The Evolution Theory, Vol. 2 of 2

by August Weismann

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.