LECTURE XXXV
THE ORIGIN AND THE EXTINCTION OF SPECIES
Adaptation does not depend upon chance—The case of eyes—Of leaf-mimicry—All persistent change depends ultimately on selection—Mutual sterility without great significance—Relative isolation (Lepus variabilis)—Influence of hybridization—Decadence of species—Differences in the duration of decadence—Natural death of individuals—Extinction due to excessive variability (Emery)?—Machairodus as interpreted by Brandes—Lower types more capable of adaptation than higher—Flightless birds—Disturbance of insular fauna and flora by cultivation—The big game of Central Europe.
In the polar hare we have a case in which the adaptations to the life conditions both of time and space are recognizable as the effect of definite causes, and thus as a necessity; but the same must be true everywhere even in regard to the most complex adaptations which seem to depend entirely upon chance; everywhere adaptation results of necessity—if it is possible at all with the given organization of the species—as certainly as the adaptation dress of the hare depends on the length of the winter, and in point of fact not less certainly than the blue colour of starch on the addition of iodine. The most delicate adaptations of the vertebrate eye to the task set for it by life in various groups have been gradually brought about as the necessary results of definite causes, just in the same way as the complex protective markings and colouring on the wing of the Kallima and other leaf-mimicking butterflies.
That adaptations can be regarded as mechanically necessitated is due to the fact that in every process of adaptation the same direction of variation on the part of the determinants concerned is guaranteed, since personal selection eliminates those which vary in a wrong direction, so that only those varying in a suitable direction survive, and they then continue to vary in the same direction. But the greatest difference between our conception of natural selection and that of Darwin lies in this: that Darwin regarded its intervention as dependent upon chance, while we consider it as necessary and conditioned by the upward and downward intra-germinal fluctuation of the determinants. Appropriate variational tendencies not only may present themselves, they must do so, if the germ-plasm contains determinants at all by whose fluctuations in a plus or minus direction the appropriate variation is attainable.
That a horse should grow wings is beyond the limits of the possibilities of equine variation—there are no determinants which could present variations directed towards this goal; but that any multicellular animal which lives in the light should develop eyes lies within the variational possibilities of its ectoderm determinants, and in point of fact almost all such animals do possess eyes, and eyes, too, whose functional capacity may be increased in any direction, and which are adaptable and modifiable in any manner in accordance with the requirements of the case. As soon as the determinants of the most primitive eye came into existence, they formed the fundamental material by whose plus- or minus-variations all the marvellous eye structures might be brought about, which we find in the different groups of the Metazoa, from a mere spot sensitive to light to a shadowy perception of a moving body, and from that again to the distinct recognition of a clear image, which we are aware of in our own eyes. And what wonderful special adaptations of the eye to near and to distant vision, to vision in the dusk and at night, or in the great ocean-depths, to recognition of mere movement or the focussing of a clear image, have been interpolated in the course of this evolution!
All such adaptations are possible, because they can proceed from variations of determinants which are in existence; and in the same way it is possible, at every stage of the evolution of organisms, for eyes to degenerate again, whether they have been high up or low down in the scale of gradations of this perhaps the most delicate of all our sense-organs. As soon as a species migrated permanently from the light into perfect darkness its eyes began to degenerate. We know blind flat worms, blind water-fleas and Isopods, also blind insects and higher Crustaceans, and even blind fishes and amphibians, the eyes of which are now to be found at very different levels of degeneration, as Eigenmann has recently shown in regard to several species of cave-dwelling salamanders of the State of Ohio. In all these cases it is only necessary for the determinants of the eye to continue to vary in the minus direction, and the disappearance of the eye must be gradually brought about.
We must picture upward development in quite a similar way. The forest butterflies of the Tropics could not possibly all have their under surfaces coloured like a leaf if the protective pattern depended solely upon the chance of a useful variation presenting itself. It always presented itself through the fluctuations of the determinants, and thus the appropriate colourings were not merely able to develop, but of necessity did so in gradually increasing perfection. If chance played any part in the matter, it would be quite unintelligible why the protective colouring should occur only where it acts as a protection, and why, for instance, it should not appear sometimes upon the upper surface of the butterfly wing, or upon the posterior wings which are covered when the butterfly is at rest. We have already studied in detail the precision with which the coloration is localized on minute points and corners of the wing: this can only be understood if natural selection works with the certainty of a perfect mechanism. Chance only comes into the matter in so far as it depends upon chance whether the relevant determinants in one id or another are to vary in the direction of plus or minus; but as the germ-plasm contains many ids, and chance may decide it differently in each of these, the presence of a majority of determinants varying in a desirable direction does not depend upon chance, for if they are not contained in one individual they are in another. It is only necessary that they should be present in some, and that these should be selected for reproduction.
We must therefore regard natural selection, that is to say, personal selection, as a mechanical process of development, which begins with the same certainty and works 'in a straight line' towards its 'goal,' just as any principle of development might be supposed to do. Fundamentally it is after all a purely internal force which gives rise to evolution, the power of the most minute vital units to vary under changing influences, and it is only the guidance of evolution along particular paths that is essentially left to personal selection, which brings together what is useful and thus determines the direction of further evolution. If we bear in mind that even the minutest variations of the biophors and determinants express nothing more or less than reactions to changed external conditions in the direction of adaptation, and that the same is true of each of the higher categories of vital units, whether they be called cell, tissue, organ, person, or corm, we see that the whole evolution of the forms of life upon the earth depends upon adaptations following each other in unbroken succession, and fitting into each other in the most complex way. The whole evolution is made possible by the power of variation of the living units of every grade, and called forth and directed by the ceaseless changes of the external influences. I said years ago that everything in organic evolution depended upon selection, for every lasting change in a vital unit means adaptation to changed external influences, and implies a preference in favour of the parts of the unit concerned, which are thereby more fitly disposed.
In this sense we can also say that the species is a complex of adaptations, for we have seen that it depends upon the co-operation of different grades of selective processes, that in many cases it is produced solely by germinal selection, but that in very many more personal selection plays the chief part, whether in bringing about sexual adaptations, or adaptations to the conditions of existence.
When we have thus recognized that the origin of a variation in a definite direction results as inevitably when it is called forth by the indirect influence of conditions, that is, through the need for a new adaptation, as when it is induced in the germ-plasm by direct causes such as those of climate, we shall not be disposed to estimate very highly the part played by mutual sterility in the origin of species. We shall rather be inclined to assign it a rôle at a later stage, after the separation of the forms has taken place, and this view is supported by the fact of the mutual sterility of most nearly related species, and by the theoretical consideration that the frequency of hybrids, even if these are always eliminated in the struggle for existence, must signify a loss for both the parent species. But no certain conclusion can be based upon either of these arguments—not upon the theoretical one, because here again we are unable to estimate the extent of this loss; and not upon the argument from fact, because the results of experiments in crossing animals have generally been overestimated, since we are apt to regard the most nearly related animals that are at our disposal as being very closely related. Thus, for instance, horse and ass, horse and zebra are undoubtedly rightly included within the same genus, but the fact that there are several species of zebra in Africa gives us an idea of the number of transition stages that may have existed between the horse and the zebra. Entomologists have sometimes reared hybrids between the most nearly related indigenous species of hawk-moth of the genus Smerinthus—hybrids of Smerinthus ocellata, the eyed hawk-moth, and Smerinthus populi, the poplar hawk-moth. I have myself made many experiments of this kind, and have often succeeded in getting the two species to pair and even to deposit eggs, but I have never seen a caterpillar emerge from them. The hybrids do occur, however, and they have been repeatedly obtained by Standfuss. In external appearance they are intermediate between the parent forms, but with marked divergences, thus, for instance, the beautiful blue eye on the posterior wing of S. ocellata ([Fig. 5], vol. i. p. 69) may have almost disappeared or be only indicated. They are sterile. But we know three species of Smerinthus in North America, which are all much nearer to S. ocellata than S. populi is, for they all possess the eye-spot referred to, although it is less well developed. The proof that the most nearly related species do not yield fertile descendants should be sought for by crossing Smerinthus ocellata with one of these American species if it is to have any decisive value.
Experiments of the same kind have been made by Standfuss with different species of indigenous Saturnia, and these have shown not only that crossing is possible, but that the hybrids are fertile in their turn. These results are to be valued the more highly because it is well known that Lepidoptera, and even the usually prolific silk-moths, do not readily reproduce in captivity, even within the same species. We have in Saturnia pyri, spini, and carpini three well-marked distinct species with no intermediate forms in nature, and with quite different colouring in the caterpillars. That these should have been successfully combined in a triple hybrid proves at least that sexual alienation cannot have advanced far in this case.
We must beware, however, of attributing too much to the constant mutual crossing which occurs in a species living on a connected area and of regarding its influence as irresistible. Undoubtedly it must go far towards securing the uniformity of individuals, but not only is it unable to achieve this, but it cannot successfully resist the stronger influences making for variation which may be exerted upon a part of the area of the species. We have already seen that it is quite erroneous to suppose that every new adaptation must be lost sight of again because of the continual crossing with other members of the species upon the same area. Other things being equal, this depends entirely upon the importance of the adaptation in question. Just as climatic influences may be so strong that they entirely overcome the influence of crossing, and give rise to a local race notwithstanding imperfect geographical isolation, so the same may happen in the case of adaptations. It is quite conceivable that the polar hare of Scandinavia may have evolved a whole series of races, each of which is adapted to the duration of the snow in its geographical range, although a crossing of these quick-footed animals must frequently occur in the course of time, even as regards forms from widely separated areas, and although the whole region is inhabited without a break by the species, so that a 'mingling' of the hares of all regions from south to north, and conversely, may take place, and indeed must be continually taking place, though of course very slowly.
It is precisely this extreme slowness with which the intermingling of racial characters take place that seems to me essential for the production of local or, as in this case, regional races. It is not difficult to calculate the rate of 'blood-distribution' if we assume that the conditions for a rapid dissemination are as favourable as possible. Let us assume that it takes place along a certain line—in this case from south to north—and that the numerical strength of the species remains constant, each pair of hares yielding a pair of surviving offspring, which will attain to reproduction. Let us suppose that one of these hares moves his home northwards to the extent of his range, that is, as far as a hare is accustomed to range from his head quarters, and that he pairs with one of the descendants on the next stretch.
Let us further suppose that this stretch is ten kilometres in extent, and that the change of quarters take place once in each year, then the blood of a South Scandinavian hare would have extended ten kilometres further north in ten years, and in a hundred years 100 kilometres; it would not, however, be quite pure, but mixed and thinned by crossing with a hundred mates of different individual bloods, that is, thinned to the extent of 2 to the 100th power, that is, to less than a millionth part. Thus even with these much too favourable assumptions the influence of a region of hares 100 kilometres distant would be actually nil upon the inhabitants of a region which was in process of new adaptation. That the assumptions are too favourable is quite obvious, since every surviving hare would not be likely to move his home, and probably the majority would remain in the old quarters and find mates there. The blood-mingling would therefore take place much more rarely, perhaps only once in ten years, and the wandering descendants of the second generation might move southward, and so neutralize the previous blood-mingling, and so on. But let us keep to our favourable assumptions, and attempt to determine how strong the assimilating influence of the blood-mingling from south to north would be upon a point A. The blood of the nearest stretch diluted to a half would affect the inhabitants of A once in each year; the second stretch would only contribute blood of ¼ strength, the third of 1/8, the fourth of 1/16, and the blood of the tenth would be diluted to 1/1024. A region B, extending over twenty such stretches, or 200 kilometres, would thus shelter within it a hare population of which the centre would only be influenced from the periphery in vanishing proportions. If the winter were of equal length over the whole area of B, all the inhabitants would be tending to vary the period for which the winter dress was worn in correspondence with the length of the winter, and the centre of the region would be the less impeded in this process because the more peripheral areas would also be approximating to the same adaptation. But since even the admixture of 1/32 of strange blood could have no hindering influence upon a variation, there would remain a region of 2 × 5 = 10 stretches upon which the influence of the non-varying regions would be without effect. There would therefore arise a new race in relation to the duration of the winter dress, and this would not cease abruptly, but would gradually pass over into the neighbouring regions, which however would be pure at their centre, just as is probably the case in reality, if we regard B as any point in the line of distribution from south to north.
The harmony of the individuals within a species will therefore depend in part upon the mingling of hereditary primary constituents associated with reproduction, but in greater part upon adaptation to the same conditions; it is a similarity of adaptation, and the strongest influence which sexual reproduction exerts lies not in the mingling of these hereditary constituents alone, but above all in the reduction in the germ-plasm of the two parental hereditary contributions—a reduction which results from and through the sexual intermingling. It is only this that prevents these primary constituents from varying at too unequal a rate in the transformations of species, and causes them ultimately to resemble each other closely again.
But while mutual sterility is not an absolutely necessary condition in the separation of species, it would be going too far in the opposite direction to regard mutual fertility as something general, or to attribute to it a rôle in the origination of new species.
Certain botanists, like Kerner von Marilaun, regard the mingling of species as a means of forming new species with better adaptations; they suppose that fertile hybrids may, in certain circumstances, crowd out the parent species, and themselves become new species. It will be admitted that such cases do occur, that, for instance, in the north of Europe the hybrid between the large and the small water-lily, Nuphar luteum and Nuphar pumilum, to which the name Nuphar intermedium has been given, has driven both the parent species from the field, because its seeds mature earlier, and it is therefore better adapted to the short vegetative period of the north, but nevertheless we must maintain that the evolution of species on the whole does not take place through hybridization. Such cases are probably nothing more than rare exceptions. This is corroborated by the entire insignificance of hybridization in animals, among which species appear in the same way as they do in plants, and where the mingling of two species occurs only sporadically and in a few species, never to any very great extent.
If species are complexes of adaptations, based in each case on the given physical constitution of the parent species, then we can readily understand the fact that they are in our experience not fixed or eternal, but that they change in the course of the earth's history. The numerous fossil remains in the various strata of the earth's crust prove that this is true in a high degree, that in almost every one of the more important geological strata new species occur, and that not only species and genera, but families, orders, indeed whole classes of animals, which lived at one time, have now completely disappeared from the face of the earth. We can understand this phenomenon when we reflect that the conditions of life have also been slowly changing through the course of the earth's history, so that the old species had only the alternative of dying out, or of becoming transformed into new species.
But simple as this conclusion is, it can hardly be deduced with certainty from the occurrence and succession of the fossil species alone. For instance, we should strive in vain to recognize the cause which led one of those regularly arranged snail-species of the Steinheim lake basin to become transformed into one or two new species at a particular time, or to find the cause which moved those curious tripartite Crustaceans of primitive times, the Trilobites, which peopled the Silurian seas with such a wealth of forms, to become suddenly scarce towards the end of the Silurian period, and to disappear altogether in the succeeding period, the Devonian. The famous geologist Neumayr sought to refer this striking phenomenon to the fact that just at that time the Cephalopods, 'the most formidable and savage marauders among the invertebrate marine fauna,' gained the ascendancy, and it is quite possible that he was right in his surmise, but who is to prove it? Can we decide even in the case of animals now living whether the losses inflicted on a much persecuted species by an abundant and greedy persecutor exceed the numbers of progeny, and are therefore driving the species gradually towards extermination? Probable as such a supposition appears, it cannot be accepted as proven.
Since in many cases of the extinction of great animal-groups we cannot even prove that there was a simultaneous ascendancy of powerful enemies, other factors must be discovered to which the apparently sudden disappearance may be attributed. Many naturalists have tried to guess at internal reasons for extinction, and have adopted the theory—associated with the tendency to assume mystical principles of evolution—that species in dying out are obeying an internal necessity, as if their birth and death were predestined, as it is in the case of multicellular individuals, as if there were a physiological death of the species as there is of the multicellular individual.
Neumayr showed, however, that the facts of palæontology afford no support for this view. I need not repeat his arguments, but will simply refer to his clear and concise exposition of the problem. It is obvious that our theory of the extinction of species as due to external causes cannot be rejected on the ground that our knowledge of the struggle that species had to maintain for their existence in past times is even mere imperfect than our knowledge of the struggle nowadays, and that we are frequently unable to judge of it at all. But the facts of geology are of value in another, quite different way. They reveal such an extraordinary dissimilarity in the duration of species, and also of the great groups of organisms, that the dissimilarity of itself is sufficient to prevent our regarding the extinction of species as regulated by internal causes. Certain genera of Echinoderms, such as starfish (Astropecten), lived in the Silurian times, and they are represented nowadays in our seas by a number of species: and in the same way the Cephalopod genus Nautilus has maintained itself among the living all through the enormous period from the Silurian sea to our own day. Formerly the Nautilids formed a predatory horde that peopled the seas, and, as we have seen, we may perhaps attribute to their dominance the disappearance of an order of Crustaceans, the Trilobites, which were equally abundant at that period. Now only a single species of nautilus (Nautilus pompilius) lives on the coral reefs of the southern seas. Similarly, the genus Lingula of the nearly extinct class of Brachiopods, somewhat mussel-like sessile marine animals, has been preserved from the grey dawn of primitive times, with its records in the oldest deposits, and is represented in the living world of to-day by the so-called 'barnacle-goose' mussel, Lingula anatina.
On the other hand, we know of numerous species which lasted for quite a short time, such as, for instance, the individual members of the series of Steinheim Planorbis species, or of the Slavonic Paludina species. Not infrequently, too, genera make their appearance and disappear again within the period of one and the same geological stratum.
These facts not only tell against an unknown vitalistic principle of evolution, but in general against the idea of the determination of the great paths of evolution by purely internal causes. If there were a principle of evolution the dissimilarity in the duration of life could not be so excessive; if there were a 'senile stage' of species and a natural death of species comparable to the natural death of individuals, it would not have been possible for most of the Nautilidæ to have been restricted to the Silurian epoch, and yet for one species to have continued to live till now; and if there were a 'tendency' of species to vary persistently onwards, and to 'become further and further removed from the primitive type,' as has been maintained, then such ancient and primitive Cephalopod forms like the Nautilus-species could not have persisted until now, but must long ago have Wen transmuted into higher forms. The converse, however, is conceivable enough, namely, that the great mass of the species of a group such as the Nautilidæ were crowded out by superior rivals in the struggle for existence, but that certain species were able to survive on specially protected or otherwise favoured areas. We have a fine example of this in the few still living species of the otherwise extinct class of Ganoid fishes. During the Primary and Secondary epochs these Ganoids peopled all the seas, but at the boundary between the Cretaceous and the Tertiary period they retrograded considerably, simultaneously with the great development of bony fishes or Teleosteans, and now they are only represented by a dozen species distributed over the earth, and most of these are purely river forms, while the others at least ascend the rivers during the spawning season to secure the safety of their progeny. For the rivers are sheltered areas as compared with the seas, and large fishes like the Ganoids will be able there to hold their own in the struggle better than they could in the incomparably more abundantly peopled sea.
Thus I can only regard it as playing with ideas to speak of birth, blossoming, standstill, decay, and death of species in any other than a figurative sense. Undoubtedly the life of the species may be compared with that of the individual, and if the comparison be used only to make clear the difference between the causes of the two kinds of phenomena, there can be no objection to it, only we must beware of thinking we have explained anything we do not know by comparing it with something else that is also unknown.
We have already shown that the natural death of multicellular organisms is a phenomenon which first made its appearance with the separation of the organism into somatic or body cells and reproductive or germ cells, and that death is not an inevitable Nemesis of every life, for unicellular organisms do not necessarily die, though they may be killed by violence. These unicellular organisms have thus no natural death, and we have to explain its occurrence among multicellular organisms as an adaptation to the cellular differentiation, which makes the unlimited continuance of the life of the whole organism unnecessary and purposeless, and even prejudicial to the continuance of the species. For the species it is enough if the germ-cells alone retain the potential immortality of the unicellulars, while, on the other hand, the high differentiation of the somatic cells necessarily involves that they should wear themselves away in the performance of their functions, and so become subject to death, or at least that they should undergo such changes that they are no longer capable of functioning properly, so that thus the organism as a whole loses the power of life.
There can be no doubt whatever that death is virtually implied in the very constitution of a multicellular organism, and is thus, so to speak, a foreseen occurrence, the inevitable end of a development which begins with the egg-cell and reaches its highest point with the liberation of the germ-cells, that is, with reproduction, and then enters on a longer or shorter period of decadence, leading to the natural death of the individual.
It is only by straining the analogy that this course of development can be compared with the origin and transformation or extinction of species. Not even the entirely external analogy of the blossoming from a small beginning and the subsequent decay is always correct; for in the fresh-water snails of Steinheim, at any rate, almost the whole of the members of the species underwent a transformation at a particular time, and became a new species, which was after a long time retransformed without any appreciable decrease in the number of individuals being observable. To speak of a 'senile stage' of the species, of a stiffening of its form, of an incapacity for further transformation, is to indulge in a play of fancy quite inadmissible in the domain of natural science.
It is admitted, however, that there is a correct idea at the base of all this, for many species have not passed over into new forms, but have simply died out because they were unable to adapt themselves to changed conditions. This did not happen because they had become incapable of variation, but because they could not produce variations of sufficient magnitude, or variations of the kind required to enable the species to remain an active competitor in the struggle for existence.
It obviously depends upon the coincidence of manifold circumstances, whether an adaptation can be successfully effected or not. Above all, it must be able to keep pace with the changes in the conditions of life, for if these advance at a more rapid rate the organisms will succumb in the midst of the attempt at adaptation. It is probably in this way that the striking disappearance of the Trilobites is to be explained, as Neumayr has pointed out, for the Nautilidæ, a new group of enemies, multiplied so quickly at their expense that they had not time to evolve any effective means of protection. It cannot be maintained for a moment that every species is able to protect itself against extermination by any other; the increased fertility, the increased rapidity of locomotion, the increased intelligence and similar qualities, may all be insufficient, and then extinction follows; not, however, because the species has become 'senile,' but because the variations possible to its organization do not suffice to maintain it in the struggle.
In discussing germinal selection I mentioned the view expressed by Emery, that excessive variation in the same direction from intra-germinal causes has not rarely been the cause of the extinction of species. I also mentioned the very similar view of Döderlein, who could not refer at that time to germinal selection, but assumed internal compelling forces, which pressed a variation irresistibly forward in the direction in which it had started, even beyond the bounds of what is useful for the desired end, and which might thus bring about the extinction of the species. I cannot entirely agree with these views, as I have already indicated, because I do not believe that the impulse to variation can ever become irresistible and uncontrollable. If it could, then we should not see, as we do, innumerable cases in which the augmentation or diminution of a part has gone on precisely to the point at which it ceases to be purposeful. Even the degeneration of organs only proceeds as far as is necessary to accomplish a particular end, as we see plainly from the parasitic Crustaceans of different orders. In many of these parasitic forms the swimming legs degenerate, but in the female only, because these attach themselves by suckers or in some other manner to their host, so that they cannot let go again. But the males need their swimming legs to seek out the females. The females too require them in their youth, in order to seek out the fish from which they are to obtain their food-supply, and thus the degeneration of the swimming legs has come to a full stop exactly at the point where they cease to be of use; they develop in early youth and degenerate later, when the animal becomes sessile. In accordance with the law of biogenesis we may say that while the degeneration is complete in the final stages of ontogeny, its retrogression was not continued back to the germ, but only to the young stages. From this it follows that the progress of a variation may at any time have a goal fixed for it, and we have seen that this is possible by means of personal selection, which accumulates the never-failing fluctuations of the variation in the direction of plus or of minus. In the individual id a determinant X may perhaps decrease and possibly also increase without limit, although we have no certain knowledge in regard to the latter point, but as this determinant is contained in all the ids, there are always plus and minus fluctuations by means of which personal selection can operate.
But of course it requires a certain amount of time for this, and in the fact that this time is often not available lies, I think, the reason why excessive differentiations have often led to the extinction of a species, not because the increase of the excessive organ must go on irresistibly, but because changes in the conditions have made the exuberant organ inappropriate, and it could not degenerate quickly enough to save the species from extinction.
Brandes has recently given a beautiful illustration of this by associating the existence of the remarkable sabre-toothed tigers (Machairodus) with enormously long canine teeth, which lived in the Diluvial period in South America, with the gigantic Armadillos which lived there at the same time, whose bony armature two yards in height now excites our admiration. He rightly points out that the dentition of Machairodus neogæus is by no means a typically perfect dentition for a beast of prey, like that of the Indian tiger or the lion; as far as incisors and molars are concerned it was much less effective than that of these predatory animals, and the great length of the dagger-like flattened canines, which protruded far beyond the mouth, entirely prevented the bringing together of the teeth of the upper and lower jaw after the fashion of a pair of pincers. He rightly infers from this that this dentition was adapted to a specialized mode of nutrition, and he regards the great mailed Armadillos, such as the three-yards-long heavy Glyptodont of the Pampas, as the victims into which they were wont to thrust their sabre-teeth in the region of the unprotected neck, and thus to master the almost invulnerable creature, which was invincible as far as all other predatory animals were concerned. Thus the remarkable dentition is explained on the one hand, and on the other the amazing extent and hardness of the victim's coat of mail. Thus, too, we can understand why there should have been at that time a whole series of cat-like animals with sabre-like teeth, in which the length and sharpness of the teeth increased with the bodily size, for these predatory animals corresponded to a whole series of Armadillos, whose size was increasing, as was also the strength of their armour.
Of course this interpretation is hypothetical, but it contains much internal probability, so that it may be taken as a good illustration of the reciprocal increase of adaptations between two animal groups. We understand now why, on the one hand, this colossal tortoise-like armour should have developed in a mammal, and, on the other hand, why these enormously long sabre-teeth should have been evolved; we also understand—and this is the point with which we are here chiefly concerned—why these two 'excessive' developments should ultimately lead to the destruction of their possessors. For a long period the Armadillos were able to save themselves from extermination by increasing their bodily size and the strength of their armour, and they thus saved themselves from persecution on the part of beasts of prey with smaller and weaker teeth. But the predatory animals followed suit and lengthened their teeth and increased their bodily size, until ultimately even the strongest armour of the victim afforded no efficient protection, and the mighty Glyptodonts were by degrees utterly exterminated. But then the death-knell of the Machairodus had also sounded, for he was so exactly adapted to this one kind of diet that he could no longer overpower other victims and feed on their flesh; the sabre-teeth prevented him from tearing his prey like other predatory animals, he could probably only suck them.
Even if this is a supposititious case, it serves to show that it was not an internal principle of variation that caused the teeth of these carnivores and the armour of their victims to increase so unlimitedly; it was the necessity of adaptation. They did not perish because armour and teeth increased so excessively, but because neither of these adaptations could be neutralized all at once, and small variations were of no use to them in their final struggle for survival.
In a certain sense we may say that simpler, more lowly organisms are more capable of adaptation than those which are highly differentiated and adapted to specialized conditions in all parts of their bodies, since from the former much that is new may arise in the course of time, while very little and nothing very novel can spring from the latter. From the simplest Protozoan the whole world of unicellular organisms could arise, and also the much more diverse Metazoa; from the lower marine worms there could arise not only many kinds of higher marine worms—the segmented worms or Annelids—but also quite new groups of animals, the Arthropods and the Vertebrates. It is hardly likely that a new class of animals will evolve from our modern birds, because these are already so perfectly adapted to their aerial life that they could hardly adapt themselves to life on land or in the water sufficiently well to be able to hold their own in regard to all the possibilities of life with the rest of the dwellers on land or in water. We do indeed know of birds which have returned entirely to a purely terrestrial life—the ostriches, for instance—and of others which have adapted themselves to a purely aquatic life, such as the penguins, but these are small groups of species, and are hardly likely to increase. On the contrary, we can prove that many have already succumbed in the struggle with man, and we anticipate the extermination of others. But the reason why they are so readily exterminated obviously lies in the fact that they have surrendered the advantage given to them by their bird-nature, by adapting themselves to terrestrial life, and that they are not able to regain it, at least not in the short time that is at their disposal if they are to be saved from extermination. The best example of this is the Dodo (Didus ineptus). This remarkable-looking bird, of about the size of a swan, lived in flocks upon the island of Mauritius until about the end of the seventeenth century. It had small wings with short quills which were useless for flight. As it could neither escape by flight nor through the water, and could only move clumsily and awkwardly upon land with its short legs and heavy body, it was hopelessly doomed as soon as a stronger enemy made his appearance. It fell a victim to the sailors who first landed on the island and clubbed it with sticks in huge numbers. Until that event it was without doubt excellently adapted to life on that fertile island, for on a volcanic island in the middle of the ocean there were no large enemies, and it was therefore not dependent on the power of flight for safety, and could pick up abundant food from the ground. But when man suddenly appeared on the scene and began to persecute it, it was not the 'senile rigidity' of its organization that prevented it from making use of its wings again; it was the slowness of variation and consequently of selection, which is common to all species, which impelled it to extinction. The same fate will probably overtake the Kiwi of New Zealand (Apteryx australis) in the near future, for though it has so far escaped the arrows of the aborigines, it is not likely in its wingless condition to be able to hold out long against European guns, unless close times and preserved forests are instituted for it, as they have been for our chamois.
Even sadder from the biologist's point of view than such extermination of individual species through the vandalism and greed of our own race is the disturbance of whole societies of animals and plants by man that is going on or has been accomplished on most of the oceanic islands, and we must briefly notice these cases while we are dealing with the decadence of species. I refer to the crowding out of the usually endemic animal and plant population on such islands through cultivation. The first step in this work of 'cultivation' is always the cutting down of the forests which for thousands of years have clothed these islands as with a mantle of green, have regulated their rainfall, secured their fertility, and allowed a medley of indigenous animals, usually peculiar to the spot, to arise. We have already spoken of St. Helena. The original and remarkable fauna and flora of this island had for the most part disappeared 200 years ago, through the cutting down of trees in the forests, and these were later wholly destroyed by the introduction of goats, which devoured all the young trees as they grew. But with the forests most of the indigenous insects and birds were doomed to destruction, so that now there is not an indigenous bird or butterfly to be found there; only a few terrestrial snails and beetles of the original fauna still survive.
But it is not only on islands that a large number of species have been decimated or entirely exterminated by deforestation, by the introduction of plants cultivated by man and of the 'weeds' associated with these, and by the importation of domesticated animals. In Central Europe not only have all the larger beasts of prey, like the bear, the lynx, and the wolf, almost completely disappeared, but the reindeer, the bison, the wild ox (Aurochs), and the elk have been exterminated as wild animals, and in North America the buffalo will soon only exist in preserved herds, if that is not already the case. Here, of course, the direct interference of the all-too-powerful enemy, man, has played the largest part in causing the disappearance of the species referred to, but the process may give us an idea of the way in which a superior animal enemy may be able gradually to exterminate a weaker species where there is no attainable or even conceivable variation which might preserve them from such a fate. Several of the mammals which I have mentioned are not yet entirely exterminated; even the Aurochs perhaps still exists in the pure white herds preserved in some British parks; but there are more instances than that of the Dodo of the utter extermination of a species through human agency within historic times. It may be doubtful whether the sea-otter (Enhydris marina) has not been already quite exterminated because of its precious fur, but it is quite certain that the huge sea-cow (Rhytina stelleri), which lived in large numbers in the Behring Straits at the end of the eighteenth and the beginning of the nineteenth centuries, was completely exterminated by sailors within a few decades.
We may therefore gain from what is going on before our eyes, so to speak, some sort of idea of the way in which the extermination of species may go on even independently of man at the present time, and how it must have gone on also in past ages of the earth's history. Migrations of species have taken place ceaselessly, although very slowly, for every species is endeavouring slowly to extend its range and to take possession of new territories, and thus the fauna and flora of any region must have changed in the course of time, new species must have settled in it from time to time, and the conditions of life must have changed, and in many cases this must have led to the extermination of species, in the same way, though not so quickly, as human interference now brings about their doom.
This is true for plants as for animals. A good example, not indeed of complete extermination, but of very considerable diminution in the numbers of individuals of a plant-species by the advent of a species of mammal, is communicated to us by Chun in regard to Kerguelen Land. A flowering plant, the Kerguelen cabbage (Pringlea antiscorbutica), has been greatly reduced in numbers since the thoughtless introduction of rabbits to this uninhabited island (1874). While, in 1840, Captain Ross used this plant in great quantities as a preventative against scurvy in his crew, and even carried away stores to last for months, the Valdivia Expedition in 1898 found rabbits in abundance, but the Kerguelen cabbage had been entirely exterminated at every spot accessible to these prolific and voracious rodents. It was only found growing upon perpendicular cliffs or upon the islands lying out in the fiords.
An avoidance of the threatened destruction of a species by its adaptation to the new circumstances can only be possible when the changes occur very slowly, and will therefore be more likely to be achieved in the case of physical changes in the conditions of life, such as climatic changes, a change in the mutual relations of land and sea, and so on. But it appears that even climatic changes do not evoke any variation and new adaptation as long as the species can avoid the changes by migrating. The often quoted case of Alpine and Arctic plants proves this at any rate, that those species which inhabited the plateaus and highlands of Europe did not all vary to suit the change when a warmer climate prevailed, but that in part at least they followed the climate to which they were already adapted, that is, that they migrated towards the north on the one hand and higher up the Alps on the other. It cannot be denied that many of the insects and plants did adapt themselves at that time to the warmer climate, and became the modern species which now inhabit the plains, for many related species occur on the Alps and in the plains, but apparently many others simply made their escape from a climate which no longer suited their requirements. Thus, as far as I am aware, there is no species of Primula in South or Central Germany which could be derived from the beautiful red Primula farinosa of the Alps, but this species occurs also upon the old glacier-soil at the northern base of the Alps, and in similar soil again in the north of Germany and on the meadows of Holstein. Similar examples might be cited in regard to the Alpine-Arctic butterflies.
It is intelligible enough that we are still very far from being able to give a precise account of the main changes in the plant and animal world during the history of the earth in regard to the special causes which have produced them. Possibly the future will throw more light upon this subject by extending our knowledge of the fossil remains of all countries. But so much at least we can say at present, that there is no reason to refer the dying out of the earlier forms to anything else than the changes in the conditions of life, the struggle for existence, and the limitation of the power of transformation and adaptation due to the organization which the species has already attained; there is no trace of any such thing as a phyletic principle of life in the vitalistic sense, as far as the decadence of species is concerned.