Let us now see whether the various facts and rules concerning the geological succession of organic beings agree better with the common view of the immutability of species, or with the theory of slow and gradual modification of the offspring by natural selection.
New species have emerged very slowly, one after the other, both in water and on land. L YELL has shown that it is scarcely possible to ignore the evidence laid down in the various Tertiary strata in this respect, and each year strives more to fill in the remaining gaps and the percentage of species still alive to those which are totally extinct step down more and more. In some of the most recent, though certainly very old, in terms of years, only 1-2 extinct species occur, and only one or two forms, either new at all or new to the locality, join the earlier ones. If we take the observations of P HILIPPI ‘S in Sicilymay be trusted, the gradual replacement of the earlier sea-dwellers in this island by other species has been exceedingly slow. The secondary formations are more broken; but in any single formation, as B RONN has remarked, neither the appearance nor the disappearance of its many now extinct species has occurred simultaneously.
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Species of different clans and classes have changed neither at the same pace nor in the same ratio. In the oldest Tertiary strata the few living species lie amidst a multitude of extinct forms. F. Alconer has reported a striking instance of the species, viz., of a surviving crocodile, encamped with a multitude of alien and extinct mammals and reptiles in the strata of the Sub – Himalayas. The Silurian Lingula species differ very little from the living species of this genus, while most of the remaining Silurian mollusks and all crustaceans undergo great changes. The land-dwellers seem to change at a faster rate than the sea-dwellers, of which apt evidence is lately given by theSwitzerland has been reported. There seems some reason to believe that such organisms, which are at a higher level of organization, change more rapidly than those imperfectly developed; but there are exceptions to this rule. According to P ICTET ‘ S , the degree of organic change corresponds toRemark not exactly the succession of our geological formations, so that between every two successive formations the forms of life changed in exactly the same degree. But if we compare any formations, even two initially allied, we find that all species have undergone some changes. Once a species has disappeared from the face of the earth we have some reason to suspect that the same species will never appear again. The apparently most striking exceptions to this rule are Barrande ‘s so -called “colonies” of species, which intersperse for a time in the midst of older formations, and then later reappear; yet I keep L YELL ‘S explanation that they are conditioned by migrations from one geographical province to the other, for perfectly sufficient.
These various facts agree well with my theory. I don’t believe in any fixed law of development that caused all the inhabitants of an area to suddenly[p. 344]or to change simultaneously or equally. The modification process must be a very slow one. The variability of each species is quite independent of that of the other species. Whether natural selection makes use of such variability, and whether the variations accumulated to a greater or lesser extent cause greater or weaker modifications in the species that are changing, depends on many complicated conditions: on the usefulness of the change, on the effect of the Crossing, from the degree of breeding, from the gradual change in the natural condition of the area, and especially from the condition of the other organisms which compete with the changing species; hence it is by no means surprising if one species preserves its form unaltered while others change it, or if it changes such to a lesser degree than these. We observe the same in geographical distribution, e.g. B. on Madeira , where the land snails and beetles differ to a considerable extent from their closest relatives in Europediverged, while birds and sea-molluscs have remained the same. The apparently more rapid change in the terrestrial and higher-organized forms, as opposed to that of the marine and lower-lying species, may perhaps be deduced from the more complex relations of the more perfect beings to their organic and inorganic conditions of life, as explained in a former section have been, deduce. When many of the inhabitants of a region have been modified and perfected, it is understood from the principle of competition, and from the all-important relationships of organism to organism, that a form which undergoes no modification and perfection at all is subject to extinction. From this it follows that all species of a region last,
For members of a class, the rate of change may average over long and equal periods of time[p. 345]be almost the same. But as the accumulation of long-lasting fossil-bearing formations is conditioned by large masses of sediment being deposited during a period of subsidence, our formations must necessarily have been formed mostly with long and irregular intervals; hence the degree of organic change which the organic residues deposited in the strata of the earth bear is not the same in successive formations. From this point of view, each formation does not designate a new and complete act of creation, but only a scene, usually torn out entirely by chance, from a slowly progressing drama.
It is easy to see that a species, once extinct, cannot reappear even if the same inorganic and organic conditions of life reappear. For although the progeny of one species may be so prepared (and certainly has been so prepared in innumerable instances) as to fill and replace the exact place of another species in the economy of nature, yet both forms, the old and the new, not be identical, because both have certainly also inherited different characters from their different progenitors. So e.g. For example, if our fantails become extinct, pigeon fanciers, through long-continued efforts aimed at the same point, will probably bring about a new breed that is scarcely distinguishable from our present fantail.
Species groups, such as genera and families, follow in[p. 346]their appearance and disappearance follow the same general rules as the separate species themselves, changing more or less rapidly, to a greater or lesser extent. A group does not reappear once it has gone under; their existence is cut off. I am well aware that there are some apparent exceptions to this rule; but there are so surprisingly few of them that E DW. FORBES , P ICTET and W OODWARD(though they otherwise dispute all of the views I defend) concede their correctness, and this rule agrees perfectly with my theory. For if all the species of a group derive from only one parent-species, then it is clear that so long as any species of the group appears in the long sequence of geologic periods, so long as members of the same group still appear in must have existed in uninterrupted succession, in order to be able to produce gradually changed and new forms, or still the old and unaltered forms. So species of the Lingula family must have been present uninterruptedly from their appearance in the lowest strata up to the present day.
We saw in the last chapter that it sometimes appears as if the species of a group had suddenly appeared en masse, and I have attempted to explain this fact, which, if rightly behaved, would be fatal to my theory. But such cases are certainly only to be regarded as exceptions; according to the general rule, the number of species in each group gradually increases to its maximum and then, sooner or later, slowly decreases again. If one expresses the species-number of a genera, or the genera-number of a family, by a vertical line traversing the successive formations with a thickness varying according to the number of species contained in each, it may sometimes seem as if beginning broadly below instead of with a sharp point;[p. 347]This gradual increase in a group is perfectly consistent with my theory, as the species in a genus and the genera in a family can only increase in number slowly and gradually, because the process of transformation and development of a number of allied forms can only be a slow one, as a species at first furnishes only one or two varieties, which gradually change into species, which in their turn give rise to other species with equal slowness, and so on (like a large tree gradually branching out) until the group grows large.
lapse. ) Up to now we have only occasionally spoken of the disappearance of species and groups of species. According to the theory of natural selection, however, the extinction of old forms and the formation of new, improved forms are intimately connected. The old opinion, that from time to time all the inhabitants of the earth were swept from the surface by great upheavals, is now quite common, and even of such geologists as ELIE DE BEAUMONT , MURCHISON , B ARRANDE etc. given up, whose more general way of looking at things would have to direct them towards the same thing. On the contrary, we have every reason to believe, from the studies made upon the Tertiary formations, that species and groups of species are very gradually disappearing, one after another, first in one place, then in another, and finally everywhere. In some cases, however, as in the breach of an isthmus and the consequent immigration of a multitude of new inhabitants, or in the submersion of an island, the extinction may have been comparatively rapid. Individual species, as well as groups of species, have lasted very unequally long periods, some groups, as we have seen, from the earliest cradle of life to the present day, while others have not even reached the close of the Palaeolithic period. There does not appear to be any specific law governing the length of the duration of a species or genera. But there seems to be reason to believe that the total extinction of the species in a group is usually a slower one[p. 348]process than itself is its origin. If one expresses the appearance and disappearance of the species of a group, as in the previous case, by a vertical line of variable thickness, it tends to taper far more gradually at its upper end, corresponding to extinction, than at its lower end, representing development. But in some cases the extinction of whole groups of beings, like that of the Ammonites at the end of the Secondary period, has been marvelously rapid compared to most other groups.
The whole question of the extinction of species has been shrouded in the most mysterious obscurity. Some writers have even suggested that species, like individuals, have regular lifespans. No one has been more astonished by the disappearance of species than has been the case with me. When I found a horse tooth in the same layer with remains of Mastodon, Megatherium, Toxodon and others in La Plata State. I was filled with astonishment when I found an enormous group of species, all of which lived together in later geological times with conchilia species that are still alive today. Because since those of the Spaniards in South Americaintroduced horses have spread wildly over the whole country and multiplied to immeasurable numbers, I had to ask myself at that discovery what, in relatively recent times, was able to wipe out the earlier horse under living conditions which lend themselves to the multiplication of the Spanish horse have proved so extraordinarily favourable? But how completely unfounded was my astonishment! Professor O WENsoon realized that the tooth, though very similar to those of living species, came from a very different now extinct species. Were this species still extant, though somewhat rare, no naturalist would be in the least surprised at its rarity, as there are many rare species of all classes in all districts. If we ask ourselves why this or that species is rare, we answer that there must be something unfavorable in the existing living conditions, although we know that something[p. 349]not easy to pin down. If the fossil horse still existed as a rare species, considering the analogy with all other mammalian species, and even with the slow-breeding elephant, and the reproductive history of the in South Americaferal domestic horse feel that that fossil species, under more favorable conditions, must be able to populate the whole continent within a few years. But we cannot say what unfavorable conditions are preventing its increase, whether there are only one or several, and in what period of life and in what degree each of them acts unfavorably. But were those conditions gradually to worsen, we would certainly not notice the fact, though that horse (fossil) species would certainly become rarer and rarer, and finally die out; for her place has already been taken by another victorious competitor.
There is much difficulty always in remembering that the increase of every living thing is continually checked by imperceptible harmful agents, and that the same imperceptible agents may quite suffice to bring about a constant diminution and final extinction. But this sentence remains so incomprehensible that I have repeatedly heard people express their astonishment that such large animals as the mastodon and the older dinosaurs could have perished, as if the large body mass was already enough to ensure victory in the struggle for existence . On the contrary, a considerable size might, in some cases, hasten the extinction, on account of the earlier insufficient need for food. Even before man East Indies and Africainhabited, some cause must have checked the continued multiplication of the elephant species living there. A very able judge believes that it is at present chiefly insects (such as Bruce has also described in Abyssinia) which, by constant distress and fatigue, chiefly check the more rapid reproduction of elephants. It is certain that both insects of various kinds and[p. 350]Blood-sucking bats condition the spread of in various parts of South Americaimported domestic mammals. We see many instances in the recent tertiary formations of rarity preceding total extinction, and we know that it has been the same case with those animal species which, by the influence of man, have locally or everywhere disappeared from the earth. I want to repeat here what I had printed in 1845: Admit that species usually become rare before they become extinct, and don’t be surprised when a species becomes rare, but then be amazed when it finally perishes – that is Same as admitting that in individuals sickness precedes death, and not feeling alienated at the sickness of an individual, but wondering when the sick man dies, and attributing his death to some unknown violence.
The theory of natural selection rests on the supposition that every new variety, and finally every new species, has been created and maintained by possessing some inherent advantage over the competing species, as a result of which the unfavored species mostly inevitably die out. It is the same with our cultural products. When a new somewhat perfected variety has been formed, it initially replaces the less perfected varieties in the vicinity; when it is more improved, it spreads near and far, as did our short-horned oxen, and takes the place of the other breeds in other parts. Thus the appearance of new and the disappearance of old forms, both natural and artificial, are closely linked. In some well-thriving groups the number of new species-forms formed at a given time is greater than the old extinct ones; but as we know that the number of species has not increased indefinitely, at least in the last geological periods, we may assume that the production of new forms caused the extinction of an approximately equal number of old ones.
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The competition, as has been previously explained and exemplified, will usually be most serious between those forms which are most alike in all respects. Hence the modified and improved offspring will usually cause the extinction of their parent-kind; and when many new forms of any single species have arisen, the nearest allies of that species, that is, those belonging to the same genus, will be most liable to extinction. And so, I imagine, a number of new species descended from a ancestral species, i. H. replace a clan, an old clan of the same family. But it must also happen often that a new species from this or that group takes the place of a species from another group, and thus causes its extinction; if then many related forms develop from the victorious intruder, many of these will have to yield to their places, and they will usually be related species, which owing to a common inherited disadvantage succumb to the others. However, whether the underlying species belong to one or to different classes, yet often one or the other of them, owing to an ability to live a somewhat different life, or because of its remote habitation, may have less severe competition to negotiate, and so still received for a long time. For example, survives B. only a single Trigonia in the if then many related forms develop from the victorious intruder, many of these will have to yield to their places, and they will usually be related species, which owing to a common inherited disadvantage succumb to the others. However, whether the underlying species belong to one or to different classes, yet often one or the other of them, owing to an ability to live a somewhat different life, or because of its remote habitation, may have less severe competition to negotiate, and so still received for a long time. For example, survives B. only a single Trigonia in the if then many related forms develop from the victorious intruder, many of these will have to yield to their places, and they will usually be related species, which owing to a common inherited disadvantage succumb to the others. However, whether the underlying species belong to one or to different classes, yet often one or the other of them, owing to an ability to live a somewhat different life, or because of its remote habitation, may have less severe competition to negotiate, and so still received for a long time. For example, survives B. only a single Trigonia in the who, as a result of a jointly inherited disadvantage, are subject to the others. However, whether the underlying species belong to one or to different classes, yet often one or the other of them, owing to an ability to live a somewhat different life, or because of its remote habitation, may have less severe competition to negotiate, and so still received for a long time. For example, survives B. only a single Trigonia in the who, as a result of a jointly inherited disadvantage, are subject to the others. However, whether the underlying species belong to one or to different classes, yet often one or the other of them, owing to an ability to live a somewhat different life, or because of its remote habitation, may have less severe competition to negotiate, and so still received for a long time. For example, survives B. only a single Trigonia in the have to drive through less severe competition and thus survive for a longer time. For example, survives B. only a single Trigonia in the have to drive through less severe competition and thus survive for a longer time. For example, survives B. only a single Trigonia in theAustralian seas the species of this genus which were numerous in the Secondary period, and a small number of species of the once rich group of ganoid-fishes occur in our fresh waters. And so the total extinction of a group is usually a slower process than its development.
As to the apparently sudden extinctions of entire families and orders, such as that of the trilobites at the end of the Paleolithic and the ammonites at the end of the Mesolithic time period, we must first recall what has already been said above about the very long intervals between ours[p. 352]various formations, during which many forms may have been slowly extinct. Furthermore, when many species of a new group take possession of a new area by sudden immigration or unusually rapid development, they may displace many of the old inhabitants in a correspondingly rapid manner; and the forms which give them their places will usually be allied participants in some common disadvantage of organization.
So the way in which individual species and entire groups of species become extinct seems to me to agree well with the theory of natural selection. Extinction cannot surprise us; what should rather surprise us is our momentarily nurtured arrogance to comprehend the many intricate conditions upon which the existence of each species depends. If we forget for a moment that every species tends to increase in an unregulated manner, and some counteraction, though seldom seen, is always at work, the whole household of nature must certainly appear very obscure to us. Only if we could state precisely why this species is more abundant in individuals than that, why this and not another can be naturalized in a region indicated,
About the almost simultaneous change of life forms on the whole surface of the earth. ) Hardly any other paleontological discovery is so startling as the fact that the forms of life undergo a simultaneous change over almost the whole surface of the earth. Thus our European chalk formation can be recognized in many distant parts of the world and in the most varied climates, where not a piece of chalk itself can be discovered. So especially in North and in tropical South America , at the Cape of Good Hope and on the East Indian Peninsula,[p. 353]because at these distant points of the earth’s surface the organic remains of certain strata bear an unmistakable resemblance to those of our chalk. Not that it’s the same species everywhere; for some of these localities have not a species in common; – but they belong to the same family, clan, sub-clan and often resemble each other down to the indifferent sculptures on the surface. Further, other forms, which do not occur in Europe , but above or below the Cretaceous formation, are absent from that formation even in those distant regions. In the successive Paleozoic formations of Russia , Western Europe and North Americaa similar parallelism in the occurrence of life forms has been noted by several authors; and likewise in the European and North American Tertiary Mountains after L YELL . Even if we forget entirely the few species which the Old and New Worlds have in common, the general parallelism of the successive forms of life in the various stocks of the widely separated Paleolithic and Tertiary formations stands so firmly that these formations can easily be compared link by link.
These observations, however, relate only to the sea-dwellers of the different parts of the world, and we have not sufficient evidence to judge whether the products of land and fresh water at points so remote also alternate in a parallel manner. One would like to doubt whether it is the case; for when the Megatherium, the Mylodon and Toxodon and the Macrauchenia from the La Plata areas to Europehad been brought without any evidence of their geological deposits, no one would have suspected that they co-existed with sea mollusks which are still alive; but as these monstrous beings are associated with the mastodon and horse, it may at least be inferred that they must have lived in one of the last stages of the Tertiary Period.
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When we spoke earlier of the simultaneous change of sea-dwellers over the whole surface of the earth, we are not dealing here with the same thousand or hundred thousand years, or even with a strict simultaneity in the geological sense of the word. For if all the marine animals now living in Europe , and all who lived there in the Pleistocene period (an immensely remote period, in terms of years, in which it includes the Ice Age), with compared to those now living in South America or Australia , the most experienced naturalist would hardly be able to say whether the present or the Pleistocene inhabitants of Europemore closely match those of the southern hemisphere. Likewise, many of the most competent observers believe that the present life-world in the United States is more closely related to the population which existed in Europe during some of the latter stages of the Tertiary Period than to that which now dwells; and if this be so, the fossil-bearing strata which are now being deposited on the North American coasts would evidently at a later period be more closely associated with somewhat older European strata. Notwithstanding this, I believe there can be little doubt that in the very distant future all recent marineFormations, especially the Upper Pliocene, the Pleistocene, and the present strata of Europe , North and South America , and Australia , because they are the remains of organisms in some degree allied, and not also those species which alone belong to the deeper older deposits belonging to, enclosing in itself, would correctly describe as being of the same age in the geological sense.
The fact that the forms of life change simultaneously, in the above broad sense of the word, even in distant parts of the world, has greatly struck the excellent observers DE VERNEUIL and D’ A RCHIAC . After reporting on the parallelism of Paleolithic life forms in different parts of Europe , they go on to say:[p. 355]”Turning our attention now to North America , we discover there a number of analogous facts, and it seems certain that all these variations in species, their extinction, and the appearance of new ones, are not mere changes in the ocean currents or others may be ascribed more and less to local and transitory causes, but depend on general laws affecting the whole animal kingdom.” Also B ARRANDEhas made similar observations and strongly emphasized them. Indeed, it is quite useless to seek the cause of these great changes in the forms of life all over the surface of the earth, and in the most varied climates, in the changes of sea-currents, of climate, or of other natural conditions of life; we must, as Barrande remarks, look for a special law for this. We shall see this more clearly when we speak of the present distribution of organic beings; we shall then find how slight is the relation between the natural conditions of life of different countries and the nature of their inhabitants.
This great fact of the parallel succession of life-forms throughout the world is explicable from the theory of natural selection. New species arise from new varieties which retain some advantage of older forms, and those forms which already predominate in number, or have some advantage over other forms, will of course most often give rise to new varieties or incipient species; for these last will be victorious against others to a still higher degree and they will survive. We find definite evidence of this in the fact that the ruling, i. H. the most common and widespread plant species in their homeland have produced the greatest number of new varieties, as compared with other species in their own homeland. Likewise, it is natural[p. 356]species have invaded, also have better prospects than others of still further expansion and of the formation of further varieties and species in the new regions. This process of dispersal may often be a very slow one, depending on climatic and geographical changes, chance events, or on the gradual acclimatization of new species to the different climates through which they pass; but in time the spread of the dominant forms will usually take hold. It will probably be slower in land-dwellers on separate continents than in organisms of contiguous seas. We may therefore expect to find a less exact degree of parallel succession in the produce of land than in that of the sea, as indeed is the case.
If dominant species spread out from an area, they will sometimes encounter even more dominant species, and then their triumphal streak and even their very existence will cease. We do not know at all exactly what are all the most favorable conditions for the propagation of new and dominant species; but we can, I think, see clearly that a large number of individuals, inasmuch as they have a better chance of producing advantageous variations, and that strict competition by means of many forms already existing, must be in the highest degree advantageous, as well as the ability itself spread to new areas. A certain degree of isolation, sometimes recurring after long intervals, might, as before explained, also be beneficial. A portion of the surface of the earth may be more favorable to the production of new and dominant species on land, and another to those of the sea. If two great regions have for very long periods been equally fitted for the production of dominant species, the struggle of their inhabitants against each other, whenever they may meet, will become a long and hard one, and some will be of one birth and some of the other -Site made victorious advance. But in and some will advance victorious from one birthplace and some from another. But in and some will advance victorious from one birthplace and some from another. But in[p. 357]In the course of time the most dominant forms, on whichever side they may have originated, will everywhere gain the upper hand. In proportion as they predominate, they will cause the extinction of other more imperfect forms; and since whole related groups have often inherited the same imperfection in common, such groups will gradually tend to die out altogether, though here and there a single member may survive for a while.
Thus, it seems to me, the parallel and, in a broad sense, simultaneous succession of the same forms of life throughout the world agrees well with the principle that new species are formed by widespread and very variable dominant species; the new species thus produced, owing to heredity, and from already possessing some advantage over their elders and over other species, become themselves dominant; these, too, now spread, vary, and again form new species. Those forms which are superseded, and give their places to the new victorious forms, will usually be group-related, having inherited some imperfection in common; therefore, as the new and more perfect groups spread over the earth, old groups must disappear before them. This succession of forms on both paths will everywhere tend to correspond.
One remark remains to be made on this subject. I have given the reasons why I believe that each of our great fossil-rich formations has been deposited at periods of continued subsidence, but that these deposits have been separated by long intervals where the sea-floor has been steady or rising, or where the deposits were not made quickly enough to enclose the organic remains and protect them from destruction. Now during these long empty intervals, I suppose, the inhabitants of every region have undergone many alterations, and suffered much from extinction, and[p. 358]Great migrations have taken place from one part of the world to another. Now, since there is reason to believe that wide fields have undergone the same motions, surely exactly simultaneous formations have often been deposited over very wide spaces of a part of the world; but we are not entitled to conclude from this that this was invariably the case, or that large areas were invariably affected by the same movements. If two formations in two regions arose at almost, but not exactly, the same time, we shall generally recognize in both, for reasons already explained above, the same succession of life-forms; but the species will not correspond exactly,
I suspect that cases of this kind occur in Europe itself. P RESTWICH , in his excellent treatise on the Eocene strata of England and France , is able to show a generally exact parallelism between the successive stocks of both regions. Although now in comparing certain hives in England with those in Francethere is a remarkable correspondence between the two species belonging to the same taxa, but these species themselves differ in a way that is difficult to explain given the small distance between the two areas, if one does not want to assume that an isthmus has separated two neighboring seas, which were inhabited by different fauna at the same time. L YELL has made similar observations on some of the later Tertiary formations, and likewise B ARRANDE has shown that between the successive Silurian strata of Bohemia’s and Scandinavia’sin general there is an exact parallelism, but notwithstanding this there is an astonishing difference between the species. Now, had the various formations of these regions not been deposited during exactly the same period, the deposition in one region being paused in the other[p. 359]coincided,—and had the species in both districts undergone slow changes both during the accumulation of strata and during the long intervals between them: the various formations of both districts would appear ordered in a like manner and in accordance with the general succession of life-forms, and their order even seem exactly parallel (without being so); notwithstanding this, not all species would agree in the individual apparently corresponding hives of both regions.
Relationship of extinct species among themselves and with living forms. ) Let us now take a look at the mutual relationships of extinct and living forms. All fall into one great natural system, which is explained by the principle of common descent. As a general rule, the older a form, the more it diverges from living forms. Yes, like B UCKLANDlong ago noticed that all fossil forms are divided into groups that are still living, or inserted between them. There is no denying that the extinct forms help fill in wide gaps between the clans, families, and orders that still exist. For if we direct our attention either to the living or to the extinct forms alone, the series is much less perfect than if we gather both into a common system. With regard to the vertebrates, many pages could be filled with the excellent explanations of our great paleontologist O WEN on the connection of living groups of animals by fossil forms. After C UVIERConsidering the ruminants and the pachyderms as two of the most distinct orders of mammals, O WEN discovered so many fossil intermediates that he was obliged to change the whole classification of these two orders, and placed certain pachyderms in equal suborder with ruminants. So e.g. B. he fills the wide gap between camel and pig with small intermediate steps. As for the invertebrates, B ARRANDE affirms , certainly the first[p. 360]authority in this respect, as he saw more clearly every day that the Paleolithic animals, though belonging in the same order, family, and clan with those now living, were not yet divided into groups as definite as these latter.
Some writers have objected to the notion that an extinct species or group of species is intermediate between living species or groups. If it were meant to say that the extinct form in all its characters is exactly the middle between two living forms, the objection would be justified. But I recognize that in a perfectly natural classification many fossil species have their place between living species, and some extinct genera between living genera, or even between genera of different families. The usual case, especially with very distinguished groups such as fish and reptiles, seems to me to be that where the same day z. B. differ from each other by a dozen characters,
It is a common opinion that the older a form is, the more apt it is by means of some of its characters to link now widely separated groups. This remark must no doubt be confined to those groups which have undergone great changes in the course of geological times, and it may be difficult to prove the truth; for here and there a living animal is also discovered, like the Lepidosiren, which is related to very different groups at the same time. But if we compare the older Reptiles and Batrachians, the old Fishes, the old Cephalopods, and the Eocene Mammalia, with the newer members of the same classes, we must admit some truth in the remark.
We shall now see to what extent these various[p. 361]Facts and conclusions consistent with the theory of variant offspring. As the subject is somewhat complicated, I must ask the reader to look again at the picture on page 131 . Suppose the numbered letters represent genera, and the rows of dots radiating from them the species belonging to them. The picture is too simple in so far as too few genera and species are assumed; but that is immaterial to us. The horizontal lines may represent the successive geological formations, and all forms below the topmost of these lines may be considered extinct. The three living clans a 14 , q 14 , p 14 may form a small family; b 14and f 14 represent a closely related or sub-family, and o 14 , e 14 , m 14 a third family. These three families, with the many extinct kinships on the various branching lines emanating from stem-form A, form an order; for all will have inherited something common from their ancient and common progenitor. According to the principle of continuing divergence of character, which is explained in that picturewas determined, the newer each form must deviate more from its first progenitor. This also explains the rule that the oldest fossils differ most from the forms that are now living. Yet we must not think that divergence of character is a necessary quality; it depends solely on the progeny of a species being capable of occupying many and varied places in nature’s economy. Hence it is also quite possible, as we have seen with some Silurian fossils, that a species, with only slight modification corresponding to but slightly altered conditions of life, may persist, and always retain the same general characters for long periods. This is represented in the picture by the letter F 14expressed.
All the many forms descended from A, extinct as well as still living, form together, according to our assumption, an order, and this order is gradual owing to continuous extinction of forms and divergence of characters[p. 362]been divided into families and subfamilies, some of which perished at earlier periods, and others of which endure to the present day.
The picture further shows us that if a number of the forms previously extinct and included in the successive formations were rediscovered at various points deep down the line, the three surviving families on the top line appear more interlinked would have to. if e.g. B. the clans a 1 , a 5 , a 10 , f 8 , m 3 , m 6 , m 9if unearthed again, the three families would appear so closely linked that they would probably be united into one large family, much as was done with the ruminants and certain pachyderms. Anyone who would object to the description of those clans connecting the three living families as “intermediate in character” would in fact be right in so far as they did not meet directly, but only in a detour created by many very different forms insert between those others. If many extinct forms were above one of the middle horizontal lines or formations, such as B. No. VI—but none have been found below this line, one would only find the two families standing on the left—viz., a 14etc. and b 14 etc. — unite into one big family, and the other two a 14 –f 14 with five and o 14 –m 14with three clans would then remain separate from it. Yet these two families would appear less distinct from each other than before the discovery of the fossil remains. If we e.g. Suppose, for example, that the extant clans of the two families differed in a dozen characters, they must have shown fewer differences in the earlier period marked VI, because at that stage of development they descended from the common progenitor of the order in character not diverged as much as later. Thus it happens then that ancient and extinct clans often hold some mean between their modified offspring, or between their collateral relatives.
[p. 363]
In nature the case will be far more complex than our picturerepresents; for the groups are much more numerous, their duration is exceedingly unequal in length, and the variations have reached various gradations. Now that we have the last volume of the geological record, with many connections broken, we have no right, excepting a few very rare cases, to expect the filling of great gaps in the system of nature, and the connection of separate families and orders. All we can hope for is to find those groups which do not undergo great changes until known geologic time somewhat nearer in the earliest formations, so that the older members differ somewhat less widely in some of their characters than do the present members of the same groups;
Thus, with progressive modification, the most important facts regarding the mutual relationship of the extinct forms of life to one another and to the forms that still exist seem to me to be explained in a sufficient manner according to the theory of common descent. From any other point of view they are completely inexplicable.
From the same theory it is evident that the fauna of a great period in the history of the earth must, in its general character, be intermediate between what at first preceded and what followed. Such are the species which are in the sixth great stratum of our picture occur, the modified descendants of those which were already present in the fifth, and are the elders of those still further modified in the seventh; they cannot, therefore, do otherwise than hold nearly the mean between the two. We must, however, bear in mind the total extinction of some earlier forms, the immigration of new forms from other districts, and the considerable alteration of forms during the long interval between two successive formations. Taking these concessions into account, the fauna must be of any great size[p. 364]geological period undoubtedly occupy exactly the middle point between the preceding and the following one. I need only cite as an example how the fossil remains of the Devonian system prompted paleontologists to construct it, recognizing their intermediate character between those of the underlying Silurian and succeeding Carboniferous systems. But not every fauna has to follow this mean exactly, because the periods of time which have elapsed between successive formations can be of unequal length.
There is no substantial objection to the truth of the assertion that the fauna of each period taken as a whole must be about the mean between the preceding and succeeding faunas, from the fact that some genera are exceptions to this rule. So e.g. B., if one mastodons and elephants according to Dr. F ALCONERfirst according to their mutual relationship and then according to their geological succession in two series, both series not coinciding with each other. The species most differing in their characters are neither the oldest nor the youngest, nor are those of middle character also middle-aged. But supposing for a moment that our knowledge of the dates of appearance and disappearance of species in this and similar cases is perfectly accurate, we have no right to believe that the successive forms must necessarily exist of equal duration ; a very ancient form may happen to have a longer duration than any later developed form, especially among those country-dwellers who inhabit quite separate districts. Comparing the small with the big, let’s take the pigeons as an example. If one were to arrange the living and extinct main breeds of our domestic pigeons as best as possible according to their relationships, this arrangement would not agree exactly either with the chronological order of their origin and still less with that of their demise. Because the progenitor rock pigeon is still alive, and there are many intermediate varieties between it and the messenger pigeon[p. 365]extinct, and messenger-pigeons, which are the ultimate in beak-length, arose earlier than the short-beaked tumblers, which form the opposite end of the beak-length-based sequence.
Closely related to the assertion that the organic remains of an intermediate formation are also of an almost intermediate character is the fact upon which all paleontologists insist, that the fossils from two successive formations are much more closely related than those from distant ones are. Pictet cites as a well-known example the general similarity of the organic remains from the different stocks of the Cretaceous Formation, although the species in all stocks are different. This fact alone, because of its generality, appears to Professor Pictet to have shaken his firm belief in the immutability of species. Well acquainted with the mode of distribution of now living species over the earth’s surface, he dares not deduce an explanation of the close resemblance of various species in closely-consecutive formations on the assumption that the physical conditions of the ancient land-areas were have remained almost the same. Let’s remember that the forms of life, at least in the sea, exist all over the world and therefore under the most diverse climates, e.g. conditions changed almost simultaneously; — and consider what an insignificant influence the most marvelous climatic changes during the Pleistocene period encompassing the entire Ice Age have exerted on the specific forms of sea-dwellers!
On the theory of common descent, the full significance of the fact that fossil remains from contiguous formations, though distinct as species, are closely related is clear. As the deposition of each formation has often been interrupted, and long pauses have taken place between the deposition of different formations, we ought not, as I have tried to show in the last chapter, to expect in any one or two formations all[p. 366]to find intermediate varieties between the species which lived at the beginning and at the end of these formations; but after more or less great intervals (very long in terms of years, but moderately long in geological terms) we should find closely related forms, or, as some writers have called them, “representative species,” and these we do find. In short we discover such evidence of a slow and almost imperceptible change of specific forms as we have a right to expect.
About the stage of development of older forms compared to those that are still alive.) We have seen in the fourth chapter that the degree of differentiation and specialization of the parts and organic beings in their mature age affords the best measure hitherto attempted of measuring the degree of perfection or height thereof. We have also seen that inasmuch as specialization of the parts and organs is an advantage to every being, Natural Selection will constantly tend to make the organization of every being more and more specialized, and thus, taken in this sense, more perfect; which does not exclude, however, that many creatures destined for simpler living conditions still keep their organization simple and unimproved.HILIPPS has emphasized, have undergone almost no change up to the present day; yet these latter have faced more vigorous competition than the denizens of the wide sea. Also in another and more general sense it follows that, according to the theory of natural selection, the newer forms strive higher than their ancestors; for every new species has developed gradually, because in the struggle for existence it has always had an advantage over other and older forms. If in a near-similar climate[p. 367]the Eocene inhabitants of one part of the world were called to compete with the present inhabitants of the same or another part of the world, the Eocene fauna or flora would certainly have to succumb and be exterminated, as a secondary fauna would be conquered by the Eocene, and a Paleolitic by the secondary. – According to the theory of natural selection, the new forms would have to prove their higher position over the old ones not only by their victory in the struggle for existence, but also by a more advanced specialization of the organs. But is this really the case? A large majority of geologists would undoubtedly agree with Diess. But my imperfect judgment, having read the discourses of L YELL on this subject, and H OOKER ‘S opinion in relation to the plants, to agree only to a limited degree. Regardless of this, the decisive proof should only be able to be supplied by later geological research. B RONN has treated this subject more fully and adequately than any other author [36] .
The task is extraordinarily involved in many respects. The geological record of creation, already incomplete at all times, does not, in my opinion, go far enough back to show with unmistakable clarity that in the known history of the earth the organization has made great strides. Even to-day natural scientists are often not unanimous as to which animals of a class are the higher. Thus some regard the sharks as the highest fishes, on account of some important organizational relations with that of the reptiles, while others regard the bony fishes as such. The ganoids are in the middle between the sharks and bony fish. Today these latter are far predominant in number, whereas formerly there were only sharks and ganoids, and in this case the fish will be said[p. 368]have progressed or retreated in their organization, depending on how they are measured with a different standard [37] . But it is a hopeless attempt to measure the height of limbs of entirely different types against one another. Who can say whether an octopus (sepia) stands higher than the bee: than that insect of which the great naturalist V . B AERsays that it is indeed organized higher than a fish, though of a different type. In the complicated struggle for existence, it is quite credible that such crustaceans, e.g. B., which are not very high in their own class, which would overcome cephalopods or most imperfect molluscs; and these crustaceans, though not highly developed, must be very high on the invertebrate ladder, judging by that most decisive of criteria, the law of competition for existence.
Apart from the difficulty inherent in deciding which forms of organization are the highest, we do not have only the highest members of a class in two different periods (although this is certainly one of the most important, or perhaps the most important, element in of deliberation), but we have all the members, high and low, to compare with one another. In ancient times it swarmed with the most perfect as well as the most imperfect molluscs, viz., cephalopods and brachiopods; while nowadays these two orders have greatly diminished, and the intermediate classes between them have greatly increased[p. 369]are. Accordingly, some naturalists have concluded that the mollusks were formerly more highly developed than they are now; while others appealed to the present considerable diminution of the imperfect mollusks, the more so as the extant cephalopods, though fewer in number, were more highly organized than their ancient substitutes. We must therefore compare the proportional numbers of the upper and lower classes of the earth’s population in two different periods. If e.g. For example, if there were now 50,000 species of Vertebrates, and we could only estimate their number at some earlier period at 10,000, we should have to use this increase in the uppermost classes, which at the same time caused a great displacement of lower forms from their place,[38] From this one can see how little hope there seems to be of ever, under such extremely complicated relationships, of judging in a perfectly correct way the relative degree of organization of imperfectly known faunas of successive periods in the earth’s history.
From another important point of view we shall appreciate this difficulty all the better when we consider certain extant faunas and floras. To judge from the extraordinary manner in which products lately imported from Europe have spread through New Zealand , and taken up places which had previously been occupied, if all the plants and animals of Great Britain were freely introduced there, a multitude of British forms would appear in time completely naturalize there, and exterminate many of the natives. Against this, what we now see happening in New Zealand , and the fact that[p. 370]scarcely any inhabitant of the southern hemisphere has gone feral in any part of Europe , causing us to doubt whether, if all the produce of New Zealand were released freely into Britain , a somewhat greater number of them would be capable of separating from native plants and animals to take vacant positions. From this point of view it may be said that the products of Great Britain are superior to those of New Zealand . And yet the ablest naturalist, after the most careful study of the species of either country, could not have foreseen this result.
A Gassiz emphasizes that the ancient animals resemble in certain respects the embryos of new animals of the same class, or that the geological succession of extinct forms parallels, as it were, the embryonic development of new forms. However, I must agree with PICTET ‘S and H UXLEY ‘S that this doctrine is remotely unproven . But I fully expect to see them later confirmed, at least with regard to those subordinate groups which have only recently branched off from one another. Because this lesson from A GASSIZagrees well with the theory of natural selection. In a later chapter I shall try to show that the ancients differ from their embryos in consequence of variations which do not take place in earliest youth and are also inherited to a correspondingly later age. While this process leaves the embryo almost unchanged, it accumulates more and more diversity in the old over the course of successive generations.
Thus the embryo appears like a portrait preserved by nature of the earlier and not yet much modified state of each animal. This view may be true, but it is never capable of perfect proof. Because if we also found that e.g. For example, if the oldest known forms of mammals, reptiles, and fish correspond exactly to these classes, but stand somewhat closer to each other than the present typical representatives of these classes, we would[p. 371]but look in vain for animals which still bear the common embryo-character of the vertebrates, until we have discovered fossil-bearing strata deep below the Silurian ones, of which there is very little prospect indeed.
Succession of the same types within the same areas during the later Tertiary periods. ) C LIFT showed many years ago that the fossil mammals from the bone caves of New Holland were very closely related to the marsupials still living there. In South America a similar relation has been shown, even to the untrained eye, in the armadilla-like pieces of armor of gigantic size, which have been found in various parts of la Plata ; and Professor O WEN has proved most conclusively that most of the animals so numerous in fossils found there are South Americanbelong to types. This relationship is seen more clearly in the wonderful collections of fossil bones brought back from the Brazilian caves by L AND and C LAUSEN . These facts made such an impression on me that in the years 1839 and 1845 I emphasized very emphatically this “law of the succession of like types,” this “wonderful relation between the dead and the living in the same continent.” Professor O WEN later extended the same generalization to the mammals of the old world . We find the same law in the giant birds of New Zealand , which he restored . We also see it in the birds of theBrazilian caves. W OODWARD has shown that the same law applies to the sea conchylia, although he has not developed it well because of the wide distribution of most genera of molluscs. Other instances could be given, such as the relations between the extinct and living land snails of Madeira , and between the ancient and present brackish-water conchylia of the Aral-Caspian Sea.
But what does this strange law of the succession of the same types in the same country-areas mean? Compares[p. 372]If one compares the present climate of New Holland and the parts of South America lying at the same latitude , it would seem a foolish undertaking, on the one hand from the dissimilarity of the natural conditions the dissimilarity of the inhabitants of these two continents, and on the other hand from the similarity of the conditions that To explain the consistency of the types in each of them during the later tertiary periods. Nor can it be asserted that, according to an immutable law, marsupials are found chiefly or solely in New Holland , or Edentaten, etc. of the current American types can only be produced in America . Because it is known thatEurope was in ancient times populated by numerous marsupials, and I have shown in the writings cited above that in America the law of distribution for the land mammals was formerly different from what it is now. North America formerly shared much in the present character of the southern half of the continent, and the southern half was formerly more akin to the northern half than it is now. By Falconer and Cautley ‘s discoveries we know that North India , as regards her mammals, was formerly more closely related than is now to Africahour Analogous facts could also be given of the distribution of sea-animals.
According to the theory of common descent with progressive modification, the great law of long-lasting but not unchangeable succession of the same types in one and the same field is immediately explained. For the inhabitants of every part of the world will evidently tend to leave closely related but somewhat modified descendants in that part during the next period of time. If the inhabitants of one continent were formerly very different from those of another continent, their modified descendants will still differ in almost the same way and degree. But after very long periods of time and geographic changes permitting very large migrations, the weaker forms become the dominant ones[p. 373]give way and so nothing is immutable in propagation laws of past and present time.
I may be mocked if I believe that the Megatherium and the other monsters allied to it left in South America the sloth, the armadilla, and the ant-eater as modified offspring. One cannot admit this for a moment. Those large animals have completely died out without leaving any offspring. But in the caves of Brazil there are many extinct species, in size a.o. Characters closely related to those still in South Americaliving species, and some of the fossil ones may actually be the producers of species still living there. It must not be forgotten that according to my theory all species of a genea are descended from one and the same species, so that if out of six genera each contains eight species in the same geological formation, and in the next-following formation there are again six other related or representative genera with it same number of species occur, we may then conclude that only one species from each of the six older genera left modified descendants which formed the six newer genera. The other seven species of the ancient genera are all extinct without leaving any heirs. But it would probably happen far more often that two or three species from only two or three of the old genera have been the ancestors of the six new genera, and the other old species and all the remaining old genera have entirely died out. In declining orders with decreasing numbers of clans and species, as evidently the EdentatenSouth America’s will leave fewer genera and species modified offspring in a straight line.
Compilation of the previous and current chapter. ) I have tried to show that the geological record of creation is extremely imperfect; that only a small portion of the surface of the earth has yet been carefully examined; that only certain classes of organic beings survive in abundance in the fossil state; that the number of in ours[p. 374]Individuals and species preserved in museums mean nothing compared with the incalculable number of generations which must have succeeded one another during only one formation time; that, as a rule, immense periods of time must have elapsed between any two successive formations, because fossil-rich formations powerful enough to resist future destruction can usually be deposited only during periods of subsidence; that consequently there was probably more extinction during periods of subsidence, and more alteration of organic forms during periods of uplift; that the creation record is the most imperfectly preserved from these latter periods; that each individual formation has not been deposited in uninterrupted continuity; that the duration of each formation is perhaps short compared with the average duration of species-forms; that migrations have played a large part in the first appearance of new forms in the formation of a region; that the most widespread species have also varied the most, and have most often given rise to new species; and that varieties have often been local at first. All these causes taken together must make the geological record extremely incomplete, and can explain in large part why we find single intermediate forms between the members of a group of organisms, but do not see endless series of varieties linking the extinct and living forms together in the finest gradations. that migrations have played a large part in the first appearance of new forms in the formation of a region; that the most widespread species have also varied the most, and have most often given rise to new species; and that varieties have often been local at first. All these causes taken together must make the geological record extremely incomplete, and can explain in large part why we find single intermediate forms between the members of a group of organisms, but do not see endless series of varieties linking the extinct and living forms together in the finest gradations. that migrations have played a large part in the first appearance of new forms in the formation of a region; that the most widespread species have also varied the most, and have most often given rise to new species; and that varieties have often been local at first. All these causes taken together must make the geological record extremely incomplete, and can explain in large part why we find single intermediate forms between the members of a group of organisms, but do not see endless series of varieties linking the extinct and living forms together in the finest gradations. that the most widespread species have also varied the most, and have most often given rise to new species; and that varieties have often been local at first. All these causes taken together must make the geological record extremely incomplete, and can explain in large part why we find single intermediate forms between the members of a group of organisms, but do not see endless series of varieties linking the extinct and living forms together in the finest gradations. that the most widespread species have also varied the most, and have most often given rise to new species; and that varieties have often been local at first. All these causes taken together must make the geological record extremely incomplete, and can explain in large part why we find single intermediate forms between the members of a group of organisms, but do not see endless series of varieties linking the extinct and living forms together in the finest gradations.
Whoever wants to reject these views of the nature of the geological record must also reject my whole theory. For in vain will he then ask where the innumerable transitional links have remained, which must once have linked together the most closely related or representative species, and which one finds one above the other in the various stocks of a large formation. He will not believe in the immeasurable intervals which have elapsed between our successive formations; he will overlook what an essential part the migrations have played since the first[p. 375] Appearance of organisms in the formations of a large part of the world like Europehave considered for themselves; he will refer to the apparently, but often only apparently, sudden appearance of whole groups of species. If he were to ask where the remains of those infinitely numerous organisms remained, which must have been deposited long before the formation of the oldest Silurian layers, I can only answer hypothetically that, as far as can be seen, our oceans are already changing have lain in their present places for untold ages, and that where our continents now stand they have surely stood since the Silurian period; but that the surface of the earth must have had a completely different appearance long before this period,
But these difficulties aside, it seems to me that all the other great and guiding facts in palaeontology follow simply from the theory of descent from common ancestors, with progressive modification by Natural Selection. It explains why new species are slow to emerge; why species of different classes do not necessarily alternate in the same proportion or degree, but are all subject to change only over long periods. The extinction of old forms is the most inevitable consequence of the emergence of new ones. It explains why a species, once gone, never appears again. Groups of species (clans, etc.) increase in number only slowly and last for unequally long periods; for the process of modification is necessarily a slow one, and dependent on many complicated accidents. The dominant species of the larger dominant groups strive to leave many modified offspring, and so new subgroups and groups are again formed. In proportion as these arise, the species of less vigorous groups, owing to their commonly inherited imperfection, incline to the common[p. 376]extinction without leaving any modified offspring anywhere on the surface of the earth. But the complete extinction of an entire group of species may often be a very slow process if individual species can survive for a while in sheltered or isolated locations. Once a group has perished it can never reappear because a link in the generational line has been broken.
So it is understandable that the spread of dominant forms of life, which vary most frequently, populate the earth with closely related but modified forms over the course of time, which then usually succeed in taking the places of those groups of species which they had in the struggle for life subject to existence. Therefore, after long intervals, it will appear as if the inhabitants of the earth’s surface have changed everywhere at the same time.
So it is also understandable why it is that the old and new forms of life form a large system with each other, since they are connected with each other through procreation. It is understandable, from the continuing tendency to divergence of character, why the older the fossil forms differ the more from those now living; why ancient and extinct forms are often fitted to fill gaps between living ones, and sometimes unite two groups which have previously been set up separately, although as a rule they are only somewhat nearer to each other. The older a form is, the more often it seems to develop characters which are more or less intermediate between groups now separate; because the older a form is, the more closely related and consequently more like it will be to the common progenitor of such groups which have since diverged widely. Extinct forms seldom hold exactly the middle ground between living ones, but only stand in their midst as a result of an extensive concatenation of many extinct and divergent forms. We see clearly why the organic remains of closely spaced formations resemble each other more closely than those far from[p. 377] have to be more distant from each other; for those forms are more closely related by blood than these are to one another. We are finally able to see why the organic remains of middle formations also keep the middle in their characters.
The earth-dwellers of each later period must have conquered the earlier ones in the struggle for existence and must in that respect be at a higher level of perfection than these, and their bodily structure has generally become more specialized since then, and it may seem vague and misinterpreted sentiment that many paleontologists believe in progress of the organization as a whole. Should it later turn out that old animal forms in a certain degree resemble the embryos of newer ones from the same class, this would also be understandable. The succession of the same types of organization in the same field during the last geological periods ceases to be mysterious and is a simple consequence of heredity.
If, therefore, the geological record of creation is as incomplete as I believe it to be (and it can at least be asserted that the contrary cannot be proved), the chief objections to the theory of natural selection will be greatly diminished, or disappear altogether. On the other hand, the main laws of paleontology seem to me to prove clearly that species arose through ordinary procreation. Former forms of life have been replaced by more perfect forms which have arisen and been preserved through Natural Selection, by the Laws of Variation still constantly at work around us.
