The monogenists, who have based the demonstration of the unity of the human species upon the physiological character of the prolificacy of the cross-breeds, have not taken into account these elements. They have confined themselves to the assertions that all human races can produce cross-breeds, and that all these breeds are prolific. Now, admitting for a moment that these assertions are exact, the conclusion they have drawn from them is still contestable, until they can demonstrate that the study of these cross-breeds reveals no genital inequality between the parent races. But what becomes of their argumentation, if it be proved that all intermixtures are not eugenistic, that is to say, that certain mongrels are not between themselves indefinitely prolific; that other cross-breeds become sterile in the first generation; and, finally, that certain races are so little homogenesic, that the birth of cross-breeds of the first degree is more or less exceptional? If one of these propositions can be effectually established, the monogenists would have little cause to congratulate themselves for having appealed to physiology. They would, on the contrary, have furnished their adversaries with deadly weapons, and their doctrine would be demolished on the battle field they have themselves chosen. The facts I intend to exhibit tend to prove that it was a great error to consider all intermixtures of men as eugenesic. Obliged as I am to refer to testimonies which, perhaps, do not always exhibit a desirable precision, some doubts may hover over my conclusion; this much, however, will result from this sketch, that the examination of the laws of hybridity is far from being favourable to the doctrine of monogenists. We shall study the cross-breeds both in relation to their fecundity and their physical and moral validity; for, from our point of view, it is sufficient to prove that certain cross-breeds are inferior to the parent races, as regards longevity, vigour, health, and intelligence, to render it very probable that the two races are not of the same species. When a monogenist is called upon to demonstrate that all human intermixtures are eugenesic, the first example which he ordinarily cites is that of the Mulattoes in America, the issue of the union of European colonists and African negresses. This example, which has for a long time been considered as decisive, might not be without a reply; for there exist races differing much more from us than the races of the western coast of Africa; but the question here is, whether it be quite true that all American Mulattoes are eugenesic.