We meet, first, with this fact, namely, the union of the Negro with a white woman is frequently sterile, whilst that of a white man with a negress is perfectly fecund. This might tend to establish between these two races a species of hybridity analogous to that existing between goats and sheep, which we have termed _unilateral hybridity_. Professor Serres, fully alive to the gravity of this fact has given the following explanation: “One of the characters of the Ethiopian race[24] consists in the length of the penis compared with that of the Caucasian race. This dimension coincides with the length of the uterine canal in the Ethiopian female, and both have their cause in the form of the pelvis in the Negro race. There results from this physical disposition, that the union of the Caucasian man with an Ethiopian woman is easy and without any inconvenience for the latter. The case is different in the union of the Ethiopian with a Caucasian woman, who suffers in the act, the neck of the uterus is pressed against the sacrum, so that the act of reproduction is not merely painful, but _frequently non-productive_.” This explanation, though based upon an anatomical character perfectly correct, is yet far from being satisfactory; but we have quoted it here to show that one of the two most eminent monogenists of our epoch has admitted as a perfectly authentic fact, that the union of Caucasian women with Negroes is very frequently non-productive. Mr. Theodore Waitz, author of a scientific treatise on Anthropology (the first volume is entirely devoted to the study of general doctrines), has carefully examined the question of the intermixture of races, and endeavoured to reconcile the results of these crossings with the system of monogenists. He was, nevertheless, obliged to admit, from the numerous documents collected, that in many cases the cross-breeds are feebly constituted, Thus, in Senegal the offspring of the Foulahs and the Negroes are handsome and more intelligent than the latter, but there are amongst them many stammerers, blind, hunchbacks, and idiots. The children of Arabs and the women of Darfour are debilitated and little vivacious, and the author adds _that the children of a European woman and a Negro are rarely vigorous_.[25] It seems thus to result from these various investigations, that the union between the Negro and a white woman is little productive, and that their offspring is neither vigorous nor vivacious. Nevertheless, we admit this conclusion with some reserve, because the avowed unions of Negroes with white women are comparatively rare, and consequently the authors who have spoken of them could only have their inferences upon a few facts. The inverse intermixture between the white man and the negress is, on the contrary, very frequent, and as prolific in the first generation as in the direct alliances between individuals of the same race.