As far as we know, there are no human races who, when intermingled, are entirely sterile. But as regards the degree of fertility of first crosses and of mongrels, the opinions of different anthropologists vary considerably. Those who do not believe in the unity of the human race have been especially solicitous to prove that crosses are almost inevitably followed by bad results in that respect. Thus Dr. Knox thinks that the half-breeds, if they were abandoned to themselves and no longer had access to pure races, would rapidly disappear, the “hybrid” being rejected by nature as a degradation of humanity.[1658] Dr. Nott asserts that, when two proximate species of mankind, two races bearing a general resemblance to each other in type, are bred together, they produce offspring perfectly prolific; but that, when species the most widely separated, such as the Anglo-Saxon and the negro, are crossed, the mulatto offspring are but partially prolific, and acquire an inherent tendency to run out, and become eventually extinct, when kept apart from the parent stocks.[1659] The same opinion is entertained by M. Broca, and by M. Pouchet, who thinks that the crossed race will exist only if it continues to be supported by the two creating types remaining in the midst of it.[1660] On the other hand, Dr. Prichard believes it may be asserted, without the least chance of valid contradiction, that mankind, of all races and varieties, are equally capable of having offspring by intermarriage, and that such connections are equally prolific whether contracted between individuals of the same variety or of the most dissimilar varieties. “If there is any difference,” he says, “it is probably in favour of the latter.”[1661] According to M. Godron, the mongrels have generally shown a higher degree of fertility than their parent races;[1662] and M. Quatrefages asserts that mulattoes are as fruitful as pure breeds.[1663]