CHAPTER II[ToC]
EVOLUTION OR DESCENT OF LIVING ORGANISMS
The theory of evolution is intimately associated with the name of Darwin, for it was he who established it in the scientific world. In reality, the idea of the transformation of organisms was put forward by Lamarck more than a century ago, but he did not sufficiently support it. The theory of evolution states that the different animal and vegetable species are not each of them specially created as such from the first, but that they are connected with each other by a real and profound relationship, and derived progressively one from another; generally from more simple forms, by engraphia and selection. Man himself is no exception to this rule, for he is closely related to the higher apes.
It is no longer possible to-day to deny the fundamental fact which we have just stated. Since Darwin, and as the result of the powerful impulse which this man of genius gave to natural science, innumerable observations and experiments have confirmed the truth of the progressive evolution of living beings. Comparative anatomy, comparative geography of plants and animals, comparative embryology, and the study of the morphology and biology of a number of recently discovered plants and animals, have built up more and more the genealogical tree, or phylogeny, of living beings, that is to say their ancestral lineage. The number of varieties and races or sub-species increases indefinitely, the more closely they are examined.
Researches on the fossil remains of species of animals and plants which have been extinct for thousands and millions of years (palæontology) have also contributed to determine the trunk of the great tree of former life. The numerous gaps which still exist between these fragmentary documents of former ages are nevertheless too considerable for continuous connections to be established in the past by the aid of fossils.
We not only know that the different forms of living beings are connected to each other by a real relationship, but we can fathom more and more deeply the degrees of this relationship, and can often prove from which group of animals a given group is descended. In many cases we can determine at which period the fauna and flora of two continents have been separated from each other, and in what manner they have been transformed, each in its own way, while still preserving the general characters which were common before their separation. The specialist can soon discover what species belong to the old geographically differentiated fauna and flora of the country, and what have been ulteriorily imported.
I record these facts for the benefit of those persons who have not yet understood that it is absolutely useless at the present day to dispute the evolution of living beings. Deceived by the divergent opinions of scientists concerning hypotheses which endeavor to explain the details of evolution, these persons confound the details with the fundamental facts of evolution.
Ontogeny. Phylogeny.—In the light of the facts of evolution, heredity takes quite a new aspect when removed from the old biblical idea of the independent creation of species. Haeckel launched into the scientific world, under the name of "fundamental biogenetic law," a theory which, without having the right to the title of an immutable dogma, explains the facts in a general way, and gives us a guiding line along the phylogenetic history of living beings. "Ontogeny," that is the history of the embryological development of each individual, always consists in a summary and fragmentary repetition of phylogeny, or the history of the ancestors of the species to which the individual belongs. This signifies that, as embryos, we repeat in an abridged form the series of types or morphological stages through which has passed the series of our animal ancestors, from the primitive cell to man. In reality this is only true in a relative way, for a considerable part of the ancestral engraphias of the embryo has disappeared without leaving any trace; also many embryos, especially those which have special conditions of existence outside the body of their mother, have acquired special complex organs and corresponding functions. Thus, the caterpillars of butterflies with their specific and generic peculiarities, hairs, horns, etc., furnish many examples of secondary acquired characters which have nothing in common with the worm, which is the ancestral type of the butterfly represented by the embryonic period when it is a caterpillar. However, many undoubted vestiges of the ancestral history are found in the embryos at different periods of their development. It is certain that insects descended from worms, and there is no doubt that the larvæ of insects, which are almost worms, represent the ontogenetic repetition of the phylogeny of insects.
It is also certain that whales, although they have whalebone instead of teeth, have descended from cetacea provided with teeth, which in their turn descended from terrestrial mammals. But we find in the embryo whale a complete denture which is of no use to it, and which disappears in the course of the embryonic period. This denture is nothing else than a phylogenetic incident in the ontogeny of the whale.