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.