But these two fundamental propositions, admitted as axioms, do they express the truth? Can this triumphant syllogism, of which they are the premises, stand? Is it true that only animals of the same species can produce a prolific progeny? Is it true that all human crossings are eugenesic? To upset the syllogism of the monogenists, and to deprive their system of any scientific base, it might be sufficient that the first of the above questions should be answered in the negative. The system would then become what it was before it came in contact with science, namely, a belief more or less respectable, founded upon a sentiment or a dogma. But if the second question were also negatived, and it could be demonstrated that all human crossings are not eugenesic, then not merely the syllogism, but the whole doctrine of the monogenists would crumble to pieces. The doctrine would then not merely be extra-scientific, but anti-scientific; it being positive that two groups of animals, so different as to be incapable of fusion by generation, do not belong to the same species. This is an incontestable and uncontested truth. We were thus led to examine successively the two fundamental propositions serving as a base to the unitarian doctrine, for which purpose a series of researches were requisite. We have, in the first place, investigated the results of certain crossings between animals of incontestably different species, such as dogs and wolves, goats and sheep, camels and dromedaries, hares and rabbits, etc.; and we have demonstrated _that these crossings produce eugenesic mongrels, that is to say, perfectly and indefinitely prolific between themselves_. It is thus not true that all animals capable of producing an eugenesic progeny are of the same species; and even if all human intermixtures were eugenesic, as is generally believed, we could not infer from this the unity of the human species. The monogenists are thus deprived of their principal basis and their sole scientific argument. It was, however, necessary to inquire, whether this popular axiom, that all human crossings are eugenesic, was a demonstrated truth or a lightly accepted hypothesis, without any verification or control? Such has been the object of our second series of investigations. We recognised at the outset that the monogenists, considering their axiom as self-evident, have made no efforts to establish its correctness, so that, strictly speaking, we might have discarded it. When, contrary to the opinion of several modern authors, we wished to establish that there were really eugenesic intermixtures in the human genus, we found in science assertions without proofs, and we believe that our investigations concerning the mixed populations of France have, in this respect, the merit of novelty. We may be mistaken as to the value of our demonstration; but we venture to assert, that this demonstration is the first that has been attempted.