 Chapter 42 Part 2 of Principles of Geology This is a LibriVox recording. All LibriVox recordings are in the public domain. For more information or to volunteer, please visit LibriVox.org Recording by Abahi in March 2018 Principles of Geology by Charles Lyle Chapter 42 Part 2 African Deserts If we attribute the origin of a great part of the desert of Africa to the gradual progress of moving sands driven eastward by the westerly winds, we may safely infer that a variety of species must have been annihilated by this cause alone. The sand flood has been inundating from time immemorial some of the rich lands on the west of the Nile, and we have only to multiply this effect the sufficient number of times in order to understand how, in the lapse of ages, a whole group of terrestrial animals and plants may become extinct. The African Desert, without including Bornu and Darfur, extends, according to the calculation of Humboldt, over 194,000 square leagues, an area nearly three times as great as that of France. In a small portion of so vast a space, we may infer from analogy that there were many peculiar species of plants and animals which must have been banished by the sand and their habitations invaded by the camel and by birds and insects formed for the arid sands. There is evidently nothing in the nature of the catastrophe to favour the escape of the former inhabitants to some adjoining province, nothing to weaken in the bordering lands that powerful barrier against immigration, pre-occupancy. Nor, even if the exclusion of a certain group of species from a given tract were compensated by an extension of their range over a new country, would that circumstance tend to the conservation of species in general, for the extirpation would merely then be transferred to the region so invaded? If it be imagined, for example, that the aboriginal quadrupeds, birds and other animals of Africa, emigrated in consequence of the advance of drift sand and colonised Arabia, the indigenous Arabian species must have given way before them and have been reduced in number or destroyed. Let us next suppose that, in some central or more elevated parts of the great African desert, the upheaving power of subterranean movements should be exerted throughout an immense series of ages. A company that certain intervals by volcanic eruptions, such as gave rise at once, in 1755, to a mountain 1,600 feet high on the Mexican plateau. When the continued repetition of these events had caused the mountain chain, it is obvious that a complete transformation in the state of the climate would be brought about throughout a vast area. We may imagine the summits of the new chain to rise so high as to be covered, like Mount Atlas, for several thousand feet with snow during a great part of the year. The melting of these snows during the greatest heat would cause the rivers to swell in the season when the greatest drought now prevails. The waters, moreover, derived from this source, would always be of lower temperature than the surrounding atmosphere and would thus contribute to cool the climate. During the numerous earthquakes and volcanic eruptions supposed to accompany the gradual formation of the chain, there would be many floods caused by the bursting of temporary lakes and by the melting of snows by lava. These inundations might deposit alluvial matter far and wide over the original sands as the country assumed varied shapes and was modified again and again by the moving power from below and the aqueous erosion of the surface above. At length the Sahara might be fertilized, irrigated by rivers and streamlets intersecting it in every direction and covered by jungle and morasses, so that the animals and plants which now people northern Africa would disappear and the region would gradually become fitted for the reception of a population of species perfectly dissimilar in their forms, habits and organization. There are always some peculiar and characteristic features in the physical geography of each large division of the globe and on these peculiarities the state of animal and vegetable life is dependent. If therefore we admit incessant fluctuations in the physical geography we must at the same time concede the successive extinction of terrestrial and aquatic species to be part of the economy of our system. When some great class of stations is in excess in certain latitudes as for example in white savannas, arid sands, lofty mountains or inland seas we find a corresponding development of species adapted for such circumstances. In North America where there is a chain of vast inland lakes of fresh water we find an extraordinary abundance and variety of aquatic birds, freshwater fish, testesia and small amphibious reptiles fitted for such a climate. The greater part of these would perish if the lakes were destroyed an event that might be brought about by some of the least of those important revolutions contemplated in geology. It might happen that no freshwater lakes of corresponding magnitude might then exist on the globe or that if they occurred elsewhere they might be situated in New Holland, Southern Africa, Eastern Asia or some region so distant as to be quite inaccessible to the North American species or they might be situated within the tropics in a climate uninhabitable by creatures fitted for a temperate zone or finally we may presume that they would be preoccupied by indigenous tribes. A vivid description has been given by Mr. Darwin and Sir W. Parrish of the great droughts which have sometimes visited the Pampas of South America for three or four years in succession during which an incredible number of wild animals, cattle, horses and birds have perished from want of food and water. Several hundred thousand animals were drowned in the Parana alone having rushed into the river to drink and being too much exhausted by hunger to escape. Such droughts are often attended in South America and other hot climates by widespreading conflagrations caused by lightning which fires the dried grass and brushwood. Thus quadrupeds, birds, insects and other creatures are destroyed by myriads. How many species, both of the animal and vegetable world which once flourished in the country between the valley of the Parana and the Straits of Magellan may not have been annihilated since the first drought or first conflagration began. To pursue this train of reasoning farther is unnecessary. The geologist has only to reflect on what has been said of the habitations and stations of organic beings in general and to consider them in relation to those effects which were contemplated in the second book as resulting from the igneous and aqueous causes now in action and he will immediately perceive that amidst the vicissitudes of the earth's surface species cannot be immortal but must perish one after the other like the individuals which compose them. There is no possibility of escaping from this conclusion without resorting to some hypothesis as violent as that of Lamarck who imagined, as we have before seen, that species are each of them endowed with indefinite powers of modifying their organization in conformity to the endless changes of circumstances to which they are exposed. Effects of a general alteration in climate on the distribution of species Some of the effects which must attend every general alteration of climate are sufficiently peculiar to claim a separate consideration before concluding the present chapter. I have before stated that during seasons of extraordinary severity many northern birds and in some countries many quadrupeds migrate southwards. If these cold seasons were to become frequent, in consequence of a gradual and general refrigeration of the atmosphere such migrations would be more and more regular until, at length, many animals now confined to the arctic regions would become the tenants of the temperate zone while the inhabitants of the temperate zone would approach nearer to the equator. At the same time many species previously established on high mountains would begin to descend in every latitude towards the middle regions and those which were confined to the flanks of mountains would make their way into the plains. Analogous changes would also take place in the vegetable kingdom. If, on the contrary, the heat of the atmosphere beyond the increase the plants and animals of low grounds would ascend to higher levels. The equatorial species would migrate into the temperate zone and those of the temperate into the arctic circle. But although some species might thus be preserved every great change of climate must be fatal to many which can find no place of retreat and their original habitations become unfit for them. For if the general temperature beyond the rise then there is no cooler region whether the polar species can take refuge if it be on the decline then the animals and plants previously established between the tropics have no resource. Suppose the general heat of the atmosphere to increase so that even the arctic region became too warm for the musk ox and reindeer it is clear that they must perish. So if the torrid zone should lose so much of its heat by the progressive refrigeration of the earth's surface as to be an unfit habitation for apes, boas, bamboos and palms these tribes of animals and plants or at least most of the species now belonging to them would become extinct for there would be no warmer latitudes for their reception. It will follow therefore that as often as the climates of the globe are passing from the extreme of heat to that of cold from the summer to the winter of the great year before alluded to the migratory movement will be directed constantly from the poles towards the equator and for this reason the species inhabiting parallel latitudes in the northern and southern hemispheres must become widely different. For I assume on grounds before explained that the original stalk of each species is introduced into one spot of the earth only and consequently no species can be at once indigenous in the arctic and Antarctic circles. But when, on the contrary, a series of changes in the physical geography of the globe or any other supposed cause occasions an elevation of the general temperature when there is a passage from the winter to one of the vernal or summer seasons of the great cycle of climate then the order of the migratory movement is inverted. The different species of animals and plants direct their course from the equator towards the poles and the northern and southern hemispheres may become peopled to a certain limited extent by identical species. I say limited because we cannot speculate on the entire transposition of a group of animals and plants from tropical to polar latitudes or the reverse as a probable or even possible event. We may believe the mean annual temperature of one zone to be transferable to another but we know that the same climate cannot be so transferred. Whatever be the general temperature of the earth's surface comparative equability of heat will characterize the tropical regions while great periodical variations will belong to the temperate and still more to the polar latitudes. These and many other peculiarities connected with heat and light depend on fixed astronomical causes such as the motion of the earth and its position in relation to the sun and not on those fluctuations of its surface which may influence the general temperature. Among many obstacles to such extensive transference of habitations we must not forget the immense lapse of time required according to the hypothesis before suggested to bring about a considerable change in climate. During a period so vast the other cause of extirpation before enumerated would exert so powerful an ignorance as to prevent all save a very few hardy species from passing from equatorial to polar regions or from the tropics to the pole. But the power of accommodation to new circumstances is great in certain species and might enable many to pass from one zone to another if the mean annual heat of the atmosphere and the ocean were greatly altered. To the marine tribes especially such a passage would be possible for there are less impeded in their migrations by barriers of land than are the terrestrial by the ocean. Add to this that the temperature of the ocean is much more uniform than that of the atmosphere investing the land so that we may easily suppose that most of the testesia, fish and other classes might pass from the equatorial into the temperate regions if the mean temperature of those regions were transposed although a second expatriation of these species of tropical origin into the arctic and Antarctic circles would probably be impossible. Let us now consider more particularly the effect of vicissitudes of climate in causing one species to give way before the increasing numbers of some other. When temperature forms the barrier which rests the progress of an animal or plant in a particular direction the individuals are fewer and less vigorous as they approach the extreme confines of the geographical range of the species. But these stragglers are ready to multiply rapidly on the slightest increase or diminution of heat that may be favorable to them. Just as particular insects increase during a hot summer and certain plants and animals gain ground after a series of congenial seasons. In almost every district, especially if it be mountainous, there are a variety of species, the limits of whose habitations are contaminous, some being unable to proceed farther without encountering too much heat, others too much cold. Individuals which are thus on the borders of the regions proper to their respective species are like the outposts of hostile armies ready to profit by every slight change of circumstances in their favour and to advance upon the ground occupied by their neighbours and opponents. The proximity of distinct climates produced by the inequalities of the earth's surface brings species possessing very different constitutions into such immediate contact that their naturalisations are very speedy whenever opportunities of advancing present themselves. Many insects and plants, for example, are common to low plains within the Arctic Circle and to lofty mountains in Scotland and other parts of Europe. If the climate, therefore, of the polar regions were transferred to our own latitudes, the species in question would immediately descend from these elevated stations to overrun the low grounds. Invasions of these kind, attended by the expulsion of the pre-occupants, are almost instantaneous, because the change of temperature not only places the one species in a more favourable position, but renders the others sickly and almost incapable of defence. These changes inconsistent with the theory of transmutation. Lamarck, when speculating on the transmutation of species, supposed every modification in organisation and instinct to be brought about slowly and insensibly in an indefinite lapse of ages. But he does not appear to have sufficiently considered how much every alteration in the physical condition of the habitable surface changes the relations of a great number of co-existing species, and that some of these would be ready instantly to avail themselves of the slightest change in their favour and to multiply to the injury of others. Even if we thought it possible that the palm or the elephant, which now flourish in equatorial regions, could ever learn to bear the variable seasons of our temperate zone or the rigours of an arctic winter, we might with no less confidence affirm that they must perish before they had time to become habituated to such new circumstances, that they would be displaced by other species as often as the climate varied, may be inferred from the data before explained, respecting the local extermination of species produced by the multiplication of others. Suppose the climate of the highest part of the woody zone of Etna to be transferred to the seashore of the base of the mountain. No botanist would anticipate that the olive, lemon tree and prickly pear, cactus opuntia, would be able to contend with the oak and chestnut, which would begin forthwith to descend to a lower level, or that these last would be able to stand their ground against a pine, which would also, in the space of a few years, begin to occupy a lower position. We might form some kind of estimate of the time which might be required for the migrations of these plants, whereas we have no data for concluding that any number of thousands of years would be sufficient for one step in the pretended metamorphosis of one species into another, possessing distinct attributes and qualities. This argument is applicable not merely to climate, but to any other cause of mutation. However, slowly a lake may be converted into a marsh, or a marsh into a meadow, it is evident that before the lecustrine plants can acquire the power of living in marshes, or the marsh plants of living in a less humid soil, other species, already existing in the region and fitted for these several stations, will intrude and keep possession of the ground. So, if a tract of salt water becomes fresh by passing through every intermediate degree of brackishness, still the marine mollusks will never be permitted to be gradually metamorphosed into fluvia-tiled species, because long before any such transformation can take place by slow and insensible degrees, other tribes, already formed to delight in brackish or fresh water, will avail themselves of the change in the fluid, and will, each in their turn, monopolize the space. It is idle, therefore, to dispute about the abstract possibility of the conversion of one species into another when there are known causes so much more active in their nature which must always intervene and prevent the actual accomplishment of such conversions. A faint image of the certain doom of a species less fitted to struggle with some new condition in a region which it previously inhabited and where it has to contend with a more vigorous species, is presented by the extirpation of savage tribes of men by the advancing colony of some civilized nation. In this case, the contest is merely between two different races, two varieties, moreover, of a species which exceeds all others in its aptitude to accommodate its habits to the most extraordinary variations of circumstances. Yet few future events are more certain than the speedy extermination of the Indians of North America and the savages of New Holland in the course of a few centuries when these tribes will be remembered only in poetry or history. Concluding Remarks We often hear astonishment expressed at the disappearance from the earth in times comparatively modern of many small as well as large animals, the remains of which have been found in a fossil state under circumstances implying that neither any great geographical revolution nor the exterminating influence of man has intervened to account for their extinction. But in all such cases we should inquire whether we are sufficiently acquainted with the numerous and complicated conditions on which the perpetuation of each species depends to entitle us to wonder if it should be suddenly cut off. Mr Darwin, when calling attention to the fact that the horse, Megatherium, Megalonix and many contemporary Mamelia had perished in South America after that continent had acquired its present configuration and when, if we may judge by the testesia, the climate very nearly resembled the present, observes that in the living creation one species is often extremely rare in a given region while another of the same genus and with closely allied habits is exceedingly common. A zoologist familiar with such phenomena, if asked to explain them, usually replies that some slight difference in climate, food or the number of its enemies must determine the relative strength of the two species in question, although we may be unable to point out the precise manner of the action of the check. We are therefore driven to the conclusion that causes generally quite inappreciable by us determine whether a given species shall be abundant or scanty in numbers. Why then should we feel astonishment if the rarity is occasionally carried a step farther to extinction? End quote. End of Chapter 42, Part 2 Chapter 43 of Principles of Geology This is a LibriVox recording. A LibriVox recordings are in the public domain for more information or to volunteer. Please visit LibriVox.org recording by Dion Giants, Salt Lake City, Utah. Principles of Geology by Charles Lyle Chapter 43 Extinction and Creation of Species Theory of the successive extinction of species consistent with a limited geographical distribution. Opinions of botanists respecting the centres from which plants have been diffused. Whether there are grounds for inferring that the loss from time to time of certain animals and plants is compensated by the introduction of new species. Whether any evidence of such new creations could be expected within the historical era. The question whether the existing species have been created in succession must be decided by geological monuments. Successive extinction of species consistent with their limited geographical distribution. In the preceding chapters I have pointed out the strict dependence of each species on animal and plant on certain physical conditions in the state of the Earth's surface and on the number and attributes of other organic beings inhabiting the same region. I have also endeavored to show that all these conditions are in a state of continual fluctuation. The igneous and aqueous agents remodeling from time to time the physical geography of the globe and the migrations of species causing new relations to spring up successively between different organic beings. I have deduced as a corollary that the species existing at any particular period must in the course of ages become extinct one after the other. They must die out to borrow an emphatical expression from Bofan because time fights against them. If the views which I have taken are just there will be no difficulty in explaining why the habitations of so many species are now restrained within exceedingly narrow limits. Every local revolution such as those contemplated in the preceding chapter tends to circumscribe the range of some species while it enlarges that of others. And if we are led to infer that new species originate in one spot only each must require time to diffuse itself over a wide area. It will follow therefore from the adoption of this hypothesis that the recent origin of some species and the high antiquity of others are equally consistent with the general fact of their limited distribution some being local because they have not existed long enough to admit of their wide dissemination. Others because circumstances in the animate or inanimate world have occurred to restrict the range which they may once have obtained. As a general rule however species common to many distant provinces and those now found to inhabit very distant parts of the globe are to be regarded as the most ancient. Numerically speaking they may not perhaps be largely represented but their wide diffusion shows that they have had a long time to spread themselves and have been able to survive many important revolutions in physical geography. After so much evidence has been brought to light by the geologist of land and sea having changed places in various regions since the existing species were in being we can feel no surprise that the zoologist and botanist have hitherto found it difficult to refer the geographical distribution of species to any clear and permanent principles since they have usually speculated on the phenomena upon the assumption that the physical geography of the globe had undergone no material alteration since the introduction of the species now living. So long as this assumption was made the facts relating to the geography of plants and animals appeared capricious and by many the subject was pronounced to be so full of mystery and anomalies that the establishment of a satisfactory theory was hopeless. Centers from which plants have been diffused some botanists conceived in accordance with the hypothesis of Wildenau that mountains were the centers of creation from which the plants now inhabiting continents have radiated to which decandole and others with much reason objected that mountains on the contrary are often the barriers between two provinces of distinct vegetation. The geologist who was acquainted with the extensive modifications which the surface of the earth has undergone in very recent geological epochs may be able perhaps to reconcile both these theories in their application to different regions. A lofty range of mountains which is so ancient as to date from a period when the species of animals and plants differed from those now living will naturally form a barrier between contiguous provinces. But a chain which has been raised in great part within the epic existing species and around which new lands have arisen from the sea within that period will be a center of peculiar vegetation. In France observes decandole the alps and savannas prevent a great number of the plants of the south from spreading themselves to the northward. But it has been remarked that some species have grown through the gorges of these chains and are found on their northern sides principally in those places where they are lower and more interrupted. Now the chains here alluded to have probably been of considerable height ever since the era when the existing vegetation began to appear and were it not for the deep fissures which divide them much more abrupt terminations to the extension of distinct assemblages of species. Parts of the Italian peninsula on the other hand have gained a considerable portion of their present height since a majority of the marine species now inhabiting the Mediterranean and probably also since the terrestrial plants of the same region were in being. Large tracts of land have been added both on the Adriatic and Mediterranean side to what originally constituted a much narrower range of mountains if not a chain of islands running nearly north and south like Corsica and Sardinia. It may therefore be presumed that the Apennines have been a center when species have diffused themselves over the contiguous lower and newer regions. In this and all analogous situations the doctrine of Wildenau that species have radiated from the mountains as from centers may be well founded introduction of new species. If the reader should infer from the facts laid before him in the preceding chapters that the active extinction of animals and plants may be part of the constant and regular course of nature, he won't naturally inquire whether there are any means provided for the repair of these losses. Is it part of the economy of our system that the habitable globe should to a certain extent become depopulated both in the ocean or that the variety of species should diminish until some new arrives when a new and extraordinary effort of created energy is to be displayed? Or is it possible that new species can be called into being from time to time and yet that so astonishing a phenomenon can escape the observation of naturalists? Humboldt has characterized these subjects as among the mysteries which natural science cannot reach and he observes that the investigation of the origin of beings does not belong to zoological or botanical geography. To geology however these topics do strictly appertain and this science is chiefly interested in inquiries into the state of the animate creation as it now exists with a view of pointing out its relations to antecedent periods when its condition was different. Before offering any hypothesis towards the solution of so difficult a problem let us consider what kind of evidence we ought to expect in the present state of science in the first appearance of new animals or plants if we could imagine the successive creation of species to constitute like their gradual extinction a regular part of the economy of nature. In the first place it is obviously more easy to prove that a species once numerously represented in a given district has ceased to be which did not pre-exist has made its appearance assuming always for reasons before stated that single stocks only of each animal and plant are originally created and that individuals of new species do not suddenly start up in many different places at once. So imperfect has the science of natural history remained down to our own times that within the memory of persons now living the numbers of known animals and plants have been doubled or even quadrupled in many classes new and often conspicuous species are annually discovered in parts of the old continent long inhabited by the most civilized nations conscious therefore of the limited extent we always infer when such discoveries are made that the beings in question had previously eluded our research or had at least existed elsewhere and only migrated at a recent period into the territories where we now find them it is difficult even in contemplation to anticipate the time when we shall be entitled to make any other hypothesis in regard to all the marine tribes and to by far the greater number of the terrestrial such as birds which possess such unlimited powers of migration insects which besides the variability of each species in number are also so capable of being diffused to vast distances and cryptogamous to which as to many other classes both of the animal and vegetable kingdom similar observations are applicable what kind of evidence of new creations could be expected what kind of proofs therefore could we reasonably expect to find of the origin at a particular period of a new species perhaps it may be said in reply that in the last two or three centuries some forest tree or new quadruped might have been observed to appear suddenly in those parts of England or France which had been most thoroughly investigated that naturalists might have been able to show that no such living being inhabited any other region of the globe and that there was no tradition of anything similar having been before observed in the district where it had made its appearance now although this objection may seem plausible yet its force will be found to depend entirely on the rate of fluctuation which we suppose to prevail in the animate world and on the proportion which such conspicuous subjects of the animal species bear to those which are less known and escape our observation there are perhaps more than a million species of plants and animals exclusive of the microscopic and infusory animal cules now inhabiting the Terracquia's globe the terrestrial plants may amount says decandole to somewhere between 110,000 and 20,000 but the data on which this conjecture is founded are considered by many botanists to be vague and unsatisfactory sprinkle only enumerated in 1827 about 31,000 known Phenogamous and 6,000 cryptogamous plants but that naturalist omitted many perhaps 7,000 Phenogamous and 1,000 cryptogamous species Mr. Lindley in a letter to the author in 1836 expressed his opinion that it would be rash to speculate on the existence of more than 80,000 Phenogamous and 10,000 cryptogamous plants if we take he says in a letter to the author on this subject 37,000 as the number of published Phenogamous species and then add for the undiscovered species in Asia and New Holland 15,000 in Africa 10,000 and in America 18,000 we have 80,000 species and if 7,000 be the number of published cryptogamous plants and we allow 3,000 for the undiscovered species making 10,000 there would then be on the whole 90,000 species but since that period one catalogue as I learned from Dr. J. Hooker contains a list of the names of 78,000 Phenogamous plants which had been published before 1841 it was supposed by Linnaeus that there were four or five species of insects in the world for each Phenogamous plant but if we may judge from the relative proportion of the two classes in Great Britain the number of insects must be still greater for the total number of British insects according to the last census is about 12,500 whereas there are only 1500 Phenogamous plants indigenous to our island as the insects are much more numerous in hot countries than in our temperate latitudes it seems difficult to avoid the conclusion that there are more than half a million species in the world the number of known mammifers when Tamanic wrote exceeded 800 and Mr. Waterhouse informs me that more than 1200 are now 1850 ascertained to exist Mr. Cuvier estimated the amount of known fishes at 6,000 and Mr. G. Gray in his genera of birds enumerates 8,000 species we have still to add the reptiles and all the invertebrated animals exclusive of insects it remains in a great degree mere matter of conjecture what proportion the aquatic tribes are the descendants of the land but the habitable surface beneath the waters can hardly be estimated at less than double that of the continents and islands even admitting that a very considerable area is destitute of life in consequence of great doubt cold darkness and other circumstances in the late polar expedition it was found that in some regions offens bay there were marine animals inhabiting the bottom at great depths where the temperature of the water was below the freezing point that there is life at much greater profundities in warmer regions may be confidently inferred the ocean teams with life the class of polyps alone are conjectured by Lamarck to be as strong in individuals as insects every tropical reef is described as covered with corals and sponges and swarming with crustacea, echinese and testesia while almost every tide washed rock in the world is carpeted with fucie and supports some corallines, actinae and mollusca there are innumerable forms in the seas of the warmer zones that scarcely begun to attract the attention of the naturalist and there are parasitic animals without number three or four of which are sometimes appropriated to one genus as to the whale Bellina for example, even though we concede therefore that the geographical range of marine species is more extensive in general than that of the terrestrial the temperature of the sea being more uniform and the land impeding less the migrations of the oceanic than the ocean those of the terrestrial species yet it seems probable that the aquatic tribes far exceed in number the inhabitants of the land without insisting on this point it may be safe to assume that in the course of microscopic beings there are between one and two millions of species now inhabiting the terraqueous globe so that if only one of these were to become extinct annually and one new one were to be every year called into being much more than a million of years might be required to bring about a organic life I am not hazarding at present any hypothesis as to the probable rate of change but none will deny that when the annual birth and the annual death of one species on the globe is proposed as a mere speculation this at least is to imagine no slight degree of instability in the animate creation if we divide the surface of the earth into twenty regions in the same area one of these might comprehend a space of land and water about equal in dimensions to Europe and might contain a twentieth part of the million of species which may be assumed to exist in the animal kingdom in this region one species only would according to the rate of mortality before assumed perish in twenty years would survive out of fifty thousand in the course of a century but as a considerable proportion of the whole would belong to the aquatic classes with which we have a very imperfect acquaintance we must exclude them from our consideration and if they constitute half of the entire number then one species only might be lost in forty years in the terrestrial tribes now the mammalia whether terrestrial or aquatic bear so small a proportion to other classes of animals forming less perhaps than one thousandth part of the whole that if the longevity of species in the different orders were equal a vast period must elapse before it would come to the turn to us to lose one of their number if one species only of the whole animal kingdom died out in forty years no more than one mammoth might disappear in forty thousand years in a region of the dimensions of Europe it is easy therefore to see that in a small portion of such an area in countries for example of the size and France periods of much greater duration must elapse before it would be possible to authenticate the first appearance of one of the larger plants and animals assuming the annual birth and death of one species to be the rate of this attitude in the animate creation throughout the world the observations of naturalists upon living species may in the course of future centuries accumulate positive data from which an insight into the laws which govern this part of our terrestrial system may be derived but in the present deficiency of historical records we have traced up the subject to that point where geological monuments alone are capable of leading us on to the discovery of ulterior truth to these therefore we must appeal carefully examining the strata of recent formation where in the remains of living species both animal and vegetable are known to occur we must study the strata in strict reference to their chronological order as deduced from their superposition and other relations from these sources we may learn that the species now our contemporaries have survived the greatest revolutions of the earth's surface which of them have coexisted with the greatest number of animals and plants now extinct and which have made their appearance only when the animate world had nearly attained its present condition from such data we may be enabled to infer that these have been called into existence in succession or all at one period whether singly or by group simultaneously whether the antiquity of man may be as high as that of any of the inferior beings which now share the planet with him or whether the human species is one of the most recent of the whole to some of these questions and even now return a satisfactory answer and with regard to the rest we have some data to guide conjecture and to enable us to speculate with advantage but in order to be fully qualified to enter upon such discussions the reader must study the ample body of materials amassed by the industry of modern geologists End of Chapter 43 Chapter 44 of Principles of Geology This is a LibriVox recording All LibriVox recordings are in the public domain For more information or to volunteer please visit LibriVox.org Principles of Geology by Charles Lyle Book 3 Chapter 44 Effects produced by the powers of vitality on the state of the earth's surface modifications in physical geography caused by organic beings why the vegetable soil does not augment in thickness The theory that vegetation is an antagonistic power counterbalancing the degradation caused by running water untenable conservative influence of vegetation rain diminished by felling of forests distribution of American forests dependent on direction of predominant winds influence of man in modifying the physical geography of the globe The second branch of our inquiry respecting the changes of the organic world relates to the processes by which the remains of animals and plants become fossil or to speak still more generally to all the effects produced by the powers of vitality on the surface and shell of the earth Before entering on the principle division of this subject the embedding and preservation of animal and vegetable remains I shall offer a few remarks on the superficial modifications caused directly by the agency of organic beings as when the growth of certain plants covers the slope of a mountain with peat or converts a swamp into dry land or when vegetation prevents the soil in certain localities from being washed away by running water In considering alterations of this kind brought about in the physical geography of particular tracts we are too apt to think exclusively of that part of the earth's surface which has emerged from beneath the waters and with which alone as terrestrial beings we are familiar Here the direct power of animals and plants to cause any important variation is of necessity very limited except in checking the progress of that decay of which the land is the chief theater but if we extend our views and instead of contemplating the dry land consider that larger portion which is assigned to the aquatic tribes We discover the great influence of the living creation from the conformation to the solid exterior which the agency of inanimate causes alone could not produce Thus when timber is floated into the sea it is often drifted to vast distances and subsides in spots where there might have been no deposit at that time in place if the earth had not been tenanted by living beings If therefore in the course of ages a hill of wood or lignite be thus formed in the sub-aqueous regions a change in the submarine geography may be said to have resulted from the action of organic powers So in regard to the growth of coral reefs it is probable that a large portion of the matter of which they are composed is supplied by mineral springs which often rise up at the bottom of the sea in which the land abound throughout volcanic regions hundreds of leagues in extent The matter thus constantly given out could not go on accumulating forever in the waters but would be precipitated in the abysses of the sea even if there were no polyps and testatia But these animals arrest and secrete the carbonite of lime on the summits of submarine mountains and form reefs many hundred feet in thickness and hundreds of miles in length where but for them none might ever have existed why the vegetable soil does not augment in thickness If no such voluminous masses are formed on the land it is not from the want of solid matter in the structure of terrestrial animals and plants but merely because as if I have so often stated the continents are those parts of the globe where ascensions of matter can scarcely ever take place where on the contrary the most solid parts already formed are each in their turn exposed to gradual degradation The quantity of timber and vegetable matter which grows in a tropical forest in the course of a century is enormous and multitudes of animal skeletons are scattered there during the same period besides innumerable land shells and other organic substances The aggregate of these materials therefore might constitute a mass greater in volume than that which is produced in any coral reef during the same lapse of years but although this process should continue on the land forever no mountains of wood or bone would be seen stretching far and wide over the country or pushing out bold promontories into the sea The whole solid mass is either devoured by animals or decomposes as does a portion of the rock and soil on which the animals and plants are supported The waste of the strata themselves accompanied by the decomposition of their organic remains and the setting free of their alkaline ingredients is one source from whence running water and the atmosphere may derive the materials which are absorbed by the roots and leaves of plants Another source is the passage into a gaseous form of even the hardest parts of animals and plants which die and putrify in the air where they are soon resolved into the elements that are proposed and while a portion of these constituents is volatilized the rest is taken up by rain water and sinks into the earth or flows towards the sea so that they enter again and again into the composition of different organic beings The principal element found in plants are hydrogen, carbon and oxygen so that water and the atmosphere contain all of them either in their own composition or in solution The constant supply of these elements is maintained not only by the putrefaction of animal and vegetable substances and the decay of rocks but also by the copious evolution of carbonic acid and other gases from volcanoes and mineral springs and by the effects of ordinary evaporation whereby aqueous vapors are made to rise from the ocean and to circulate round the globe It is well known that when two gases of different specific gravity are brought into contact even though the heavier be the lower most they soon become uniformly diffused by mutual absorption through the whole space which they occupy By virtue of this law, the heavy carbonic acid finds its way upwards through the lighter air into the atmosphere and conveys nourishment to the leechin which covers the mountaintop If the quantity of food consumed by terrestrial animals and the elements imbibed by the roots and leaves of plants were derived entirely from that supply of hydrogen, carbon, oxygen, nitrogen and other elements given out into the atmosphere and the waters by the putrescence of organic substances then we might imagine that the vegetable mold would after a series of years neither gain nor lose a single particle by the action of organic beings and this conclusion is not far from the truth but the operation which renovates the vegetable and animal mold is by no means so simple as that you're supposed Thousands of carcasses of terrestrial animals every century into the sea and together with forests of drift timber are embedded in the sub-equious deposits where their elements are imprisoned in solid strata and may they remain locked up throughout whole geological epics before they again become subservient to the purposes of life On the other hand, fresh supplies are derived by the atmosphere and the water is before stated from the disintegration of rocks in their organic contents and through the agency of mineral springs from the interior of the earth from whence all the elements before mentioned which enter principally into the composition of animals and vegetables are continually evolved even nitrogen is found by chemists to be contained very generally Vegetation not an antagonistic power counterbalancing the action of running water if we suppose that the copious supply from the nether regions by springs and volcanic vents of carbonic acid and other gases together with the decomposition of rocks may be just sufficient to counterbalance that loss of matter which having already served for the nourishment of animals and plants is annually carried down in organized forms and buried in sub-equious strata we concede the utmost that is consistent with probability an opinion however has been expressed that the processes of vegetable life by absorbing various gases from the atmosphere cause so large a mass of solid matter to accumulate on the surface of the land that this mass alone may constitute a great counter-poise to all the matter transported to lower levels by the aqueous agents of decay Torrents and rivers it is said the waves of the sea and marine currents act upon lines only but the power of vegetation to absorb the elastic and non-elastic fluids circulating on the earth extends over the whole surface of the continents but the silent but universal action of this great antagonistic power the spoilation of waste caused by running water on the land and by the movements of the ocean are neutralized and even counterbalanced in opposition to these views I concede that we shall form a juster estimate of the influence of vegetation but we consider it as being in a slight degree conservative and capable of retarding the waste of land but not of acting as an antagonistic power the vegetable mold is seldom more than a few feet in thickness and frequently does not exceed a few inches and we by no means find that its volume is more considerable in those parts of our continents which we can prove by geological data to have been elevated at more ancient periods and where consequently there has been the greatest time for the accumulation of vegetable matter produced throughout successive zoological epics on the contrary these higher and older regions are more frequently denuded so as to expose the bare rock to the action of the sun and air we find in the torrid zone where the growth of plants is most ranked and luxurious that accessions of matter due to their agency are by no means the most conspicuous indeed it is in these latitudes where the vegetation is most active that for reasons to be explained in the next chapter even though superficial peat mosses are unknown which cover a large area in some parts of our temperate zone if the operation of animal and vegetable life could restore to the general surface of the continents a portion of the elements of those disintegrated rocks of which such enormous masses are swept down annually into the sea the effects would long air this have constituted one of the most striking features in the structure and composition of our continents all the great steps and table lands of the world where the action of running water is feeble would have become the grand repositories of organic matter accumulated without the intermixter of earthy sediment which so generally characterizes the subaqueous strata I have already stated that in the known operation of the igneous causes a real antagonistic power is found which may counterbalance the leveling action of running water and there seems no good reason for presuming that the upheaving and depressing force of earthquakes together with the ejection of matter by volcanoes may not be fully adequate to restore that inequality of the surface which rivers and the waves and currents of the ocean annually tend to lessen if a counterpoise be derived from this source the quantity and elevation of land above the sea may forever remain the same in spite of the action of the aqueous causes which, if thus counteracted, may never be able to reduce the surface of the earth more nearly to a state of equilibrium than that which it has now attained and on the other hand, the force of the aqueous agents themselves might thus continue forever unimpaired conservative influence of vegetation if then, vegetation cannot act as an antagonistic power amid the mighty agents of change which are always modifying the surface of the globe let us next inquire how far its influence is conservative, how far it may retard the leveling effects of running water which it cannot oppose much less counterbalance it is well known that a covering of herbage and shrubs may protect a loose soil from being carried away by rain or even by the ordinary action of a river and may prevent hills of loose sand from being blown away by the wind for the roots bind together the separate particles into a firm mass and the leaves intercept the rain water so that it dries up gradually instead of flowing off in a mass and with great velocity the old Italian hydrographers made frequent mention of the increased degradation which has followed the clearing away of natural woods in several parts of Italy a remarkable example was afforded in the Upper Val di Arno in Tuscany on the removal of the woods clothing the steep declivities of the hills by which that valley is bounded when the ancient forest laws were abolished by the Grand Duke Joseph during the last century a considerable tract of surface in the Cassantina the Clausantinium of the Romans was denuded and immediately the quantity of sand and soil washed down into the Arno increased enormously Frisi, alluding to such occurrences observes that as soon as the bushes and plants were removed the waters flowed off more rapidly and in the manner of floods swept away the vegetable soil this effect of vegetation is of high interest to the geologist when he is considering the formation of those valleys which have been principally due to the action of rivers the space is intervening between the valleys whether they be flat or ridgey when covered with vegetation they scarcely undergo the slightest waste as the surface may be protected by the green sword of grass and this may be renewed in the manner before described from elements derived from rainwater and the atmosphere hence while the river is continually bearing down matter in the alluvial plain and undermining the cliffs on each side of every valley the height of the intervening rising grounds may remain stationary in this manner a cone of loose scoriae sand and ashes such as Monte Nuovo may, when it has once become densely clothed with herbage and shrubs suffer scarcely any further dilapidation and the perfect state of the cones of hundreds of extinct volcanoes in France the Neapolitan territory Sicily and elsewhere may prove nothing whatever either is to their relative or absolute antiquity we may be able to infer from the integrity of such conical hills of incoherent materials that no flood can have passed over the countries where they are situated since their formation but the atmospheric action alone in spots where there happen to be no torrents and where the surface was clothed with vegetation could scarcely in any lapse of ages have destroyed them during the torrents Spain in 1830 I was surprised to see a district of gently undulating ground in Catalonia consisting of red and grey sandstone and in some parts of red moral almost entirely denuded of herbage while the roots of the pines, home oaks and some other trees were half exposed as if the soil had been washed away by a flood such as the state of the forests for example between Oristo and Vich and near San Lorenzo but being overtaken by a violent thunderstorm in the month of August I saw the whole surface even the highest levels of some flat-popped hills streaming with mud while on every declivity the devastation of torrents was terrific the peculiarities and the physiognomy of the district were at once explained and I was taught that in speculating on the greater effects which the direct action of rain may once have produced on the surface of certain parts of England we need not revert to periods when the heat of the climate was tropical in the torrent zone the degradation of land is generally more rapid but the waste is by no means proportioned to the superior quantity of rain or the suddenness of its fall the transporting power of water being counteracted by a greater luxuriance of vegetation a geologist who is no stranger to tropical countries observes that the softer rocks would speedily be washed away in such regions if the numerous roots of plants were not matted together in such a manner as to produce considerable resistance to the destructive power of the rains the parasitical and creeping the plants also entwine in every possible direction so as to render the forests nearly impervious and the trees possess forms and leaves best calculated to shoot off the heavy rains which when they have thus been broken in their fall are quickly absorbed by the ground beneath or when thrown into the drainage depressions give rise to furious torrents influence of man in modifying the physical geography of the globe before concluding this chapter I shall offer a few observations on the influence of man in modifying the physical geography of the globe for he must class his agency among the powers of organic nature felling of forests the felling of forests has been attended in many countries by a diminution of rain as in Barbados and Jamaica for in tropical countries where the quantity of aqueous vapor in the atmosphere is great but where on the other hand the direct rays of the sun are most powerful any impediment to the free circulation of air or any screen which shades the earth from the solar rays becomes a source of humidity and wherever dampness and cold have begun to be generated by such causes the condensation of vapor continues the leaves moreover of all plants are alembics and some of those in the torrid zone have the remarkable property of distilling water thus contributing to prevent the earth from becoming parched up distribution of the American forests there can be no doubt in that the state of the climate especially the humidity of the atmosphere influences vegetation in that in its turn vegetation reacts upon the climate but some writers seem to have attributed too much importance to the influence of forests particularly those of America as if they were the primary cause of the moisture of the climate the theory of a modern author on this subject that forests exist in those parts of America only where the predominant winds carry with them a considerable quantity of moisture from the ocean seems far more rational in all countries he says having a summer heat exceeding 70 degrees the presence or absence of natural woods and their greater or less luxuriance may be taken as a measure of the amount of humidity and of the fertility of the soil short and heavy rains in a warm country will produce grass which having its roots near to the surface springs up in a few days and withers when the moisture is exhausted but transitory rains however heavy will not nourish trees because after the surface is saturated the remainder of the water runs off and the moisture lodged in the soil neither sinks deep enough nor is sufficient quantity to furnish the giants of the forest with the necessary sustenance it may be assumed that 20 inches of rain falling moderately or at intervals will leave a greater permanent supply in the soil than 40 inches falling as it sometimes does in the torrid zone in as many hours in all regions he continues where ranges of mountains intercept the course of the constant or predominant winds the country on the windward side of the mountains will be moist and that on the leeward dry and hence parched deserts will generally be found on the west side of countries within the tropics and on the east side of those beyond them the prevailing winds in these cases being generally in opposite directions on this principle the position of forests in north and south America may be explained thus for example in the region within the 30th parallel the moisture swept up by the trade wind from the Atlantic is precipitated and part upon the mountains of Brazil which are but low and so distributed as to extend far into the interior the portion which remains is born westward and losing a little as it proceeds is at length arrested by the Andes where it falls down in showers on their summits the aerial current now deprived of all the humidity with which it can part arrives in a state of complete execution at Peru where consequently no rain falls but in the region of America beyond the 30th parallel the Andes serve as a screen to intercept the moisture brought by the prevailing winds from the Pacific Ocean rains are copious on their summits and in Chile on their western declivities but none falls on the plains to the eastward except occasionally when the wind blows from the Atlantic I have been more particular in explaining these views because they appear to place in a true light the dependence of vegetation on climate the humidity being increased and more uniformly diffused throughout the year by the gradual spreading of wood it has been affirmed that formally when France and England were covered with wood Europe was much colder than at present that the winters in Italy were longer and that the Sina and many other rivers froze more regularly every winter than now Mr. Arago in an essay on this subject has endeavored to show by tables of observations on the conglolation of the Rhine, Danube, Rome, Poe, Sina and other rivers at different periods that there is no reason to believe the cold who have been in general more intense in ancient times he admits however that the climate of Tuscany is so far modified by the removal of wood as that the winters are less cold but the summers also he contends are less hot than of old and the summers according to him were formally hotter in France than in our own times his evidence is derived chiefly from documents showing that wine was made three centuries ago in the Ververe and several other provinces at an earlier season at greater elevation and in higher latitudes than are now found suitable to the vine there seems little doubt that in the United States of North America the rapid clearing of the country has rendered the winters less severe and the summers less hot in other words the extreme temperatures of January and July have been observed from year to year to approach somewhat nearer to each other whether in this case or in France the mean temperature has been raised seems by no means as yet decided but there is no doubt that the climate has become as Buffon would have said less excessive I have before shown when treating of the excavation of new estuaries in Holland by inroads of the ocean as also of the changes on our own coasts that although the conversion of sea into land by artificial labours may be great yet it must always be in subordination to the power of the tides and currents or to the great movements which alter the relative level of the land and sea Chapter 20 if in addition to the assistance obtained by parliamentary grants for defending Dunwich from the waves all the resources of Europe had been directed to the same end the existence of that port might perhaps have been prolonged by several centuries Chapter 3 but in the meantime the current would have continued to sweep away portions from the adjoining cliffs on each side giving to the whole line of coast its present form until at length the town projecting as a narrow promontory must have become exposed to the irresistible fury of the waves it is scarcely necessary to observe that the control which man can obtain over the igneous agents is less even than that which he may exert over the aqueous he cannot modify the upheaving or depressing force of earthquakes or the periods or degree of violence and on these causes the inequalities of the earth's surface and consequently the shape of the sea and land appear mainly to depend the utmost that man can hope to effect in this respect is occasionally to divert the course of a lava stream and to prevent the burning matter for a season from overwhelming a city or some other of the proudest works of human industry if all the nations of the earth should attempt to quarry away the lava which flowed during one eruption from the Icelandic volcanoes in 1783 and the two following years and should attempt to consign it to the deepest abysses of the ocean the might toil for thousands of years and not accomplish their task yet the matter born down to the sea by two great rivers the Ganges and the Borumputur in each quarter of a century probably equals in weight and volume the mass of Icelandic lava produced by that great eruption page 282 so insignificant is the aggregate force exerted by man when contrasted with the ordinary operations of aqueous or igneous agents in the natural world no application perhaps of human skill and labor tends so greatly to vary the state of the habitable surface is that employed in the drainage of lakes and marshes since not only the stations of many animals and plants but the general climate of a district may thus be modified it is also a kind of alteration to which it is difficult to find anything analogous in the agency of inferior beings for we ought always before we decide that any part of the influence of man is novel and anomalous carefully to consider the powers of all other animated agents which may be limited or superseded by him many who have reasoned on these subjects seem to have forgotten that the human race often succeeds to the discharge of functions previously fulfilled by other species suppose the growth of some of the larger terrestrial plants or in other words the extent of forest to be diminished by man and the climate to be thereby modified it does not follow that this kind of innovation is unprecedented it is a change in the state of vegetation and such may often have been the result of the appearance of new species upon the earth the multiplication for example of certain insects in parts of Germany during the last century destroyed more trees than man perhaps could have felt during an equal period it would be rash however to affirm that the power of man to modify the surface may not differ in kind or degree from that of other living beings although the problem is certainly more complex than many who speculated on such topics have imagined if land be raised from the sea the greatest alteration in its physical condition which could ever arise from the influence of organic beings would probably be produced by the first immigration of terrestrial plants whereby the new tract would become covered with vegetation the change next in importance would seem to be when animals first enter and modify the proportionate numbers of certain species of plants if there be any anomaly in the intervention of man in farther varying the relative numbers in the vegetable kingdom it may not so much consist in the kind or absolute quantity of alteration as in the circumstance that a single species in this case would exert by its superior power and universal distribution an influence equal to that of hundreds of other terrestrial animals if we inquire whether a man by his direct power or by the changes which he may give rise to indirectly tends upon the whole to lessen or increase the inequalities of the earth's surface we shall incline perhaps to the opinion that he is a leveling agent in mining operations he conveys upwards a certain quantity of materials from the bowels of the earth but on the other hand much rock is taken annually from the land in the shape of ballast and afterwards thrown into the sea and by this means in spite of prohibitory laws many harbors in various parts of the world have been blocked up we rarely transport heavy materials to higher levels and our pyramids and cities are chiefly constructed of stone brought down from more elevated situations by plowing up thousands of square miles and exposing a surface for part of the year to the action of the elements to assist the abrading force of rain and diminish the conservative effects of vegetation End of chapter 44