earth may have become living in a way which is outside scientific description. This answer forecloses the question and it is far too soon to do that. Science must often say ignoramus. Science should be slow to say ignorapamus. A second position held by Helmholtz, Lord Kelvin and others suggests that minute living creatures may have come to the earth from elsewhere in the cracks of a meteorite or among cosmic dust. It must be remembered that such can survive prolonged exposure to very low temperatures. That seeds can survive prolonged exposure to very low temperatures. That spores of bacteria can survive high temperature. That seeds of plants and germs of animals in a state of latent life can survive prolonged drought and absence of oxygen. It is possible, according to Berthelot, that as long as there is not molecular disintegration vital activities may be suspended for a time and may afterwards recommence when appropriate conditions are restored. Therefore one should be slow to say that a long journey through space is impossible. The obvious limitation of Lord Kelvin's theory is that it only shifts the problem of the origin of organisms that is living creatures, from the earth to elsewhere. The third answer is that living creatures of a very simple sort may have emerged on the earth's surface from not living material, for example from some semi-fluid carbon compounds activated by ferments. The terribility of this view is suggested by the achievements of the synthetic chemists who are able artificially to build up substances such as oxalic acid, indigo, salicylic acid, caffeine, and grape sugar. We don't know, indeed, what in nature's laboratory would take the place of the clever synthetic chemist but there seems to be a tendency to complexity. Corpuscules form atoms, atoms form molecules, small molecules, large ones. Various concrete suggestions have been made in regard to the possible origin of living matter which will be dealt with in a later chapter. As we know of what goes on today, there is no evidence of spontaneous generation. Organisms seem always to arise from pre-existing organisms of the same kind. Where any suggestion of the contrary has been fancied, there have been flaws in the experimenting. But it is one thing to accept the verdict om ne vivum e vivo, as a fact to which experiment has not yet discovered an exception, and another thing to maintain that this must always have been true and must always remain true. If the synthetic chemists should go on surpassing themselves, if substances like white of egg should be made artificially, and if we should get more light on possible steps by which simple living creatures may have arisen from not living materials, this would not greatly affect our general outlook on life, though it would increase our appreciation of what is often labelled as inert matter. If the dust of the earth did naturally give rise very long ago to living creatures, if they are in a real sense born of her and of the sunshine, then the whole world becomes more continuous and more vital, and all the inorganic groaning and travailing becomes more intelligible. Four. The first organisms upon the earth. We cannot have more than a speculative picture of the first living creatures upon the earth, or rather in the waters that covered the earth. Our calculation is to be found, however, in the simplest creatures living today, such as some of the bacteria and one-celled anomalculus, especially those called protists, which have not taken any very definite step towards becoming either plants or animals. No one can be sure, but there is much to be said for the theory that the first creatures were microscopic globules of living matter, not unlike the simplest bacteria of today, but able to live on air, from such a source may have originated a race of one-celled marine organisms which were able to manufacture chlorophyll, or something like chlorophyll. That is to say, the green pigment which makes it possible for plants to utilize the energy of the sunlight in breaking up carbon dioxide and in building up photosynthesis, carbon compounds like sugars and starch. These little units were probably encased in a cell wall of cellulose, but their human energy expressed itself in the undulatory movement of a lash or phlegelum, by means of which they propelled themselves energetically through the water. There are many similar organisms today, mostly in water, but some of them, simple one-celled plants, paint the tree stems and even the paving stones, green in wet weather. According to Professor A. H. Church, there was a long chapter in the history of the earth when the sea that covered everything, teamed with these green phlegelites, the originators of the vegetable kingdom. On another tech, however, there probably evolved a series of simple predatory creatures, not able to build up organic matter from air, water and salts, but devouring their neighbors. These units were not closed in with cellulose, but remained naked, with their living matter or protoplasm flowing out in changeful processes, such as we see in the amoeba, in the ditch, or in our own white blood corpuscules or other amoeboid cells. These were the originators of the animal kingdom. Thus, from very simple protists, the first animals and the first plants may have arisen. All were still very minute and it is worth remembering that had there been any scientific spectator after our kind upon the earth during these long ages, he would have lamented the entire absence of life, although the seas were teeming. The simplest forms of life and the protoplasm which Huxley called the physical basis of life will be dealt with in the chapter on biology in a later section of this work. First great steps in evolution, the first plants, the first animals, beginnings of bodies, evolution of sex, beginning of natural death. One, the contrast between plants and animals. However it may have come about, there is no doubt at all that one of the first great steps in organic evolution was the forking of the genealogical tree into plants and animals, the most important parting of the ways in the whole history of nature. Typical plants have chlorophyll. They are able to feed at a low chemical level on air, water and salts using the energy of the sunlight in their photosynthesis. They have their cells boxed in by cellulose walls so that their opportunities for motility are greatly restricted. They manufacture much more nutritive material than they need and live far below their income. They have no ready way of getting rid of any nitrogenous waste matter that they may form and this probably helps to keep them sluggish. Animals, on the other hand, feed at a high chemical level on the carbohydrates, for example starch and sugar, fats and proteins, for example gluten, albumin, casein, which are manufactured by other animals or to begin with by plants. Their cells have not cellulose walls nor in most cases much wool of any kind and motility in the majority is unrestricted. Animals live much more nearly up to their income if we could make for an animal and a plant of equal weight two fractions showing the ratio of the up-building constructive chemical processes to the down-breaking disruptive chemical processes that go on in their respective bodies, the ratio for the plant would be much greater than the corresponding ratio for the animal. In other words, animals take the munitions which plants laboriously manufacture and explode them in locomotion and work and the entire system of animate nature depends upon the photosynthesis that goes on in green plants. As the result of much more explosive life animals have to deal with much in the way of nitrogenous waste products, the ashes of the living fire but these are usually got rid of very effectively for example in the kidney filters and do not clog the system by being deposited in crystals and the like as happens in plants. Sluggish animals like sea squirts which have no kidneys are exceptions that prove the rule and it need hardly be said that the statements that have been made in regard to the contrasts between plants and animals are general statements. There is often a good deal of the plant about the animal as in sedentary sponges, zoophytes, corals and sea squirts and there is often a little of the animal about the plant as we see in the movements of all shoots and roots and leaves and occasionally in the parts of the flower. But the important fact is that on the early forking of the genealogical tree that is the divergence of plants and animals that are dependent and depends all the higher life of the animal kingdom not to speak of mankind. The continuance of civilization, the upkeep of the human and animal population of the globe and even the supply of oxygen to the air we breathe depend on the silent laboratories of the green leaves which are able with the help of the sunlight to use carbonic acid, water and salts to build up the bread of life. 2. The beginning of land plants It is highly probable that for long ages the waters covered the earth and that all the primeval vegetation consisted of simple flat blades in the universal open sea. But contraction of the earth's crust brought about elevations and depressions of the sea floor and in places the solid substratum was brought near enough the surface to allow the floating plants to begin to settle down without getting out of the light. This is how Professor Church pictures the beginning of a fixed vegetation a very momentous step in evolution. It was perhaps among this early vegetation that animals had their first successes. As the floor of the sea in these shallow areas was raised higher and higher there was a beginning of dry land. The sedentary plants already spoken of were the ancestors of the shore seaweeds and there is no doubt that when we go down at the lowest tide and wade cautiously out among the jungle vegetation only exposed on such occasions we are getting a glimpse of very ancient days. This is the forest primeval. 3. The protozoa Animals below the level of zoophytes and sponges are called protozoa. The word obviously means first animals but all that we can say is that the very simplest of them may give us some hint of the simplicity of the original first animals for it is quite certain that the vast majority of the protozoa today are far too complicated to be thought of as primitive. Though most of them are microscopic each is an animal complete in itself with the same fundamental bodily attributes as are manifested in ourselves. They differ from animals of higher degree in not being built up of the unit areas or corpuscules called cells. They have no cells, no tissues, no organs in the ordinary acceptation of these words but many of them show a great complexity of internal structure far exceeding that of the ordinary cells that build up the tissues of higher animals. They are complete living creatures which have not gone in for body making. In the dim and distant past there was a time when the only animals were of the nature of protozoa and it is safe to say that one of the great steps in evolution was the establishment of three great types of protozoa. A. Somewhere very active the infusurians like the slipper animalcule, the nightlight, noctilutia which makes the sea phosphorescent at night and the deadly trypanosome which causes sleeping sickness. B. Others were very sluggish the parasitic sporozoa like the malaria organism which the mosquito introduces into man's body. C. Others were neither very active nor very passive the riserpots with outflowing processes of living matter. This amoeboid line of evolution has been very successful it is represented by the riserpots such as amoeba and the chalk forming foraminifera and the exquisitely beautiful flincheld radiolarians of the open sea. They have their counterparts in the amoeboid cells the most multicellular animals such as the phagocytes which migrate about in the body and gulfing and digesting and treating bacteria serving as sappers and miners when something has to be broken down and built up again and performing other useful offices. 3. The making of a body The great naturalist Louis Agassiz once said that the biggest gulf in organic nature was that between the unicellular and the multicellular animals protozoa and metozoa but the gulf was bridged very long ago when sponges, stinging animals and simple worms were evolved and showed for the first time a body. What would one not give to be able to account for the making of a body one of the great steps in evolution? No one knows but the problem is not altogether obscure. When an ordinary protozoan or one celled animal divides into two or more which is its way of multiplying the daughter units thus formed, flowed apart and live independent lives but there are a few protozoa in which the daughter units are not quite separated off from one another but remain coherent. Thus volvox, a beautiful green ball found in some canals and alike is a colony of a thousand or even ten thousand cells it has almost formed the body but in this colony making protozoan and in others like it the component cells are all of one kind whereas in true multicellular animals there are different kinds of cells showing division of labour there are some other protozoa in which the nucleus or carnal divides into many nuclei within the cell this is seen in the giant amoeba, Pelomixa sometimes found in duck ponds or the beautiful opalina which always lives in the hind part of the frog's foot canal if a portion of the living matter of these protozoa should gather around each of the nuclei then that would be the beginning of a body it would be still near the beginning of a body if a division of labour set in and if there was a setting apart of egg cells and sperm cells distinct from body cells it was possibly in some such way that animals and plants with a body were first evolved two points should be noticed that body making is not essentially a matter of size though it made large size possible for the body of a many-celled wheel anomalkyl or rotifer is no bigger than many a protozoan yet the rotifer, we are thinking of hydratina has 900 odd cells whereas the protozoan has only one except in forms like volvox secondly it is a luminous fact that every many-celled animal from sponge to man that multiplies in the ordinary way begins at the beginning again as a single cell the fertilised egg cell it is of course not an ordinary single cell that develops into an earthworm or a butterfly an eagle or a man it is a cell in which a rich inheritance the fruition of ages is somehow condensed but it is interesting to bear in mind the elementary fact that every many-celled creature reproduced in the ordinary way and not by budding or the like starts as a fertilised egg cell the coherence of the daughter cells into which the fertilised egg cell divides is a reminiscence, as it were of the primeval coherence of daughter units that made the first body possible the beginning of sexual reproduction a fresh water hydra growing on the duckweed usually multiplies by budding it forms daughter buds living images of itself a check comes to nutrition and these daughter buds go free a big sea anemone may divide in two or more parts and become separate animals this is asexual reproduction which means that the multiplication takes place by dividing into two or many portions and not by liberating egg cells and sperm cells among animals as among plants asexual reproduction is very common but it has great disadvantages for it is apt to be physiologically expensive and it is beset with difficulties when the body shows great division of labour and is very intimately bound into unity thus no one can think of a bee or a bird multiplying by division or by budding moreover if the body of the parent has suffered from injury or deterioration the result of this is bound to be handed on to the next generation if asexual reproduction is the only method splitting into two or many parts was the old fashioned way of multiplying but one of the great steps in evolution was a discovery of a better method namely sexual reproduction the gist of this is simply that during the process of bodybuilding by the development of the fertilised egg cell certain units, the germ cells do not share in forming ordinary tissues or organs but remain apart continuing the full inheritance which was condensed in the fertilised egg cell these cells kept by themselves are the originators of the future reproductive cells of the mature animal they give rise to the egg cells and the sperm cells the advantages of this method are great one, the new generation is started less expensively for it is easier to shed germ cells into the cradle of the water than to separate off half of the body two, it is possible to start a great many new lives at once and this may be of vital importance when the struggle for existence is very keen and when parental care is impossible three, the germ cells are little likely to be prejudicially affected by disadvantageous dints impressed on the body of the parent little likely unless the dints have peculiarly penetrating consequences as in the case of poisons four, a further advantage is implied in the formation of two kinds of germ cells the ovum or egg cell with a considerable amount of building material and often with a legacy of nutritive yolk the spermatozoan or sperm cell adapted to move in fluids and to find the ovum from a distance thus securing change provoking cross fertilization four, the evolution of sex another of the great steps in organic evolution was a differentiation of two different physiological types the male or sperm producer and the female or egg producer it seems to be a deep-seated difference in constitution which leads one egg to develop into a male and another lying beside it in the nest into a female in the case of pigeons it seems almost certain from the work of professor Oscar Riddle that there are two kinds of egg a male-producing egg and a female-producing egg which differ in their yolk-forming and other physiological characters in sea urgence we often find two creatures superficially indistinguishable but the one is a female with large ovaries and the other is a male with equally large testes here the physiological difference does not affect the body as a whole but the reproductive organs are gonads only though more intimate physiology would doubtless discover differences in the blood or in the chemical routine metabolism in a large number of cases however there are marked superficial differences between the sexes and everyone is familiar with such contrasts as peacock and peahen, stag and hind in such cases the physiological difference between the sperm producer and the ovum producer for this is the essential difference saturates through the body and expresses itself in masculine and feminine structures and modes of behaviour the expression of the masculine and feminine characters is in some cases under the control of hormones or chemical messengers which are carried by the blood from the reproductive organs throughout the body and pull the trigger which brings about the development of an antler or a wattle or decorative plume or capacity for vocal and salutatory display in some cases it is certain that the female carries in a latent state the masculine features but these are kept from expressing themselves by other chemical messengers from the ovary of these chemical messengers more must be said later on recent research has shown that while the difference between male and female is very deep rooted corresponding to a difference in gearing there is always clear cut thus a hand pigeon may be very masculine and the cock pigeon very feminine the difference is in degree not in kind five what is the meaning of the universal or almost universal inevitableness of death a sequoia or big tree of california has been known to live for over two thousand years but eventually it died a centenarian tortoise has been known and a sea anemone sixty years of age but eventually they die what is the meaning of this apparently inevitable stoppage of bodily life the beginning of natural death there are three chief kinds of death A. the great majority of animals come to a violent end being devoured by others or killed by sudden and extreme changes in their surroundings B. when an animal enters a new habitat or comes into new associations with other organisms it may be invaded by a microbe or by some larger parasite to which it is unaccustomed and to which it can offer no resistance with many parasites a live and let live compromise is arrived at but new parasites are apt to be fatal as man knows to his cost when he's bitten by a tete fly which infects him with a microscopic animal a triphanosome that causes sleeping sickness in many animals that are not troublesome as long as the host is vigorous but if the host is out of condition the parasites may get the upper hand as in the so-called grouse disease and become fatal C. but besides violent death and microbe or parasitic death there is natural death this is in great part to be regarded as the price paid for a body a body worth having implies complexity or division of labour and internal furnishings of a more or less stable kind in which the effects of wear and tear are apt to accumulate it is not the living matter itself that grows old so much as the framework in which it works the furnishings are the vital laboratory there are various processes of rejuvenation for example, rest repair, change, reorganization which work against the inevitable processes of senescence but sooner or later recovery is with aging another deep reason for natural death is to be found in the physiological expansiveness of reproduction for many animals from worms to eels illustrate natural death as a nemesis of starting new lives now it is a very striking fact that to a large degree the simplest animals of protozoa are exempt from natural death they are so relatively simple that they can continually recuperate by rest and repair they do not accumulate any bad death moreover their modes of multiplying by dividing into two or many units are very inexpensive physiologically it seems that in some measure this bodily immortality of the protozoa is shared by some simple many-seld animals like the freshwater hydra and planarian worms here is an interesting chapter in evolution the evolution of means of evading or staving off natural death thus there is the well-known case of the pellet of the worm of the coral reefs where the body breaks up in liberating the germ cells but the head end remains fixed in a crevice of the coral and buds out a new body at leisure along with the evolution of the ways of avoiding death should be considered also the gradual establishment of the length of life best suited to the welfare of the species and the punctuation of the life history to suit various conditions 6. Great Acquisitions In animals like sea anemones and jellyfishes the general symmetry of the body is radial that is to say there is no right or left and the body might be halfed along many planes it is a kind of symmetry well suited for sedentary or for drifting life but worms begin the profitable habit of moving with one end of the body always in front and from worms to man the great majority of animals have bilateral symmetry they have a right and a left side and there is only one cut that halves the body this kind of symmetry is suited for a more strenuous life than radial animals show it is suited for pursuing food for avoiding enemies for chasing mates and with the establishment of bilateral symmetry must be associated the establishment of head brains the beginning of which is to be found in some simple worm types among the other great acquisitions that have evolved we may notice a well-developed head with sense organs the establishment of large internal surfaces such as the digestive and absorptive wall of the food canal the origin of quickly contracting striped muscle and of muscular appendages the formation of blood as a distributing medium throughout the body from which all the parts take what they need and to which they also contribute another very important acquisition almost confined so far as is known to black-boned animals was the evolution of what are called glands of internal secretion such as the thyroid and the supranrenal these manufacturers subtle chemical substances which are distributed by the blood throughout the body and have a manifold influence in regulating and harmonizing the vital processes some of these chemical messengers are called hormones which stimulate organs and tissues to greater activity others are called calones which put on a break and relate growth and others rapidly alter the pressure and composition of the blood some of them call into active development certain parts of the body which have been as it were waiting for an appropriate trigger pooling thus at the proper time the milk glands of a mammalian mother are awakened from their dormancy this very interesting outcome of evolution will be dealt with in another portion of this work End of Section 3 Section 4 of the Outline of Science, Volume 1 This is a LibriVox recording all LibriVox recordings are in the public domain for more information or to volunteer please visit LibriVox.org The Outline of Science, Volume 1 by J. Arthur Thompson Section 4 Part 2 The Story of Evolution Continued The Inclined Plane of Animal Behavior 1. Before passing to a connected story of the gradual emergence of higher and higher forms of life in the course of the successive ages the procession of life as it may be called it will be useful to consider the evolution of animal behavior Evolution of Mind A human being begins as a microscopic fertilized egg cell within which there is condensed the long result of time man's inheritance the long period of nine months before birth with its intimate partnership between mother and offspring is passed as if it were in sleep and no one can make any statement in regard to the mind of the unborn child even after birth the dawn of mind is as slow as it is wonderful to begin with there is in the ovum and early embryo no nervous system at all and it develops very gradually from simple beginnings yet as mentality cannot come in from outside we seem bound to conclude that the potentiality of it whatever that means resides in the individual from the very first the particular kind of activity known to us as thinking feeling and willing is the most intimate part of our experience known to us directly apart from our senses and the possibility of that must be implicit in the germ cell just as the genius of newton was implicit in a very miserable specimen of an infant now what is true of the individual is true also of the race there is a gradual evolution of that aspect of the living creatures activity which we call mind we cannot put our finger on any point and say before this stage there was no mind indeed many facts suggest the conclusion that wherever there is life there is some degree of mind even in the plants or it might be more accurate to put the conclusion in another way that the activity we call life has always in some degree an inner or mental aspect in another part of this book there is an account of the dawn of mind in backboneed animals what we aim at here is an outline that may be called the inclined plane of animal behavior a very simple animal accumulates a little store of potential energy and it proceeds to expend this like an explosive by acting on its environment it does so in a very characteristic self-preservative fashion so that it burns without being consumed and explodes without being blown to bits it is characteristic of the organism that it remains a going concern for a longer or shorter period its length of life living creatures that expended their energy ineffectively or self-destructively would be eliminated in the struggle for existence when a simple one-celled organism explores a corner of the field seen under a microscope behaving to all appearance very like a dog scouring a field seen through a telescope it seems permissible to think of something corresponding to mental endeavor associated with its activity this impression is strengthened when an amoeba pursues another overtakes it engulfs it loses it pursues it again recaptures it and so on what is quite certain is that the behavior of the animal cule is not like that of a potassium pill fizzing about in a basin of water nor like the lurching movements of a gun that has got loose and taken charge on board ship another feature is that the locomotor activity of an animal cule often shows a distinct individuality it may swim for instance in a loose spiral but there is another side to vital activity besides acting upon surrounding world the living creature is acted on by influences from without the organism acts on its environment that is the one side of the shield the environment acts upon the organism that is the other side if we are to see life whole we must recognize these two sides of what we call living and it is missing an important part of the history of animal life if we fail to see that evolution implies becoming more advantageously sensitive to the environment making more of its influences shutting out profitless stimuli and opening more gateways to knowledge the birds world is a larger and finer world than an earthworms the world means more to the bird than to the worm the trial and error method simple creatures act with a certain degree of spontaneity on their environment and they likewise react effectively to surrounding stimuli animals come to have definite answers back sometimes several sometimes only one as in the case of the slipper anima-cule which reverses its cilia when it comes within the sphere of some disturbing influence retreats and turning upon itself tentatively sets off again in the same general direction as before but at an angle to the previous line if it misses the disturbing influence well and good if it strikes it again the tactics are repeated until a satisfactory way out is discovered or the stimulation proves fatal it may be said that the slipper anima-cule has but one answer to every question but there are many protozoa which have several in registered reactions when there are alternative reactions which are tried one after another the animal is pursuing what is called the trial and error method and a higher note is struck there is an endeavor after satisfaction and a trial of answers when the creature profits by experience to the extent of giving the right answer first there is the beginning of learning reflex actions among simple multi-cellular animals such as sea anemones we find the beginnings of reflex actions and a considerable part of the behavior of lower animals is reflex that is to say there are laid down in the animal in the course of its development certain prearrangements of nerve cells and muscle cells which secure that a fit and proper answer is given to a frequently recurrent stimulus an earthworm half out of its burrow becomes aware of the light tread of a thrush's foot and jerks itself back into its hole before anyone can say reflex action what is it that happens certain sensory nerve cells in the earthworm's skin are stimulated by vibrations in the earth the message travels down a sensory nerve fiber from each of the stimulated cells and enters the nerve cord the sensory fibers come into vital connection with branches of intermediary associative or communicating cells which are likewise connected with motor nerve cells to these the message is thus shunted from the motor nerve cells an impulse or command travels by motor nerve fibers one from each cell to the muscles which contract if this took as long to happen as it takes to describe even an outline it would not be of much use to the earthworm but the motor answer follows the sensory stimulus almost instantaneously the great advantage of establishing or in registering these reflex chains is that the answers are practically ready-made or inborn not requiring to be learned it is not necessary that the brain should be stimulated if there is a brain nor does the animal will to act though in certain cases it may by means of higher controlling nerve centers keep the natural reflex response from being given as happens for instance when we control a cough or a sneeze on some solemn occasion the evolutionary method of it if we may use the expression has been to in register ready-made responses and as we ascend the animal kingdom we find reflex actions becoming complicated and often linked together so that the occurrence of one pulls the trigger of another and so on in a chain the behavior of the insectivorous plant called venus' fly trap when it shuts on an insect is like a reflex action in an animal but plants have no definite nervous system what are called tropisms a somewhat higher level on the inclined plane is illustrated by what are called tropisms obligatory movements which the animal makes adjusting its whole body so that physiological equilibrium results in relation to gravity pressure, currents, moisture heat, light, electricity and surfaces of contact a moth is flying past a candle the eye next to light is more illumined than the other a physiological in equilibrium results affecting nerve cells and muscle cells the outcome is that the moth automatically adjusts its flight so that both eyes become equally illumined in doing this it often flies into the candle it may seem bad business that the moth should fly into the candle but the flame is an utterly artificial item in its environment to which no one can expect it to be adapted these tropisms play an important role in animal behavior 2. Instinctive behavior on a higher level is instinctive behavior which reaches such remarkable perfection in ants, bees and wasps in its typical expression instinctive behavior depends on inborn capacities it does not require to be learned it is independent of practice or experience though it may be improved by both it is shared equally by all members of the species of the same sex for the females instincts are often different from the males it refers to particular conditions of life that are of vital importance though they may occur only once in a lifetime the female yucca moth emerges from the cocoon when the yucca flower puts forth its bell-like blossoms she flies to a flower collects some pollen from the stamens needs it into a pill-like ball and stows this away under her chin she flies to an older yucca flower and lays her eggs in some of the ovules within the seed box but before she does so we can pause it on the stigma the ball of pollen from this the pollen tubes grow down and the pollen nucleus of a tube fertilizes the egg cell in an ovule so that the possible seeds become real seeds for it is only a fraction of them that the yucca moth has destroyed by using them as cradles for her eggs now it is plain that the yucca moth has no individual experience of yucca flowers yet she secures the continuance of her race by a concatenation of actions which form part of her instinctive repertory from a physiological point of view instinctive behavior is like a chain of compound reflex actions but in some cases at least there is reason to believe that the behavior is suffused with awareness and backed by endeavor this is suggested in exceptional cases where the stereotyped routine is departed from to meet exceptional conditions it should also be noted that just as ants, hive bees and wasps exhibit in most cases pure instinctive behavior but move on occasion on the main line of trial and error or of experimental initiative so among birds and mammals the intelligent behavior is sometimes replaced by instinctive routine perhaps there is no instinctive behavior without a spice of intelligence and no intelligent behavior without an instinctive element the old view that instinctive behavior was originally intelligent and that instinct is lapsed intelligence is attempting one is suggested by the way in which habitual intelligent actions cease in the individual to require intelligent control but it rests on the unproved hypothesis that the acquisitions of the individual can be entailed on the race it is almost certain that instinct is on a line of evolution quite different from intelligence and that it is nearer to the inborn inspirations of the calculating boy or the musical genius than to the plotting methods of intelligent learning animal intelligence the higher reaches of the inclined plane of behavior show intelligence in the strict sense they include those kinds of behavior which cannot be described without the suggestion that the animal makes some sort of perceptual inference not only profiting by experience but learning by ideas such intelligent actions so great individual variability they are plastic and adjustable in a manner rarely hinted at in connection with instincts without the creature being nonplussed they are not bound up with particular circumstances as instinctive actions are but imply an appreciative awareness of relations when there is an experimenting with general ideas when there is conceptual as contrasted with perceptual inference we speak of reason but there is no evidence of this below the level of man it is not indeed always that we can credit man with rational conduct but he has the possibility of it ever within his reach animal instinct and intelligence will be illustrated in another part of this work we are here concerned simply with the general question of the evolution of behavior there is a main line of tentative experimental behavior both below and above the level of intelligence and it has been part of the tactics of evolution to bring about the hereditary and registration of capacities of effective response the advantages being that the answers come more rapidly and that the creature is left free if it chooses for higher adventures there is no doubt as to the big fact that in the course of evolution animals have shown an increasing complexity and masterfulness of behavior that they have become at once more controlled and more definitely free agents and that the inner aspect of the behavior experimenting, learning, thinking feeling and willing has come to count for more and more 3. Evolution of parental care mammals furnish a crowning instance of a trend of evolution which expresses itself at many levels the tendency to bring forth the young at a well advanced stage and to an increase of parental care associated with a decrease in the number of offspring there is a British starfish called Ludia which has 200 millions of eggs in a year and there are said to be several millions of eggs in conger eels and some other fishes these illustrate the spawning method of solving the problem of survival some animals are naturally prolific and the number of eggs which they sow broadcast in the waters allows for enormous infantile mortality and obviates any necessity for parental care but some other creatures by nature less prolific have found an entirely different solution of the problem they practice parental care and they secure survival with greatly economized reproduction this is a trend of evolution particularly characteristic of the higher animals so much so that Herbert Spencer formulated the generalization that the size and frequency of the animal family is inverse ratio to the degree of evolution to which the animal has attained now there are many different methods of parental care which secure the safety of the young and one of these is called vivaparity the young ones are not liberated from the parent until they are relatively well advanced and more or less able to look after themselves this gives the young a good send-off in life and their chances of death are greatly reduced in other words the animals that have varied in the direction of the economized reproduction may keep their foothold in the struggle for existence if they have varied at the same time in the direction of parental care in other cases it may have worked the other way round in the interesting archaic animal called parapatis which has to face a modern world too severe for it one of the methods of meeting the enviring difficulties is the retention of the offspring for many months within the mother so that it is born a fully formed creature there are only a few offspring at a time and although there are exceptional cases like the summer green flies which are very prolific though vivaparis the general rule is that vivaparity is associated with a very small family the case of flowering plants stands by itself for although they illustrate a kind of vivaparity the seed being embryos an individual plant may have a large number of flowers and therefore a huge family vivaparity naturally finds its best illustrations among terrestrial animals where the risks to the young life are many and it finds its climax among mammals now it is an interesting fact that the three lowest mammals the duck mall and two spiny anteaters lay eggs that is our ova paris that the marsupials on the next grade bring forth their young as it were prematurely and in most cases stow them away in an external pouch while all the others the placentals show a more prolonged antinatal life and an intimate partnership between the mother and the unborn young four there is another way of looking at the sublime process of evolution it has implied a mastery of all the possible haunts of life it has been a progressive conquest of the environment one it is highly probable that living organisms found their foothold in the stimulating conditions of the shore of the sea the shallow water brightly illumined seaweed growing shelf fringing the continents this literal zone was a propitious environment where sea and fresh water earth and air all meet where there is stimulating change abundant oxygenation and a copious supply of nutritive material in what the streams bring down and in the rich seaweed vegetation it is not an easy haunt of life but none the worse for that and it is tenanted today by representatives of practically every class of animals from infusorians to seashore birds and mammals the cradle of the open sea two the open sea or pelagic haunt includes all the brightly illumined surface waters beyond the shallow water of the shore area it is perhaps the easiest of all haunts of life for there is no crowding there is considerable uniformity and an abundance of food for animals is afforded by the inexhaustible floating sea metals of microscopic algae these are reincarnated in minute animals like the open sea crustaceans which again are utilized by fishes these in turn making life possible for higher forms like carnivorous turtles and toothed whales it is quite possible that the open sea was the original cradle of life and perhaps professor church is right in picturing a long period of pelagic life before there was any sufficiently shallow water to allow the floating plants to anchor it is rather in favor of this view that many shore animals such as crabs starfishes spend their useful stages in the relatively safe cradle of the open sea and only return to the more strenuous conditions of their birthplace after they have gained considerable strength of body it is probably safe to say that the honor of being the original cradle of life lies between the shore of the sea and the open sea the great deeps three a third haunt of life is the floor of the deep sea the abyssal area which occupies more than a half of the surface of the globe it is a region of extreme cold and eternal winter of utter darkness and eternal night relieved only by the fitful gleams of phosphorescent animals of enormous pressure two and a half tons on the square inch at a depth of two thousand five hundred fathoms of profound calm unbroken silence immense monotony and as there are no plants in the great abysses the animals must live on one another and in the long run on the rain more abundant animicules which sink from the surface through the miles of water it seems a very unpromising haunt of life but it is abundantly tenanted and it gives us a glimpse of the insurgent nature of the living creature that the difficulties of the deep sea should have been so effectively conquered it is probable that the colonizing of the great abysses took place in relatively recent times for the fauna does not include many very antique types it is practically certain that the colonization was due to literal animals which followed the food debris millennium after millennium further and further down the long slope from the shore the fresh waters four a fourth haunt of life is that of the fresh waters including river and lake pond and pool swamp and marsh it may have been colonized by gradual migration up estuaries and rivers or by more direct passage from the seashore into the brackish swamp or it may have been in some cases that partially blocked corners of ancient seas became gradually turned into fresh water basins the animal population of the fresh waters is very representative and is diversely adapted to meet the characteristic contingencies the risk of being dried up the risk of being frozen hard in winter and the risk of being left high and dry after floods or of being swept down to the sea conquest of the dry land five the terrestrial haunt has been invaded age after age by contingents from the sea or from the fresh waters we must recognize the worm invasion which led eventually to the making of the fertile soil the invasion due to air breathing arthropods which led eventually to the important linkage between flowers and their insect visitors and the invasion due to air breathing amphibians which led eventually to the higher terrestrial animals and to the development of intelligence and family affection besides these three great invasions there were minor ones such as that leading to land snails for there has been a widespread and persistent tendency among aquatic animals to try to possess the dry land getting onto dry land had a manifold significance it implied getting into a medium with a much larger supply of oxygen than there is dissolved in the water but the oxygen of the air is more difficult to capture especially when the skin becomes hard or well protected as it is almost bound to become in animals living on dry ground thus this leads to the development of internal surfaces such as those of lungs where the oxygen taken into the body may be absorbed by the blood in most animals the blood goes to the surface of oxygen capture but in insects and their relatives there is a different idea of taking the air to the blood or in greater part to the area of oxygen consumption the living tissues a system of branching air tubes takes air into every hole and corner of the insects body and this thorough aeration is doubtless in part the secret of the insects intense activity the blood never becomes impure the conquest of the dry land also implied a predominance of that kind of locomotion which may be compared to punting when the body is pushed along by pressing a lever against a hard substratum and it also followed that with few exceptions the body of the terrestrial animal tended to be compact readily lifted off the ground by the limbs or adjusted in some other way so that there may not be too large a surface on the ground an animal like a jellyfish easily supported in the water would be impossible on land such apparent exceptions as earthworms centipedes and snakes are not difficult to explain for the earthworm is a burr which eats its way through the soil the centipedes long body is supported by numerous hard legs and the snake pushes itself along by means of the large ventral scales to which the lower ends of very numerous ribs are attached methods of mastering the difficulties of terrestrial life a great restriction attendant on the invasion of the dry land is that locomotion becomes limited to one plane namely the surface of the earth this is in great contrast to what is true in the water where the animal can move up or down to right or to left at any angle and in three dimensions it surely follows from this that the movements of land animals must be rapid and precise unless indeed safety is secured in some other way hence it is easy to understand why most land animals have very finely developed striped muscles and why Abido running on the ground has far more numerous muscles than a lobster swimming in the sea land animals were also handicapped by the risks of drought and a frost but these were met by defences of the most diverse description from the hairs of woolly caterpillars to the fur of mammals from the carapace of tortoises to the armor of armadillos in other cases it is hardly to say the difficulties may be met in other ways as frogs meet the winter by falling into a lethargic state in some secluded retreat another consequence of getting onto dry land is that the eggs or young can no longer be set free anyhow as is possible when the animal is surrounded by water which is in itself more or less of a cradle if the eggs were laid or the young liberated on dry ground the chances are many that they would be dried up or devoured so there are numerous ways in which land animals secure the safety of their young for example by burying them in the ground or by hiding them in nests or by carrying them about for a prolonged period either before or after birth this may mean great safety for the young this may make it possible to have only a small family and this may tend to the evolution of parental care and the kindly emotions thus it may be understood that from the conquest of the land many far reaching consequences have followed finally it is worth dwelling on the risks of terrestrial life because they enable us better to understand why so many land animals have become burrowers and others climbers of trees why some have returned to the water and others have taken to the air it may be asked perhaps why the land should have been colonized at all when the risk and difficulties are so great the answer must be that necessity and curiosity are the mother and father of invention animals left the water because the pools dried up or because they were overcrowded or because of inveterate enemies but also because of that curiosity and spirit of adventure which from first to last has been one of the spurs of progress conquering the air six the last great haunt of life is the air a mastery of which must be placed to the credit of insects pterodactyls birds and bats these have been the successes but it should be noted that there have been many brilliant failures which have not attained to much more than parachuting these include the flying fishes which take leaps from the water and are carried for many yards and to considerable heights holding their enlarged pectoral fins taught or with little more than a slight fluttering there is a so-called flying frog rachophorus that skims from branch to branch and the more effective flying dragon dracovolans of the far east which has been mentioned already among mammals there are flying phalangers flying lemurs and more besides all attaining to great skill as parachutists and illustrating the endeavor to master the air which man has realized in a way of his own the power of flight rings obvious advantages a bird feeding on the ground is able to evade the stalking carnivore by suddenly rising into the air food and water can be followed rapidly and to great distances the eggs or the young can be placed in safe situations and birds in their migrations have made a brilliant conquest both of time and space many of them know no winter in their year and the migratory flight of the pacific golden plover from Hawaii to Alaska and back again does not stand alone the procession of life through the ages one the rock record how do we know when the various classes of animals and plants were established on the earth how do we know the order of their appearance and the succession of their advances the answer is by reading the rock record in the course of time the crust of the earth has been elevated into continents and depressed into ocean troughs and the surface of the land has been buckled up into mountain ranges and folded in gentler hills and valleys the high places of the land have been weathered by air and water in many forms and the results of the weathering have been born away by rivers and seas to be laid down again elsewhere as deposits which eventually formed sandstones and similar sedimentary rocks much of the material of the original crust has thus been broken down and worked up again many times over and if the total thickness of the sedimentary rocks is added up it amounts according to some geologist to a total of 67 miles in most cases however only a small part of this thickness is to be seen in one place for the deposits were usually formed in limited areas at any one time the use of fossils when the sediments were accumulating age after age it naturally came about that remains of the plants and animals living at the time were buried and these formed the fossils by the aid of which it is possible to read the story of the past by careful piecing together of evidence the geologist is able to determine the order in which the different sedimentary rocks were laid down and thus to say for instance that the Devonian period was the time of the origin of amphibians the geologist utilizes the fossils in his attempt to work out the order of the strata when these have been much disarranged for the simpler fossil forms of any type must be older than those that are more complex there is no vicious circle here for the general succession of strata is clear and it is quite certain that there were fishes before there were amphibians and amphibians before there were reptiles and reptiles before there were birds and mammals in certain cases for example there were animals and elephants the actual historical succession has been clearly worked out if the successive strata contained good samples of all the plants and animals living at the time when the beds were formed then it would be easy to read the record of the rocks but many animals were too soft to become satisfactory fossils many were eaten or dissolved away many were destroyed by heat and pressure so that the rock record is like a library very much damaged by fire and looting and decay the geological timetable the long history of the earth and its inhabitants is conveniently divided into errors thus just as we speak of the ancient, medieval and modern history of mankind so we may speak of paleozoic, mesozoic and senozoic errors in the history of the earth as a whole geologists cannot tell us except in an approximate way how long the process of evolution has taken one of the methods is to estimate how long has been required for the accumulation of the salts of the sea for all these have been dissolved out of the rocks since rain began to fall on the earth dividing the total amount of saline matter by what is contributed every year in modern times we get about a hundred million years as the age of the sea but as the present rate of salt accumulation is probably much greater than it was during many of the geological periods the prodigious age just mentioned is in all likelihood far below the mark another method is to calculate how long it would take to form the sedimentary rocks like sandstones and mudstones which have a total thickness of over 50 miles though the local thickness is rarely over a mile as most of the materials have come from the weathering of the earth's crust and as the annual amount of weathering now going on can be estimated the time required for the formation of the sedimentary rocks of the world can be approximately calculated there are some other ways of trying to tell the earth's age and the length of the successive periods but no certainty has been reached the errors before the Cambrian correspond to about 32 miles of thickness of strata and all the subsequent errors with fossil-bearing rocks to a thickness of about 21 miles in itself an astounding fact perhaps 30 million years must be allotted to the pre-Cambrian errors 18 to the Paleozoic 9 to the Mesozoic 3 to the Cenozoic making a grand total of 60 millions the establishment of invertebrate stocks it is an astounding fact that at least half of geological time the Archeozoic and Proterozoic eras passed before there were living creatures with parts sufficiently hard to form fossils in the latter part of the Proterozoic era there are traces of one-celled marine animals Radiolarians with shells of flint and of worms that wallowed in the primal mud it is plain that as regards the most primitive creatures the rock record tells us little the rarity of direct traces of life in the oldest rocks is partly due to the fact that the primitive animals would be of delicate build but it must also be remembered that the ancient rocks have been profoundly and repeatedly changed by pressure and heat so that the traces which did exist would be very liable to obliteration and if it be asked what right we have to suppose the presence of living creatures in the absence or extreme rarity of fossils we must point to great accumulations of limestone which indicate the existence of Calcerius algae and to deposits of iron which probably indicate the activity of iron-forming bacteria ancient beds of graphite similarly suggest that green plants flourished in these ancient days three the era of ancient life Paleozoic the Cambrian period was the time of the establishment of the chief stocks of backboneless animals such as sponges, jellyfishes, worms sea cucumbers, lamp shells, trilobites crustaceans and mollusks there is something very eloquent in the broad fact that the peopling of the seas had definitely begun some 30 million years ago for professor H. F. Osborne points out that in the Cambrian period there was already a colonization of the shore of the sea, the open sea and the deep waters the Ordovisian period was marked by abundant representation of the once very successful class of trilobites the ancient footed antennae bearing segmented marine animals with numerous appendages and a covering of chitin they died away entirely with the end of the paleozoic era also very notable was the abundance of predatory cuttlefishes the bullies of the ancient seas but it was in this period that the first backboneed animals made their appearance an epic-making step in evolution in other words, true fishes were evolved destined in the course of ages of the cuttlefishes which are mere mollusks in dominating the seas in the Silurian period in which the peopling of the seas went on a pace there was the first known attempt at colonizing the dry land for in the Silurian rocks there are fossil scorpions and that implies ability to breathe dry air by means of internal surfaces in this case known as lung books it was also towards the end of the Silurian when a period of great eridity set in that fishes appeared related to our mudfishes or double breathers dipnoi which have lungs as well as gills this again meant utilizing dry air just as the present day mudfishes do when the water disappears from the pools in hot weather the lung fishes or mudfishes of today are but three in number one in Queensland, one in South America and one in Africa but they are extremely interesting living fossils binding the class of fishes to that of amphibians it is highly probable that the first invasion of the dry land should be put to the credit of some adventurous worms but the second great invasion was certainly due to air breathing arthropods like the pioneer scorpion we mentioned the Devonian period including that of the old red sandstone was one of the most significant periods in the earth's history for it was the time of the establishment of flowering plants upon the earth and of terrestrial back boned animals one would like to have been the discoverer of the Devonian footprint of Thinopus the first known amphibian footprint an eloquent vestige of the third great invasion of the dry land it was probably from a stock of Devonian lung fishes that the first amphibians sprang but it was not till the next period that they came to their own while they were still feeling their way there was a remarkable exuberance of shark-like and heavily armored fishes in the Devonian seas End of Section 4 Section 5 of the Outline of Science, Volume 1 this is a LibriVox recording all LibriVox recordings are in the public domain for more information or to volunteer please visit LibriVox.org The Outline of Science, Volume 1 by J. Arthur Thompson Section 5 Part 2 The Story of Evolution continued Evolution of Land Animals Giant Anphibians and Coal Measures The Carboniferous period was marked by a mild moist climate and a luxuriant vegetation in the swampy low grounds it was a much less strenuous time than the Devonian period it was like a very long summer there were no trees of the type we see now but there were forests of club mosses and horsetails that grew to gigantic size compared with their pygmy representatives of today in these forests the jointed footed invaders of the dry land ran riot in the form of centipedes spiders scorpions and insects and on these the primeval amphibians fed the appearance of insects made possible a new linkage of far reaching importance namely the cross-fertilization of flowering plants by their insect visitors and from this time onwards it may be said that flowers and their visitors have evolved hand in hand cross-fertilization is much sure less than by the wind and cross-fertilization is more advantageous than self-fertilization because it promotes both fertility and plasticity it was probably in this period that colored flowers attractive to insect visitors began to justify themselves as beauty became useful and began to relieve the monotonous green of the horsetail and club moss forests which covered great tracks of the earth for millions of years there were also land snails representing one of the minor invasions of the dry land tending on the hull to check vegetation they too were probably preyed upon by the amphibians some of which attained a large size each age has had its giants and those of the carboniferous were amphibians called labyrinthodonts some of which were almost as big as donkeys it need hardly be said that it was in this period that most of the coal measures were laid down by the immense accumulation of the spores and debris of the club moss forests ages afterwards it was given to man to tap this great source of energy traceable back to the sunshine of millions of years ago even then it was true that no plant or animal lives or dies to itself the acquisitions of amphibians as amphibians had their golden age in the carboniferous period we may fitfully use this opportunity of indicating the advances in evolution which the emergence of amphibians implied 1. In the first place the passage from water to dry land was the beginning of a higher and more promiseful life, taxed no doubt by increased difficulties the natural question rises why animals should have migrated from water to dry land at all when great difficulties were involved in the transition the answers must be of water basins or elevations of the land surface often made the old haunts untenable b. that there may have been great congestion and competition in the old quarters and c. that there has been an undeniable endeavor after well-being throughout the history of animal life in the same way with mankind migrations were prompted by the setting in of prolonged drought by overpopulation and by the spirit of adventure 2. In amphibians for the first time the non-digitate paired fins of fishes were replaced by limbs with fingers and toes this implied an aventacious power of grasping of holding firm of putting food into the mouth of feeling things in three dimensions 3. We cannot be positive in regard to the soft parts of the ancient amphibians known only as fossils but if they were in a general way like the frogs and toads newts and salamanders of the present day we may say that they made among other acquisitions the following true ventral lungs a three-chambered heart a movable tongue a drum to the ear and lids to the eyes it is very interesting to find that though the tongue of the tadpole has some muscle fibers in it they are not strong enough to affect movement recalling the tongue of fishes which has not any muscles at all gradually as the tadpole becomes a frog the muscle fibers grow in strength it is possible for the full grown creature to shoot out its tongue upon insects this is probably a recapitulation of what was accomplished in the course of millennia in the history of the amphibian race 4. Another acquisition made by amphibians was a voice due, as in ourselves to the rapid passage of air over taught membranes, focal cords stretched in the larynx it is an interesting fact that for millions of years there was upon the earth no sound of life at all only the noise of wind and wave thunder and avalanche apart from the instrumental music of some insects perhaps beginning in the carboniferous the first vital sounds were due to amphibians and there certainly was the first voice surely one of the great steps in organic evolution Evolution of the voice the first use of the voice was probably that indicated by our frogs and toads it serves as a sex call that is the meaning of the trumpeting with which frogs herald the spring and it is often only in the males that the voice is well developed but if we look forward past amphibians altogether we find the voice becoming a maternal call helping to secure the safety of the young a use very obvious when young birds squat motionless at the sound of the parent's danger note later on probably the voice became an infantile call as when the unhatched crocodile pipes from within the deeply buried egg signalling to the mother that it is time to be unearthed higher still the voice expresses emotion as in the song of birds often outside the limits of the breeding time later still particular sounds become words signifying particular things or feelings such as food danger home anger and joy finally words become a medium of social intercourse and as symbols help to make it possible for man to reason two the early reptiles in the Permian period reptiles appeared or perhaps one should say began to assert themselves that is to say there was an emergence of backboneed animals which were free from water and relinquished the method of breathing by gills which amphibians retained in their young stages at least the unhatched or unborn reptile breathed by means of a vascular hood spread underneath the eggshell and absorbing dry air from without it is an interesting point that this vascular hood called the Alantois is represented in amphibians by an unimportant bladder growing out from the hind end of the food canal a great step in evolution was implied in the origin of this antenatal hood or fetal membrane and another one of protective significance called the amnion which forms a water bag over the delicate embryo the step meant total emancipation from the water and from gill breathing and the two fetal membranes the amnion and the Alantois persists not only in all reptiles but in birds and mammals as well these higher vertebrates are therefore called amniata in contrast to the lower vertebrates or anamnia the amphibians fishes and primitive types it is a suggestive fact that the embryos of all reptiles, birds and mammals show gill cliffs a telltale evidence of their distant aquatic ancestry but these embryonic gill cliffs are not used for respiration and show no trace of gills except in a few embryonic reptiles and birds where their dwindled vestiges have been recently discovered as to the gill cliffs they are of no use in higher vertebrates except that the first becomes the eustachian tube leading from the ear passage to the back of the mouth the reason why they persist is because they are of no use and that in a transformed guise would be difficult to interpret except in terms of the evolution theory they illustrate the lingering influence of a long pedigree the living hand of the past the tendency that individual development has to recapitulate racial evolution in a condensed and telescope manner of course for what took the race a million years may be recapitulated by the individual in a week in the Permian period the warm moist climate of most of the carboniferous period was replaced by severe conditions culminating in an ice age which spread from the southern hemisphere throughout the world with this was associated a waning of the carboniferous flora and the appearance of a new one consisting of ferns, conifers, ginkos and cycads which persisted until near the end of the Mesozoic era the Permian Ice Age lasted for millions of years and was severe in the far south of course it was a very different world then for North Europe was joined to North America Africa to South America and Australia to Asia it was probably during the Permian Ice Age that many of the insects divided their life history into two main chapters the feeding, growing, molting, immature, larval stages that is caterpillars and the more ascetic non-growing, non-molting winged face adapted for reproduction between these they're intervened the quiescent, well protected pupa stage or chrysalis probably adapted to begin with as a means of surviving the severe winter for it is easier for an animal to survive when the vital processes are more or less in abeyance disappearance of many ancient types we cannot leave the last period of the Paleozoic era and its prolonged ice age without noticing that it meant the entire cessation of a large number of ancient types especially among plants and backboneless animals which now disappear forever it is necessary to understand that the animals of ancient days stand in three different relations to those of today A. there are ancient types that have living representatives sometimes few and sometimes many sometimes much changed and sometimes but slightly changed the lamp shell, lingrulella of the Cambrian or Ordovisian period has a very near relative in the lingrulella of today there are a few extremely conservative animals B. there are ancient types which have no living representatives except in the guise of transform descendants as the king crab limelus may be said to be a transform descendant of the otherwise quite extinct race to which euripterids or sea scorpions belong C. there are altogether extinct types lost races which have left not a rack behind for there is not any representation today of such races as graptolites and trilobites looking backwards over the many millions of years comprised in the Paleozoic era what may we emphasize as the most salient features? there was in the Cambrian the establishment of the chief classes of backboneless animals in the Ordovisian the first fishes and perhaps the first terrestrial plants in the Silurian the emergence of air breathing invertebrates and mudfishes in the Devonian the appearance of the first amphibians from which all higher land animals are descended and the establishment of a land flora in the carboniferous the great club moss forest and an exuberance of air breathing insects and their allies in the Permian the first reptiles and a new flora the geological middle ages one the Mesozoic era in a broad way the Mesozoic era corresponds with the golden age of reptiles and with the climax of the conifer and cycad flora which was established in the Permian but among the conifers and cycads our modern flowering plants were beginning to show face tentatively just like birds and mammals among the great reptiles in the Triassic period the exuberance of reptilian life which marked the Permian was continued besides turtles which still persist there were ichthyosaurs plesiosaurus dinosaurs and pterosaurs beyond the Mesozoic era of great importance was the rise of the dinosaurs in the Triassic for it is highly probable that within the limits of this vigorous and plastic stock some of them bipeds we must look for the ancestors of both birds and mammals both land and water were dominated by reptiles some of which attained to gigantic size had there been any zoologist in those days he would have been very sagacious indeed if he had suspected that reptiles did not represent the climax of creation the flying dragons the Jurassic period showed a continuance of the reptilian splendor they radiated in many directions becoming adapted to many haunts thus there were many fish lizards paddling in the seas many types of terrestrial dragons stalking about on land many swiftly gliding alligator-like forms and the flying dragons which began in the Triassic attained to remarkable success in variety was formed by the extension of a great fold of skin on the enormously elongated outermost finger and they varied from the size of a sparrow to a spread of over five feet a soldering of the dorsal vertebrae as in our flying birds was an adaptation to striking the air with some force but as there is not more than a slight keel if any on the breast bone it is unlikely that they could fly far for we know from our modern birds that the power of flight may be to some extent gauge from the degree of development of the keel which is simply a great ridge for the better insertion of the muscles of flight it is absent of course in the running birds like the ostrich and it has degenerated in an interesting way in the burrowing parrot stringops and a few other birds that have gone back the first known bird but the Jurassic is particularly memorable because its strata have yielded two fine specimens of the first known bird archaeopteryx these were entombed in the deposits from the fine grain lithographic stones of Bavaria and practically every bone in the body is preserved except the breast bone even the feathers have left their marks with the stinkness this oldest known bird too far advanced to be the first bird was about the size of a crow and was probably of arboreal habits of great interest are its reptilian features so pronounced that one cannot evade the evolutionist suggestion it had teeth in both jaws which no modern bird has it had a long lizard-like tail which no modern bird has it had claws on three fingers and a sort of half-made wing that is to say it does not show what all modern birds show a fusion of half the wrist bones with the whole of the palm bones the well-known carpometacarpus bone which forms a basis for the longest pinions in many reptiles such as crocodiles there are peculiar bones running across the abdomen beneath the skin the so-called abdominal ribs and it seems an eloquent detail to find these represented in Archaeopteryx, the earliest known bird no modern bird shows any trace of them there is no warrant for supposing that the flying reptiles or pterodactyls gave rise to birds for the two groups are on different lines and the structure of the wings is entirely different thus the long-fingered pterodactyl wing was a parachute wing while the secret of the bird's wing has its center in the feathers it is highly probable that birds evolved from certain dinosaurs which had become bipeds and it is possible that they were for a time swift runners that took flying jumps along the ground thereafter perhaps came a period of arboreal apprenticeship during which there was much gliding from tree to tree before a true flight was achieved it is an interesting fact that the problem of flight has been solved four times among animals by insects and that the four solutions are on entirely different lines in the Cretaceous period the outstanding events included the waning of giant reptiles, the modernizing of the flowering plants and the multiplication of small mammals some of the Permian reptiles such as the dog tooth synodonts were extraordinarily mammal like and it was probably from among them that definite mammals emerged in the Triassic comparatively little is known of the early Triassic mammals the back teeth were marked by numerous tubercles on the crown but they were gaining strength in the late Triassic when small arboreal insectifors not very distant from the modern tree shrews to peya began to branch out in many directions indicative of the great divisions of modern mammals such as the clawed mammals, hoofed mammals and the race of monkeys or primates in the upper Cretaceous there was an exuberant radiation of mammals adaptive to the conquest of all sorts of haunts and this was vigorously continued in tertiary times there is no difficulty in the fact that the earliest remains of definite mammals in the Triassic precede the first known bird in the Jurassic for although we usually rank mammals as higher than birds being mammals ourselves how could we do otherwise there are many ways in which birds are preeminent for example in skeletal musculature, integumentary structures and respiratory system the fact is that birds and mammals have two quite different tax of evolution not related to one another safe in having a common ancestry in extinct reptiles moreover there is no reason to believe that the Jurassic Archaeopteryx was the first bird in any sense except that it was the first of which we have any record in any case it is safe to say that birds came to their own before mammals did looking backwards we may perhaps sum up what is most essential in the Mesozoic era in Professor Shukard's sentence the Mesozoic is the age of reptiles and yet the little mammals and the toothed birds are storing up intelligence and strength to replace the reptiles when the psychids and conifers shall give way to the higher flowering plants 2. The Cenozoic or Tertiary Era in the Eocene period there was a replacement of the small-brained archaic mammals by big-brained modernized types and with this must be associated the covering of the earth and the garment of grass and dry pasture marshes were replaced by meadows and browsing by grazing animals in the spreading meadows an opportunity was also offered for a richer evolution of insects and birds during the Oligocene the elevation of the land continued the climate became less moist and the grazing herds extended their range the Miocene was the mammalian golden age and there were crowning examples of what Osborne calls adaptive radiation that is to say, mammals like the reptiles before them conquer every haunt of life there are flying bats vole-plating parachutists climbers in trees like sloths and squirrels quickly moving hoofed animals burrowers like the moles freshwater mammals like duck-mull and beaver shore-frequenting seals and manatees and open sea cetaceans some of which die far more than full fathoms 5 it is important to realize the perennial tendency of animals to conquer every corner and to fill every niche of opportunity and to notice that this has been done by successive sets of animals in succeeding ages most notably the mammals repeat all the experiments of reptiles on a higher turn of the spiral thus arises what is called convergence the superficial resemblance of unrelated types like whales and fishes the resemblance being due to the fact that the different types are similarly adapted to similar conditions of life professor H. F. Osborn points out that mammals may seek any one of the 12 different habitat zones and that in each of these there may be six quite different kinds of food living creatures penetrate everywhere like the flowing waters of a great river in flood 3 the Pliocene period was a more strenuous time with less genial climatic conditions and with more intense competition old land bridges were broken and new ones were destroyed and the geographical distribution underwent great changes professor R. S. Lull describes the Pliocene as a period of great unrest many migrations occurred the world over new competitions arose and the weaker stocks began to show the effects of the strenuous life one momentous event seems to have occurred in the Pliocene and that was the transformation of the precursor of humanity into man the culmination of the highest the Pliocene period was a time of sifting there was a continued elevation of the continental masses and ice ages set in relieved by less severe interglacial times when the ice sheets retreated northwards for a time many types like the mammoth, the woolly rhinoceros, the sabertooth tiger the cave lion and the cave bear became extinct others which formerly had a wide range became restricted to the far north or were left isolated here and there by mountains like the snowmouse which now occurs on isolated alpine heights above the snowline perhaps it was during this period that many birds of the northern hemisphere learned to evade the winter by the sublime device of migration looking backwards we may quote professor Schuchert again the lands in the Cenozoic began to bloom with more and more flowering plants and grand hardwood forests the atmosphere is scented with sweet odors out of new kinds of insects appear and the places of the once dominant reptiles of the lands and seas are taken by the mammals out of these struggles there rises a greater intelligence seen in nearly all of the mammal stocks but particularly in one the monkey ape man brute man appears on the scene with the introduction of the last glacial climate a most trying time for all things endowed with life and finally there results the dominance of reasoning man over all his brute associates in man and human society the story of evolution has its climax the ascent of man man stands apart from animals in his power of building up general ideas and of using these in the guidance of his behavior and the control of his conduct this is essentially wrapped up with his development of language as an instrument of thought some animals have words but man has language logos some animals show evidence of perceptual inference but man often gets beyond this to conceptual inference reason many animals are affectionate and brave self forgetful and industrious but man thinks the ought definitely guiding his conduct in the light of ideals which in turn are wrapped up with the fact that he is a social person besides his big brain which may be three times as heavy as that of a gorilla man has various physical peculiarities he walks erect he plants the sole of his foot flat on the ground he has a chin and a good heel a big forehead and a non-pertrusive face a relatively uniform set of teeth without conspicuous canines and a relatively naked body but in spite of man's undeniable apartness there is no doubt as to his solidarity with the rest of creation there is an all-pervading similitude of structure between man and the anthropoid apes we are certain that it is not from any living form that he took his origin none of the anatomical distinctions except the heavy brain could be called momentous man's body is a veritable museum of relics vestigial structures inherited from pre-human ancestors in his everyday bodily life and in some of its disturbances man's pedigree is often revealed even his facial expression as Darwin showed is not always human some fossil remains bring modern man nearer to the anthropoid type it is difficult not to admit the ring of truth in the closing words of Darwin's descent of man we must however acknowledge as it seems to me that man with all his noble qualities with sympathy which feels for the most debased with benevolence which extends not only to other men but to the humblest living creature with his god-like intellect which has penetrated into the movements and constitution of the solar system with all these exalted powers man still bears in his bodily frame the indelible stamp of his lowly origin the evolving system of nature there is another side of evolution so obvious that it is often overlooked the tendency to link lives together in vital interrelations thus flowers and their insect visitors are often virtually interlinked in mutual dependence many birds feed on berries and distribute the seeds the male is the host of the juvenile stages of the liver fluke of the sheep the mosquito is the vehicle of malaria from man to man and the T.C. fly spreads sleeping sickness the freshwater muscle cannot continue its race without the unconscious cooperation of the minnow and the freshwater fish called the bitterling cannot continue its race without the unconscious cooperation of the muscle there are numerous mutually beneficial partnerships between different kinds of creatures and other interrelations where the benefit is one-sided as in the case of insects that make gulls on plants there are also among kindred animals many forms of colonies communities and societies nutritive chains bind long series of animals together the cod feeding on the welk the welk on the worm the worm on the organic dust of the sea there is a system of successive incarnations and matter is continually passing from one embodiment to another these instances must suffice to illustrate the central biological idea of the web of life the interlinked system of animate nature Linnea spoke of the system of nature meaning the orderly hierarchy of classes orders, families, genera and species but we owe to Darwin in particular some knowledge of a more dynamic system of nature the network of vital interrelations this has become more and more complex as evolution has continued and man's web is most complex of all it means making animate nature more of a unity it means an external method of registering steps of progress it means an evolving set of sieves by which new variations are sifted and living creatures are kept from slipping down the steep ladder of evolution parasitism it sometimes happens that the interrelation established between one living creature and another works in a retrograde direction this is the case with many thoroughgoing internal parasites which have sunk into an easygoing kind of life utterly dependent on their host for food requiring no exertions running no risks and receiving no spur to effort thus we see that evolution is not necessarily progressive everything depends on the conditions in reference to which the living creatures have been evolved when the conditions are too easygoing the animal may be thoroughly well adapted to them as a tapeworm certainly is but it slips down the rungs of the ladder of evolution this is an interesting minor chapter in the story of evolution the establishment of different kinds of parasites casual and constant temporary and lifelong external hangers on and internal unpaying borders those that live in the food canal and depend on the host's food and those that inhabit the blood or the tissues and find their food there the ignuement grubs and the like which hatch inside a caterpillar and eat it alive are not so much parasites as beasts of prey working from within but there are two sides to this minor chapter there is the evolution of the parasite and there is also the evolution of counteractive measures on the part of the host thus there is the maintenance of a bodyguard of wandering amoeboid cells which tackle the microbes invading the body and often succeed in overpowering and digesting them thus again there is the protective capacity the blood has of making antagonistic substances or antibodies which counteract poisons including the poisons which the intruding parasites often make the evidences of evolution how it came about 1. progress in evolution there has often been slipping back into generacy in the course of evolution but the big fact is that there has been progress for millions of years life has been slowly creeping upwards and if we compare the highest animals birds and mammals with their predecessors we must admit that they are more controlled more masters of their fate with more mentality evolution is on the whole integrative that is to say it makes against instability and disorder and towards harmony and progress even in the rise of birds and mammals we can discern that the evolutionary process was making towards a fuller embodiment of their expression of what man values most control, freedom, understanding and love the advance of animal life through the ages has been checkered but on the whole it has been an advance towards increasing fullness, freedom and fitness of life in the study of this advance the central fact of organic evolution there is assuredly much for man's instruction and much for his encouragement evidences of evolution the fact of evolution has been taken for granted but what are the evidences perhaps it should be frankly answered that the idea of evolution that the present is the child of the past and the parent of the future cannot be proved as one may prove the law of gravitation all that can be done is to show that it is a key a way of looking at things that fits the facts there is no lock that it does not open but if the facts that the evolution theory vividly interprets be called the evidences of its validity there is no lack of them there is historical evidence and what is more eloquent than the general fact that fishes emerge before amphibians and these before reptiles and these before birds and so on there are wonderfully complete fossil series for example among cuttlefishes in which we can almost see evolution in process the pedigree of horse and elephant and crocodile is in general very convincing but it is to be confessed that there are other cases in regard to which we have no light who can tell for instance how vertebrates arose or from what origin there is embryological evidence for the individual development often reads like an abbreviated recapitulation of the presumed evolution of the race the mammals visceral clefs are tell-tale evidence of remote aquatic ancestors breathing by gills something is known in regard to the historical evolution of antlers the red deer of today recapitulates at least the general outlines of the history the individual development of an asymmetrical flatfish like a place or soul which rests and swims on one side tells us plainly that its ancestors were symmetrical fishes there is what might be called physiological evidence for many plants and animals are variable before our eyes and evolution is going on around us today this is familiarly seen among domesticated animals and cultivated plants but there is abundant flux in wild nature it need hardly be said that some organisms are very conservative and that change need not be expected when a position of stable equilibrium has been secured there is also anatomical evidence of a most convincing quality in the forelimbs of backbone animals say the paddle of a turtle the wing of a bird the flipper of a whale the foreleg of a horse the same essential bones and muscles are used to such diverse results what could it mean save blood relationship and as to the two sets of teeth in whale bone whales which never even cut the gum is there any alternative but to regard them as relics of useful teeth which ancestral forms possessed in short the evolution theory is justified by the way in which it works two factors in evolution if it be said much for the fact of evolution but what of the factors the answer is not easy for not only is the problem the greatest of all scientific problems but the inquiry is still very young the scientific study of evolution practically dates from the publication of the origin of species in 1859 heritable novelties or variations often crop up in living creatures and these form the raw material of evolution these variations are the outcome of the changes in the germ cells that develop into organisms but why should there be changes in the constitution of the germ cells perhaps because the living material is very complex and inherently liable to change perhaps because it is the vehicle of a multitude of hereditary items among which there are very likely to be reshufflings or rearrangements perhaps because the germ cells have very changeful surroundings the blood, the body cavity fluid those deeply saturating outside influences such as change of climate and habitat penetrate through the body to its germ cells and provoke them to vary but we must be patient with the worrisome reiteration of perhaps moreover every many celled organism reproduced in the usual way arises from an egg cell fertilized by a sperm cell and the changes involved in and preparatory to this fertilization may make new permutations and hereditary qualities not only possible but necessary it is something like shuffling a pack of cards but the cards are living as to the changes wrought on the body during its lifetime by peculiarities in nurture, habits and surroundings these stents or modifications are often very important for the individual but it does not follow that they are directly important for the race since it is not certain that they are transmissible given a crop of variations or different partures or mutations whatever the inborn novelties may be called we have then to inquire how these are sifted the sifting which means the elimination of the relatively less fit variations and the selection of the relatively more fit effected in many different ways in the course of the struggle for existence the organism plays its new card in the game of life and the consequences may determine survival the relatively less fit to given conditions will tend to be eliminated while the relatively more fit will tend to survive if the variations are hereditary and reappear perhaps increased in amount generation after generation and if the process of sifting continue consistently the result will be the evolution of the species the sifting process may be helped by various forms of isolation which lessen the range of free intercrossing between members of a species for example by geographical barriers interbreeding of similar forms tends to make a stable stock outbreeding among dissimilars tends to promote variability but for an outline like this it is enough to suggest the general method of organic evolution throughout the ages organisms have been making tentatives new departures of varying magnitude and these tentatives have been tested the method is that of testing all things and holding fast that which is good end of section 5