After having rendered, if not quite certain, at least extremely probable, that _certain_ human crossings are eugenesic, we have inquired whether _all_ human crossings are in the same condition. From the documents collected it results, that _certain_ human crossings yield results notably inferior to such as constitute in animals eugenesic hybridity. The whole of the known facts permit us to consider as very probable, that certain human races taken two by two are less homœogenesic; as, for instance, the species of the dog and the wolf. If we are to make any reservation, and leave some doubts upon this conclusion, it is that we cannot admit, without numerous verifications, a fact which definitively demonstrates the plurality of human species; a fact, by the presence of which, all other discussion is rendered superfluous; a fact, finally, of which the political and social consequences would be immense. We cannot too much insist upon drawing the attention of observers upon this subject. But whatever be the result of ulterior researches on human hybridity, it remains well attested that animals of different species may produce an eugenesic progeny, and that consequently we cannot, from the fecundity of human intermixture, however disparate the races may be, draw a physiological argument in favour of the unity of species, even if the fecundity were as certain as it is doubtful. The great problem we have investigated in this essay is one of those which have caused great agitation, and most difficult to approach with a mind unbiassed by any extra-scientific preconception. This was almost inevitable; but science must keep aloof from anything not within its province. There is no faith, however respectable, no interest, however legitimate, which must not accommodate itself to the progress of human knowledge and bend before truth, if that truth be demonstrated. Hence it is always hazardous to mix up theological arguments with discussions of this kind, and to stigmatise in the name of religion any scientific opinion, since, if that opinion, sooner or later gains ground, religion has been uselessly compromised. The unskilful intervention of theologians in astronomical questions (rotation of the earth), in physiology (pre-existence of germs), in medicine (possessions), etc., has formed more infidels than the writings of philosophers. Why should men be placed in the dilemma of choosing between science and faith? And when so many striking examples have placed theologians under the necessity to acknowledge that revelation is not applicable to science, why do they obstinately continue to place the Bible before the wheels of progress?[83]