Accident, without rule or law, when invoked as the immediate cause of special transmutations, is obviously incapable of explaining this important fact; it gives no explanation whatever of the generality of fundamental types, and of the direct or lateral affinities which exist between their derivatives. It is different with the theories which favour gradual transmutation. They deal with all these important questions, and give a more or less plausible solution of them. They start from a certain number of principles whose consequences more or less explain the whole question and many of its details. In a word, they constitute genuine doctrines and it is but natural that they should have gained a certain number of adherents. Unfortunately these theories all have the same radical fault. They agree with a certain number of important facts, connected essentially with the morphology of beings; but they are in direct contradiction with the fundamental phenomena of general physiology, which are no less general or fixed than the former. This contradiction is not evident at first sight. This is the reason why these doctrines have influenced not only the public at large, but even men of the highest intellect, whose sole error consists in their having allowed themselves to consider one side of the question only. All these theories have been consolidated into the doctrine which rightly bears the name of Darwin. At the hands of this illustrious naturalist, the hypothesis of gradual transmutation has assumed a force and appearance of truth which it never possessed before. Doubtless, long before Darwin, Lamarck had formulated his _law of heredity_ and his _law of development of organs_, to which the English naturalist has added nothing; M. Naudin had compared _natural selection_ to _artificial selection_; Etienne Geoffroy St.-Hilaire had promulgated the principle of the _balance of organs_; Serres and Agassiz had recognized in embryogenic phenomena the representation of the genesis of beings. But by taking as a starting point the _struggle for existence_; by explaining in this manner _selection_; by fixing the results of _heredity_; by replacing the _pre-established laws_ of Lamarck by the laws of _divergence_, _continuity_, _permanent characters_ and of _finite heredity_; by giving by these means an explanation of the _adaptation_ of beings to all the conditions of existence, the _expansive power_ of some, the _localisation_ of others, the successive _modifications_ of all, under the dominion of the _laws of compensation_, _economy_ and of _correlation of increase_; by applying these facts to the past, present and future of animate creation, Darwin has formed a complete and systematic theory, the whole, and often the details, of which it is impossible not to admire.