 CHAPTER II. 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-trops, and the surface of the land has been buckled up into mountain ranges and folded into 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 borne away by rivers and seas to be laid down again elsewhere as deposits which eventually formed sandstones, mudstones, and similar sedimentary rocks. Much of the material of the original crust has been thus broken down and worked up again many times over, and if the total thickness of the sedimentary rocks is added up in amounts, according to some geologists, to a total of sixty-seven 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. In other cases 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 instance, of fossil horses 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 Long History of the Earth and its Inhabitants is conveniently divided into eras. Thus, just as we speak of the ancient, medieval, and modern history of mankind, so we may speak of Paleozoic, Mesozoic, and Cenozoic eras in the history of the Earth as a whole. Geologists cannot tell us, except in an approximate way, how long the process of evolution is 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 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 fifty 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 eras marked on the table as, before the Cambrian, correspond to about thirty-two miles of thickness of strata, and all the subsequent eras with fossil-bearing rocks to a thickness of about twenty-one miles, in itself an astounding fact. Perhaps thirty million years must be allotted to the pre-Cambrian eras, eighteen to the Paleozoic, nine to the Mesozoic, three to the Senozoic, making a grand total of sixty million. The Establishment of Invertebrate Stocks It is an astounding fact that at least half of the geological time, the Archaeozoic 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 calcareous algae, and do 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. 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 elegant in the broad fact that the peopling of the seas had definitely begun some thirty 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 Ordovician period was marked by abundant representation of the once very successful class of trilobites, joint-footed, antenna-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 back-boned animals made their appearance, an epic-making step in evolution. In other words, true fishes were evolved, destined in the course of ages to replace the cuttlefishes, which are mere mollusks, in dominating the seas. Here is a breakdown of geological times from the most recent to the most ancient. First is called the Cenozoic Era, and it is divided into several times. The most recent is the Pleistocene or Glacial Time, which was the time of the last great Ice Age. Before that were the Myocene and Pliocene times, which saw the emergence of man, and before them the Eocene and Oligocene times, which saw the rise of higher mammals. Prior to the Cenozoic Era was the Mesozoic Era, with three periods, the Cretaceous, the Jurassic and Triassic periods, in order from most recent to most ancient. The Cretaceous period saw the rise of primitive mammals, flowering plants, and higher insects. The Jurassic period the rise of birds and flying reptiles. The Triassic period the rise of dinosaur reptiles. Before the Mesozoic Era was the Paleozoic Era, divided into six periods. From most recent to most ancient they are the Permian, the Carboniferous, the Devonian, the Celerian, the Ordovician, and the Cambrian periods. The Permian period saw the rise of reptiles. The Carboniferous period the rise of insects. The Devonian period our first amphibians. The Celerian period land animals began. The Ordovician period our first fishes. The Cambrian period saw the peepling of the sea. Before the Paleozoic Era came the Proterozoic ages, where many of the backboneless stocks began, and prior to that were the Archeozoic ages, where living creatures began to be upon the earth. Before that we recognize what we call the formative times. During this period were the makings of continents and ocean basins, the beginnings of an atmosphere and hydrosphere, the cooling of the earth, and the establishment of the solar system. In the Celerian period in which the peepling of the seas went on a pace there was the first known attempt at colonizing the dry land. For in Celerian 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 Celerian, when a period of great aridity set in, that fishes appeared related to our mud-fishes, or double-breeders, the dipnoi, which have lungs as well as gills. This again meant utilizing dry air, just as the present day mud-fishes do when the water disappears from the pools in hot weather. The lung-fishes, or mud-fishes of today, are about 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 Finipus, 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. Evolution of Land Animals, Giant Amphibians, and Coal Measures The Carboniferous period was marked by a mild moist climate and a luxuriant vegetation and 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 which grew to a gigantic size compared with their pygmy representatives of today. In these forests the joint-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 sureer by insects 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 in club moss forests, which covered great tracks of the earth for millions of years. In the carboniferous forest there were also land snails representing one of the minor invasions of the dry land, tending on the whole 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 fitly 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, A. that local drying up of water basins or elevations of the land's surface often made the old haunts untenable, B. that there may have been great congestion in 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-digitization of the eight paired fins of fishes were replaced by limbs with fingers and toes. This implied an advantageous 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 is not any muscles at all. Gradually, as the tadpole becomes a frog, the muscle fibers grow in strength and make it 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 of voice, due, as in ourselves, to the rapid passage of air overtaught membranes, vocal 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 carbeniferous, the first vital sounds were due to amphibians, and theirs certainly was the first voice, surely one of the great steps in organic evolution. 5. 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 that 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. 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 back-boned 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 breathes 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 the 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 antinatal 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, persist not only in all reptiles, but in birds and mammals as well. These higher vertebrates are therefore called amniota, 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 clefs, a telltale evidence of their distant aquatic ancestry. But these embryonic gill clefs 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 clefs, 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 we persist when only one is of any 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 telescoped manner, of course, for what took the race of 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, ginkgos, and cycads, which persisted until near the end of the Mesozoic Era. The Permian ice age lasted for millions of years, and was most 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, for instance caterpillars, and the more acetic, non-growing, non-molting winged face adapted for reproduction. Between these there 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 noting 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 lap shell, lingulella, of the Cambrian and Ordovician period has a very near relative in the lingula of today. There are a few extremely conservative animals. B. There are ancient types that have no living representatives, except in the guise of transform descendants as the king crab, Limulus, may be said to be a transform descendant of the otherwise quite extinct race to which Euryptorids and C. Scorpions belonged. 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 be emphasized as the most salient features. There was in the Cambrian Era the establishment of the chief classes of backboneless animals, in the Ordovician the first fishes and perhaps the first terrestrial plants, in the Silurian the emergence of air-breathing invertebrates and mud-fishes, 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 forests and an exuberance of air-breathing insects and their allies, in the Permian the first reptiles and a new flora, the geological middle ages, 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 are modern flowering plants where 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, plesiosaures, dinosaurs, and pterosaurs, none of which lasted 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-lizzards 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 and variety. Their wing 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 breastbone, 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 gauged 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, and 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. They were entombed in the deposits which formed the fine-grained lithographic stones of Bavaria, and practically every bone in the body is preserved except the breastbone. Even the feathers have left their marks with distinctness. 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 wristbones 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 true flight was achieved. It is an interesting fact that the problem of flight has been solved four times among animals, by insects, by pterodactyls, by birds, and by bats, 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, save that their back teeth were marked by numerous tubercles on the crown, but they were gaining strength in the late Triassic when small arboreal insectivores, not very distant from the modern tree shrews, Tupeia, 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 hots, 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 instance in skeleton, musculature, integumentary structures, and respiratory system. The fact is that birds and mammals are on two quite different types of evolution, not related to one another, save 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 is 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 Shutechurch'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 cycads and conifers shall give way to the higher flowering plants. 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 with a garment of grass and dry pasture. Marshes were replaced by meadows and browsing by grazing 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 much less moist, and the grazing herds extended their range. The Myocene 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, vulplating parachutists, climbers and trees like sloths and squirrels, quickly moving hoofed mammals, burrowers like the moles, freshwater mammals like duck-mull and beaver, shore-frequenting seals and manatees, and open-sea cetaceans, some of which dive far more than full fathoms five. 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. Osborne points out that mammals may seek any one of the twelve different habitat zones, and that in each of these there may be six quite different kinds of food. Living creatures penetrate everywhere like the overflowing waters of a great river in flood. 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 made, 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 line of evolution. 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 on the high mountains, like the snow mouse, which now occurs on isolated alpine heights above the snow line. 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 assented with sweet odors. A vast crowd 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 arises a greater intelligence, seen in nearly all 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 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 or 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, although it is 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 the anthropoid type. It is difficult not to admit the ring of truth in the closing words of Darwin's Descent of Man. Quote, 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 godlike intellect which is penetrated into the movements and constitution of the solar system, with all these exalted powers, and still bears in his bodily frame the indelible stamp of his lowly origin. Quote, 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 vitally interlinked in mutual dependence. Many birds feed on berries and distribute the seeds. The tiny freshwater snail 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 its seetsey 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 galls 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. Linnaeus spoke of the sistema naturae, meaning the orderly hierarchy of classes, orders, genera, and species. But we owe to Darwin in particular some knowledge of a more dynamic system of naturae, 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 this 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 involved. 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, eternal 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. It seems clear that ichnum and 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 ameboid 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 substance or antibodies which counteract poisons, including the poisons which the intruding parasites often make. The evidences of evolution, how it came about. Progress in evolution. There has often been slipping back and degeneracy 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 or expression of what man values most. Control, freedom, 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. In all this it may be said, 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 might 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 instance, among cuttlefishes, in which we can almost see evolution in process. The pedigree of horse and elephant and crocodile is in general very convincing, though 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 and by gone ages, 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 rest 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 back-boned animals, say, the paddle of a turtle, the wing of a bird, the flipper of a whale, the foreleg of a horse, and the arm of a man, 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 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. FACTORS IN EVOLUTION If it be said so much for the fact of evolution, but what are 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 expression of 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, the seawater. Perhaps because 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 wearisome 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 combinations of the living items 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 dents 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 new departures 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, affected 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 give in 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 instance 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 Chapter 2 Chapter 3 Part 1 of The Outline of Science This is a LibriVox recording. All LibriVox recordings are in the public domain. For more information or to volunteer, please visit LibriVox.org. We saw in a previous chapter how the process of evolution led to a mastery of all the haunts of life, but it is necessary to return to these haunts or homes of animals in some detail, so as to understand the peculiar circumstances of each, and to see how in the course of ages of struggle all sorts of self-preserving and race-continuing adaptations or fitnesses have been wrought out and firmly established. Living creatures have spread over all the earth and in the waters under the earth. Some of them have conquered the underground world and others the air. It is possible, however, as has been indicated, to distinguish six great haunts of life, each teneted by a distinctive fauna, namely the shore of the sea, the open sea, the depths of the sea, the fresh waters, the dry land, and the air. In the deep sea there are no plants at all. In the air the only plants are floating bacteria, though there is a sense in which a tree is very aerial, and the orchid perched on its branches still more so. In the other four haunts there is a flora as well as a fauna, the two working into one another's hands in interesting and often subtle interrelations, the subject of a separate study. 1. The Shore of the Sea The seaweed area. By the shore of the sea the zoologist means much more than the narrow zone between tide-marks. He means the whole of the relatively shallow, well-illumined seaweed-growing shelf around the continents and continental islands. Technically this is called the latoral area, and it is divisible into zones, each with its characteristic population. It may be noted that the green seaweeds are highest up on the shore, the brown ones come next, the beautiful red ones are lowest. All of them have got green chlorophyll, which enables them to utilize the sun's rays in photosynthesis, that is building up carbon compounds from air, water, and salts. But in the brown and red seaweeds the green pigment is masked by others. It is maintained by some botanists that these other pigments enable their possessors to make more of the scantier life in the deeper waters. However this may be we must always think of the shore haunt as the seaweed-growing area. Directly and indirectly the life of the shore animals is closely wrapped up with the seaweeds, which afford food and foothold, and temper the force of the waves. The minute fragments broken off from seaweeds and from the seagrass, a flowering plant called sostera, form a sort of nutritive sea dust, which is swept slowly down the slope from the shore to form a very useful deposit in the quietness of deepest water. It is often found in the stomachs of marine animals living a long way offshore. Conditions of shore life The littoral area as defined is not a large haunt of life. It occupies only about nine million square miles, a small fraction of the 197 million of the whole earth's surface. But it is a very long haunt, some 150,000 miles, winding in and out by bay and fjord, estuary and creek. Where deep water comes close to cliffs there may be no shore at all. In other places the relatively shallow water, with seaweeds growing over the bottom, may extend outwards for miles. The nature of the shore varies greatly according to the nature of the rocks, according to what the streams bring down from inland, and according to the jetsam that is brought in by the tides. The shore is a changeful place. There is, in the upper reaches, a striking difference between tide in and tide out. There are vicissitudes due to storms, to freshwater floods, to windblown sand, and to slow changes of level up and down. The shore is a very crowded haunt, for it is comparatively narrow, and every niche among the rocks may be precious. Keen struggle for existence It follows that the shore must be the scene of a keen struggle for existence, which includes all the answers back that living creatures make to environing difficulties and limitations. There is struggle for food, accentuated by the fact that small items tend to be swept away by the outgoing tide, or to sink down the slope to deep water. Apart from direct competition, for instance between hungry hermit crabs, it often involves hard work to get a meal. This is true even of apparently sluggish creatures. Thus the crumb of bread sponge, or any other seashore sponge, has to lash large quantities of water through the intricate canal system of its body before it can get a sufficient supply of the microscopic organisms and organic particles on which it feeds. An index of the intensity of the struggle for food is afforded by the nutritive chains which bind animals together. The shore is almost noisy with the conjugation of the verb to eat in its many tenses. One pound of rock cod requires for its formation ten pounds of welk, one pound of welk requires ten pounds of sea worms, and one pound of worms requires ten pounds of sea dust. Such is the circulation of matter ever passing from one embodiment or incarnation to another. In struggle for food there is struggle for foothold and for fresh air, struggle against the scouring tide and against the pounding breakers. The risk of dislodgement is often great and the fracture of limbs is a common accident. Of kinds of armor, the sea urchins hedgehog-like test, the crabs shard, the limpets shell, there is great variety, surpassed only by that of weapons, the sea anemones stinging cells, the sea urchins snapping blades, the hermit crabs forceps, the grappling tentacles and parrot-speak jaws of the octopus. SHIFTS FOR A LIVING We get another glimpse of the intensity of the seashore's struggle for existence in the frequency of shifts for a living, adaptations of structure or of behavior which meet frequently recurrent vicissitudes. The starfish is often in the dilemma of losing a limb or its life. By a reflex action it jettisons the captured arm and escapes. And what is lost is gradually regrown. The crab gets its leg broken past all mending. It casts off the leg across the weak breakage plane near the base, and within a preformed bandage which prevents bleeding a new leg is formed in miniature. Such is the adapted device, more reflex than reflective, which is called self-mutilation or autonomy. In another part of this book there is a discussion of camouflaging and protective resemblance. How abundantly these are illustrated on the shore. But there are other shifts for a living. Some of the sandhoppers and their relatives illustrate the puzzling phenomenon of fainting death, becoming suddenly so motionless that they escape the eyes of their enemies. Cuddle fishes, by discharging sepia from their ink bags, are able to throw dust in the eyes of their enemies. Some undisguised shore animals, for instance crabs, are adepts in a hide-and-seek game. Some fishes, like the butterfish or gull, escape between stones where there seem no opening and are almost un-catchable in their slipperiness. Sutlist of all perhaps is the habit some hermit crabs have of entering into mutually beneficial partnership, commensalism, with sea anemones, which mask their bearers and also serve as mounted batteries, getting transport as their reward and likewise crumbs from the frequently spread table. But enough has been said to show that the shore-haut exhibits an extraordinary variety of shifts for a living. Parental Care on the Shore According to Darwin the struggle for existence, as a big fact in the economy of animate nature, includes not only competition but all the endeavors which secure the welfare of the offspring and give them a good send-off in life. So it is without a jolt that we pass from struggle for food and foothold to parental care. The marine leech called Pantabdella, an interesting greenish warty creature fond of fixing itself to skate, places its egg cocoons in the empty shell of a bivalve mollusk and guards them for weeks, removing any mud that might injure their development. We have seen a British starfish with its fully formed young ones creeping about on its body, though the usual mode of development for shore of starfishes is that the young ones pass through a free-swimming larval period in the open water. The father sea-spider carries about the eggs, attached to two of his limbs. The father seahorse puts his mate's eggs into his breast-pocket and carries them there in safety until they are hatched. The father's stickle-back of the shore-pools makes a seaweed nest and guards the eggs which his wives are induced to lay there. The father lump-sucker mounts guard over the bunch of pinkish eggs which is made as laid in the nook of a rocky shore-pool and drives off intruders with zest. He also aerates the developing eggs by frequent paddling with his pectoral fins and tail, as the Scots name cock-patle probably suggests. It is interesting that the salient examples of parental care in the shore-haunt are mostly on the male-parent side, but there is maternal virtue as well. The fauna of the shore is remarkably representative, from unicellular protozoa to birds like the oyster-catcher and mammals like the seals. Almost all the great groups of animals have apparently served in apprenticeship in the shore-haunt and since lessons learned for millions of years sink in and become organically in-registered, it is justifiable to look to the shore as a great school in which were gained racial qualities of endurance, patience, and alertness. 2. The Open Sea In great contrast to the narrow, crowded, difficult conditions of the shore-haunt, the toral area, are the spacious, bountiful, and relatively easygoing conditions of the open sea, pelagic area, which means the well-lighted surface waters quite away from land. Many small organisms have their maximum abundance at about fifty fathoms so that the word surface is to be taken generously. The light becomes very dim at two hundred fifty fathoms and the open sea, as a zoological haunt, stops with the light. It is hardly necessary to say that the pelagic plants are more abundant near the surface and that below a certain depth the population consists almost exclusively of animals. Not a few of the animals sink and rise in the water periodically. There are some that come near the surface by day and others that come near the surface by night. Of great interest is the habit of the extremely delicate stenophores or sea gooseberries, which the splash of a wave would tear into shreds. Whenever there is any hint of a storm they sink beyond its reach, and the ocean's surface must have remained flat as a mirror for many hours before they can be lured upwards from the calm of their deep retreat. The Floating Sea Meadows To understand the vital economy of the open sea we must recognize the incalculable abundance of minute unicellular plants, for they form the fundamental food supply. Along with these must also be included numerous microscopic animals which have got possession of chlorophyll, or have entered into internal partnership with unicellular algae symbiosis. These green or greenish plants and animals are the producers, using the energy of the sunlight to help them in building up carbon compounds out of air, water, and salts. The animals which feed on the producers, or on other animals, are the consumers. Between the two come those open sea bacteria that convert nitrogenous material, for instance from dead plants or animals that other bacteria have rotted, into forms, for instance nitrates, which plants can reutilize. The importance of these middlemen is great in keeping the circulation of matter a-going. The Floating Sea Meadows, as Sir John Murray called them, are always receiving contributions from inshore waters, where the conditions are favorable for the prolific multiplication of unicellular algae, and there is also a certain amount of non-living sea dust always being swept out from the seaweed and seagrass area. Swimmers and Drifters The animals of the open sea are conveniently divided into the active swimmers, necton, and the more passive drifters, plankton. The swimmers include whales great and small, such birds as the storm petrol, the fish eating turtles and sea snakes, such fishes as mackerel and herring, the winged snails or sea butterflies on which whale bone whales largely feed, some of the active cuddles or squids, various open sea prawns and their relatives, some worms like the transparent arrow worm, and such active protozoa as noctoluca, whose luminescence makes the waves sparkle in the short summer darkness. Very striking as an instance of the insurgence of life are the sea skimmers, helubatidae, wingless insects related to the water-measures in the ditch. They are found hundreds of miles from land, skimming on the surface of the open sea and diving in storming weather. They feed on floating dead animals. The drifters or easy-going swimmers, for there is no hard and fast line, are represented, for instance, by the flinty-shelled radiolarians and certain of the chalk-forming animals, globa geronid for monifera. By jellyfishes, swimming-bells, and Portuguese men of war, by the comb-bearers or stenophores, by legions of minute crustaceans, by strange animals called salps related to the sedentary sea squirts, and by some sluggish fishes like globe fishes which often float idly on the surface. Open sea animals tend to be delicately built, with a specific gravity near that of seawater, with adaptations such as projecting filaments which help flotation, and with capacities of rising and sinking according to the surrounding conditions. Many of them are luminescent, and many of them are very inconspicuous in the water, owing to their transparency or their bluish color. In both cases the significance is obscure. Hunger and Love Hunger is often very much in evidence in the open sea, especially in areas where the plankton is poor. For there is great diversity in this respect, most of the Mediterranean, for instance, having a scanty plankton as compared with the North Sea. In the South Pacific, west of Patagonia, there is said to be an immense sea desert where there is little plankton, and therefore little in the way of fishes. The success of fisheries in the North, for instance on the Atlantic cod banks, is due to the richness of the floating sea meadows and the abundance of the smaller constituents of the animal plankton. Hunger is plain enough when the baleen whale rushes through the water with open jaws, engulfing in the huge cavern of its mouth where the pendant whalebone plates form a huge sieve, incalculable millions of small fry. But there is love as well as hunger in the open sea. The maternal care exhibited by the whale reaches a very high level, and the delicate shell of the female paper nautilus or argonaut, in which the eggs and the young ones are sheltered, may well be described as the most beautiful cradle in the world. Besides the permanent inhabitants of the open sea, there are the larval stages of many shore animals which are there only for a short time. For there is an interesting give and take between the shore haunt and the open sea. From the shore come nutritive contributions and minute organisms which multiply quickly in the open waters. But not less important is the fact that the open waters afford a safe cradle or nursery for many a delicate larvae, for instance of crab and starfish, acorn shell and sea urchin, which could not survive for a day in the rough and tumble conditions of the shore and the shallow water. After undergoing radical changes in gaining strength, the young creatures return to the shore in various ways. 3. The Deep Sea Very different from all the other haunts are the depths of the sea, including the floor of the abysses and the zones of water near the bottom. This haunt, forever unseen, occupies more than a third of the earth's surface, and it is thickly peopled. It came into emphatic noticing connection with the mending of telegraph cables, but the results of the Challenger expedition of 1873 through 1876 gave the first impressive picture of what was practically a new world. 3. Physical Conditions The average depth of the ocean is about two and a half miles. Therefore, since many parts are relatively shallow, there must be enormous depths. A few of these, technically called deeps, are about six miles deep, in which Mount Everest would be engulfed. There is enormous pressure in such depths. Even at 2,500 fathoms it is two and a half tons on the square inch. The temperature is on and off the freezing point of fresh water, 28 to 34 degrees Fahrenheit, due to the continual sinking down of cold water from the poles, especially from the south. Apart from the fitful gleams of luminescent animals, there is utter darkness in the deep waters. The rays of sunlight are practically extinguished at 250 fathoms. The very sensitive bromo-gelatin plates, exposed at 500 fathoms, have shown faint indications even at that depth. It is a world of absolute calm and silence, and there is no scenery on the floor, a deep cold dark silent monotonous world. 4. Biological Conditions While some parts of the floor of the abysses are more thickly peopled than others, there was no depth limit to the distribution of life. Wherever the long arm of the dredge has reached, animals have been found—for instance, protozoa, sponges, corals, worms, starfishes, sea urchins, sea lilies, crustaceans, lamp-shells, mollusks, acidians, and fishes—a very representative fauna. In the absence of life there can be no chlorophyll-possessing plants, and as the animals cannot all be eating one another there must be an extraneous source of food supply. This is found in the sinking down of binute organisms which are killed on the surface by changes of temperature and other causes. What is left of them, before or after being swallowed, and of sea dust and mineral particles of various kinds, forms the diversified ooze of the sea floor—a soft muddy precipitate, which is said to have in places the consistency of butter in summer weather. There seems to be no bacteria in the abysses, so there can be no rotting. Everything that sinks down, even the huge carcass of a whale, must be nibbled away by hungry animals and digested, or else, in the case of most bones, slowly dissolve the way. Of the whale there are left only the ear bones, of the shark, his teeth. Adaptations to Deep Sea Life In adaptation to the great pressure the bodies of deep sea animals are usually very permeable, so that the water gets through and through them, as in the case of Venus's flower basket, a flinty sponge which a child's finger would shiver. But when the pressure inside is the same as that outside, nothing happens. In adaptation to the treacherous ooze so apt to smother, many of the active deep sea animals have very long, stilt-like legs, and many of the sedentary types are lifted into safety on the end of long stalks which have their bases embedded in the mud. In adaptation to the darkness, in which there is only luminescence that eyes could use, there is a great development of tactility. The interesting problem of luminescence will be discussed elsewhere. As to the origin of the deep sea fauna, there seems no doubt that it has arisen by many contributions from the various shore-hots. Following the down-drifting food, many shore animals have, in the course of many generations, reached the world of eternal night and winter, and become adapted to its strange conditions. For the animals of the deep sea are as fit, beautiful, and vigorous as those elsewhere. There are no slums in nature. 4. The Fresh Waters On the whole earth's surface the fresh waters form a very small fraction, about a hundredth, but they make up for their smallness by their variety. We think of deep lake and shallow pond, of the great river and the pearling brook, of lagoon and swamp, and more besides. There is a striking resemblance in the animal population of widely separated freshwater basins, and this is partly because birds carry many small creatures on their muddy feet from one watershed to another, partly because some of the freshwater animals are descended from types which make their way from the sea and the seashore through estuaries and marshes, and only certain kinds of constitution could survive the migration. And partly because some lakes are landlocked dwindling relics of ancient seas, and similar forms again would survive the change. A typical assemblage of freshwater animals would include many protozoa, like amoebae and the bell animicules, a representative of one family of sponges, the spongilidae, the common hydra, many unsegmented worms, notably planarians and nematodes, many antelids related to the earthworms, many crustaceans, insects and mites, many bivalves and snails, various fishes, a newt or two, perhaps a little mud turtle or in the warm country is a huge crocodilian, various interesting birds like the water oozle or dipper, and mammals like the water vole and the water shrew. Freshwater animals have to face certain difficulties, the greatest of which are drought, frost, and being washed away in times of flood. There is no more interesting study in the world than an inquiry into the adaptations by which freshwater animals overcome the difficulties of the situation. We cannot give more than a few illustrations. One. Drought is circumvented by the capacity that many freshwater animals have, of lying low and saying nothing. Thus the African mudfish may spend half the year encased in the mud, and many minute crustaceans can survive being dried up for years. Two. Escape from the danger of being frozen hard in the pool is largely due to the almost unique property of water that it expands as it approaches the freezing point. Thus the colder water rises to the surface and forms or adds to the protecting blanket of ice. The warmer water remains unfrozen at the bottom and the animals live on. Three. The risk of being washed away, for instance to the sea, is lessened by all sorts of gripping, grappling, and anchoring structures, and by shortening the juvenile stages when the risks are greatest. Five. The dry land. Over and over again in the history of animal life there have been attempts to get out of the water on to terra firma, and many of these have been successful, notably those made one by worms, two by air-breathing arthropods, and three by amphibians. In thinking of the conquest of the dry land by animals we must recognize the indispensable role of plants in preparing the way. The dry ground would have proved too inhospitable. Had not terrestrial plants begun to establish themselves, affording food, shelter, and humidity, there had to be plants before there could be earthworms, which feed on decaying leaves and the like, but how soon was the debt repaid when the earthworms began their world-wide task of forming vegetable mold, opening up the earth with their burrows, circulating the soil by means of their castings, and bruising the particles in their gizzard, certainly the most important mill in the world. Another important idea is that littoral haunts, both on the seashore and in the fresh waters, afforded the necessary apprenticeship and transitional experience for the more strenuous life on dry land. Much that was perfected on land had its beginnings on the shore. Let us inquire, however, what the passage from water to dry land actually implied. This has been briefly discussed in a previous article on evolution, but the subject is one of great interest and importance. Difficulties and results of the transition from water to land Leaving the water for dry land implied a loss of freedom of movement, for the terrestrial animal is primarily restricted to the surface of the earth. Thus it became essential that movements should be very rapid and very precise, needs with which we may associate the acquisition of fine cross-striped, quickly contracting muscles, and also, in time, their multiplication into very numerous separate engines. We exercise fifty-four muscles in the half-second that elapses between raising the heel of our foot in walking and planting it firmly on the ground again. Moreover, the need for rapid, precisely controlled movements implied an improved nervous system, for the brain was a movement-controlling organ for ages before it did much in the way of thinking. The transition to terra firma also involved a greater compactness of body so that there should not be too great friction on the surface. An animal like the jellyfish is unthinkable on land, and the elongated bodies of some land animals like centipedes and snakes are specially adapted so that they do not sprawl. They are exceptions that prove the rule. Getting on to dry land meant entering a kingdom where the differences between day and night, between summer and winter, are more felt than in the sea. This made it advantageous to have protections against evaporation and loss of heat or other such dangers. Hence a variety of ways in which the surface of the body acquired a thickened skin or a dead cuticle or a shell or a growth of hair and so forth. In many cases there is an increase of the protection before the winter sets in, for instance by growing thicker fur or by accumulating a layer of fat below the skin. But the thickening or protection of the skin involved a partial or total loss of the skin as a respiratory surface. There is more oxygen available on dry land than in the water, but it is not so readily captured. Thus we see the importance of moist internal surfaces for capturing the oxygen which has been drawn into the interior of the body into some sort of lung. A unique solution was offered by tracheate arthropods such as peripatis, centipedes, millipedes and insects, where the air is carried to every hole and corner of the body by a ramifying system of air tubes or tracheae. In most animals the blood goes to the air. In insects the air goes to the blood. In the robber crab, which is migrated from the shore inland, the dry air is absorbed by vascular tufts growing under the shelter of the gill cover. The problem of disposing of eggs or young ones is obviously much more difficult on land than in the water, for the water offers an immediate cradle, whereas on the dry land there are many dangers, for instance of drought, extremes of temperature and hungry, sharp-eyed enemies which had to be circumvented. So we find all manner of ways in which land animals hide their eggs or their young ones in holes and nests, on herbs and on trees. Some carry their young ones about after they are born, like the Surinam toad and the kangaroo, while others have prolonged the period of antinatal life during which the young ones develop in safety within their mother and in very intimate partnership with her in the case of the placental mammals. It is very interesting to find that the pioneer animal called peripatis, which bridges the gap between worms and insects, carries its young for almost a year before birth. Enough has been said to show that the successive conquests of the dry land had great evolutionary results. It is hardly too much to say that the invasion which the amphibians led was the beginning of better brains, more controlled activities, and higher expressions of family life. 6. The Air There are no animals thoroughly aerial, but many insects spend much of their adult life in the free air, and the swift hardly pauses in its flight from dawn to dusk of the long summer day, alighting only for brief moments at the nest to deliver insects to the young. All the active life of bats certainly deserves to be called aerial. The air was the last haunt of life to be conquered, and it is interesting to inquire what the conquest implied. 1. It meant transcending the radical difficulty of terrestrial life, which confines the creatures of the dry land to moving on one plane, the surface of the earth. But the power of flight brought its possessors back to the universal freedom of movement which water animals enjoy. When we watch a sparrow rise into the air, just as the cat has completed her stealthy stalking, we see that flight implies an enormous increase of safety. 2. The power of flight also opened up new possibilities of following the prey, of exploring new territories, of prospecting for water. 3. Of great importance too was the practicability of placing the eggs on the young, perhaps in a nest, in some place inaccessible to most enemies. When one thinks of it, the rook's nest swaying on the treetops expressed the climax of a brilliant experiment. 4. The crowning advantage was the possibility of migrating, of conquering time, by circumventing the arid summer and the severe winter, and of conquering space by passing quickly from one country to another and sometimes almost girdling the globe. There are not many acquisitions that have meant more to their possessors than the power of flight. It was a key opening the doors of a new freedom. The problem of flight, as has been said in a previous chapter, has been solved four times, and the solution has been different in each case. The four solutions are those offered by insects, extinct pterodactyls, birds, and bats. Moreover, as has been pointed out, there have been numerous attempts at flight which remain glorious failures, notably the flying fishes, which take a great leap and hold their pectoral fins taut. The flying treetode, whose webbed fingers and toes form a parachute. The flying lizard, Dracovolans, which has its skin pushed out on five or six greatly elongated mobile ribs, and various flying mammals, for instance flying phalangers and flying squirrels, which take great swooping leaps from tree to tree. The wings of an insect are hollow, flattened sacs which grow out from the upper parts of the sides of the second and third rings of the region called the thorax. They are worked by powerful muscles and are supported, like a fan, by ribs of chitin, which may be accompanied by air tubes, blood channels, and nerves. The insect's body is lightly built and very perfectly aerated, and the principle of the insect's flight is the extremely rapid striking of the air by means of the lightly built elastic wings. Many an insect has over 200 strokes of its wings in one second, hence, in many cases, the familiar hum, comparable on a small scale to that produced by the rapidly revolving blades of an airplane's propeller. For a short distance a bee can outfly a pigeon, but few insects can fly far, and they are easily blown away or blown back by the wind. Dragonflies and bees may be sighted as examples of insects that often fly for two or three miles, but this is exceptional, and the usual shortness of insect flight is an important fact for man, since it limits the range of insects like houseflies and mosquitoes, which are vehicles of typhoid fever and malaria, respectively. The most primitive insects, springtails and bristletails, show no trace of wings, while fleas and lice have become secondarily wingless. It is interesting to notice that some insects only fly once in their lifetime, namely in connection with mating. The evolution of the insect's wing remains quite obscure, but it is probable that insects could run, leap, and parachute before they could actually fly. The extinct flying dragons or pterodactyls had their golden age in the Cretaceous Era, after which they disappeared, leaving no descendants. A fold of skin was spread out from the sides of the body by the enormously elongated outermost finger, usually regarded as corresponding to our little finger. It was continued to the hind legs and thence to the tail. It is unlikely that the pterodactyls could fly far, for they have at most a weak keel on their breastbone. On the other hand some of them showed a marked fusion of dorsal vertebrae, which, as in flying birds, must have served as a firm fulcrum for the stroke of the wings. The quaint creatures varied from the size of a sparrow up to a magnificent spread of fifteen to twenty feet from tip to tip of the wings. They were the largest of all flying creatures. The bird's solution of the problem of flight, which will be discussed separately, is centered in the feather, which forms a coherent vein for striking the air. In pterodactyl in bath the wing is a web wing, or patagium, and a small web is to be seen on the front side of the bird's wing. But the bird's patagium is unimportant, and the bird's wing is on an evolutionary tack of its own. A forelimb transformed for bearing the feathers of flight. Feathers are in a general way comparable to the scales of reptiles, but only in a general way, and no transition stage is known between the two. Birds evolve from a bipedal dinosaur stock, as has been noticed already, and it is highly probable that they began their assent by taking running leaps along the ground, flapping their scaly forelimbs, and balancing themselves in kangaroo-like fashion with an extended tail. A second chapter was probably an abereal apprenticeship, during which they made a fine art of parachuting, a persistence of which is to be seen in the pigeon gliding from the dove-cut to the ground. It is in birds that the mastery of the air reaches its climax, and the mysterious sailing of the albatross and the vulture is surely the most remarkable locomotor triumph that has ever been achieved. Without any apparent stroke of the wings the bird sails for half an hour at a time, with the wind and against the wind, around the ship and in majestic spirals in the sky, probably taking advantage of currents of air of different velocities, and continually changing energy of position into energy of motion as it sinks, and energy of motion into energy of position as it rises. It is interesting to note that some dragonflies are also able to sail. The web-wing of bats involves much more than the forearm. The double fold of skin begins on the side of the neck, passes along the front of the arm, skips the thumb, and is continued over the elongated palm bones and fingers to the sides of the body again, and to the hind legs, and to the tail, if there is a tail. It is interesting to find that the bones of the bat's skeleton tend to be lightly built, as in birds, that the breastbone has likewise a keel for the better insertion of the pectoral muscles, and that there is a solidifying of the vertebrae of the back, affording, as in birds, a firm basis for the wing action. Such similar adaptations to similar needs, occurring in animals not nearly related to one another, are called convergences, and form a very interesting study. In addition to adaptations which the bat shares with the flying bird, it has many of its own. There are so many nerve endings on the wing, and often also on special skin leaves about the ears and nose, that the bat flying in the dusk does not knock against branches or other obstacles. Some say that it is helped by the echoes of its high-pitched voice, but there is no doubt as to its exquisite tactility. That it usually produces only a single young one at a time, is a clear adaptation to flight, and similarly the sharp mountain-top-like cusps on the back teeth are adapted in insectivorous bats for crunching insects. Whether we think of the triumphant flight of birds, reaching a climax in migration, or of the marvel that a creature of the earth, as a mammal essentially is, should evolve such a mastery of the air as we see in bats, or even of the repeated but splendid failures which parachuting animals illustrate, we gain an impression of the insurgence of living creatures in their characteristic endeavor after fuller well-being. We have set enough to show how well-adapted many animals are to meet the particular difficulties of the haunt which they tenet, but difficulties and limitations are ever arising afresh, and so one fitness follows on another. It is natural, therefore, to pass to the frequent occurrence of protective resemblance, camouflage, and mimicry, the subject of the next article. End of chapter 3