These facts are significant. In no case of crossing between species has fertility been observed to increase; on the contrary it is almost always diminished, and often, as we have seen above, in an immense proportion. Crossings between races have alone presented facts analogous to those mentioned by Hombron and Le Vaillant.
III. Thus, in every case crossings between human groups exhibit the phenomena characteristic of mongrels and never those of hybrids.
Therefore, these human groups, however different they may be, or appear to be, are only races of one and the same species and not distinct species.
Therefore, there is but one human species, taking this term species in the acceptation employed when speaking of animals and plants.
IV. Anyone who refuses to accept these conclusions must either deny all the facts of which it is the necessary consequence, or reject the method employed in the examination and appreciation of these facts.
But these facts are borrowed entirely either from scientific experiments, made without any discussion or controversy by men of the highest authority, or drawn from the innumerable experiments which are daily practised by agriculturalists, horticulturists, and breeders. It is therefore very difficult to deny them.
As to the method, it is evident that it rests entirely upon the identity of the general laws governing all organised and living beings. Few true men of science will, I am sure, refuse to admit such a starting point as this.
Now I wish that candid men, who are free from party-spirit or prejudices, would follow me in this view, and study for themselves all these facts, a few of which I have only touched upon, and I am perfectly convinced that they will, with the great men of whom I am only the disciple,—with Linnæus, Buffon, Lamarck, Cuvier, Geoffroy, Humboldt and Müller, arrive at the conclusion that all men belong to the same species, and that there is but one species of man.