In the fins of cetacea, as in the four limbs of other mammals, we find the same bones, which are derived from the bones of the wings and legs of their bird ancestors. In birds, the same bones are the phylogenetic derivatives of the limbs of reptiles.
All these facts demonstrate with certainty the descent of animal forms, a descent which we can follow in all its details. In certain ants whose bodies show their close relationship with a slave-keeping group, but which have become the parasitic hosts of other ants, we find not only the arched mandibles, shaped for rape, but the undoubted rudiments of the slave instinct, although this instinct has, perhaps, not been exercised by them for thousands of years.
These examples suffice to show that the form and functions of a living organism, as well as its mental faculties, are derived not only from the most recent direct ancestors of this organism, but that they partly mount much higher in the genealogical tree.
Our coccyx is a vestige of the tail of animals. It is from them also that we have inherited anger and jealousy, sexual appetites, fear, cunning, etc. As long as they remain in use, the oldest inherited characters normally remain the most tenacious and are preserved the longest. When they cease to be utilized, or become useless, they still remain for a long time as rudiments before finally disappearing; for instance the vermiform appendix of the intestine and the pineal gland of the brain. These rudiments often persist for still a longer time in the embryo, as we have seen in the case of the ancestral teeth of the embryo whales. We also meet with the stumps of wings in the chrysalis of certain ants (Anergates), the males of which have lost their wings.
Natural Selection.—The artificial selection practiced by gardeners and cattle breeders led Darwin to his hypothesis of natural selection by the struggle for existence. Confirmed in his idea by the observation of tropical nature, Darwin thought he could explain the origin of living beings by natural selection. It is this hypothesis which is properly called Darwinism. But the name Darwinism has also been given to evolution as a whole, which has been the cause of endless confusion. All the mystic and narrow-minded, full of biblical prejudice, naturally profit by this confusion to attack the facts of evolution and science itself.
The Struggle for Existence.—The struggle for existence and natural selection are absolutely positive facts, which can be constantly verified by the observation of living nature as it is presented to us. All living beings eat one another or at any rate struggle against each other, plants as well as animals; and, apart from air and water, animals are almost entirely nourished by plants and other animals. It is obvious that in this perpetual struggle the less adapted and the less armed—and by arms we include the powers of reproduction, resistance to diseases and to cold, etc.—disappear, while the better adapted and the better armed persist. I confess I cannot understand the detractors of Darwin who are blind in face of these facts and hypnotized by certain conventional suggestions.
On the other hand, what always has been and still remains hypothetical is the explanation of the descent of all plants and animals by natural selection alone. We have already spoken of the mutations of de Vries, and the theory of the mneme elaborated by Semon, and need not repeat them here. Thanks to the idea of Hering, worked out by Semon, the facts are now explained in a satisfactory manner. Engraphia, produced in the organisms by the irritating agents of the external world, prepares and builds up little by little their increasing complications, while selection, by continually eliminating the unfit, directs the elaborating work of the mneme and adapts it to the surrounding local circumstances.
De Vries has objected that the variations produced by artificial and natural selections are mutable, while sudden mutations have a much more stable character. But we have just seen that these mutations themselves are evidently only the delayed ecphoria of a long ancestral engraphia accumulated.
On the other hand, the variations obtained by selection are themselves only due to more rapid ecphorias, derived from repeated conjugations in a certain direction. Plate and others have shown that they may become more and more fixed, if they are well adapted, and thus become more tenacious. There is, therefore, no contradiction between the fundamental facts, and all is simply and naturally explained by the combination of hereditary mnemic engraphia with selection.
Recent study on the transformations of living beings have shown that they do not take place in a regularly progressive manner, as Darwin at first believed, but that periods of relatively rapid transformation alternate with periods of relative arrest, both in a general way and for each particular species. We see certain species remaining almost stationary for an immense time and tending rather to disappear, while others vary enormously, showing actual transformation. The transplantation of one species to a new environment, for instance to a new continent, provokes, as has been proved, a relatively rapid transformation. It is evident that mnemic engraphia transforms organisms the more rapidly as it changes in nature itself, which is the case in the migrations we have just mentioned, and which also changes the factors of selection.