It is impossible to believe that a law which holds good for the rest of the animal kingdom, as well as for plants, does not apply to man also. But it is difficult to adduce direct evidence for the evil effects of consanguineous marriages. We cannot expect very conspicuous results from other alliances than those between the nearest relations—between brothers and sisters, parents and children. And the injurious results even of such unions would not necessarily appear at once. Sir J. Sebright remarks that there may be families of domestic animals which go through several generations without sustaining much injury from having been bred in-and-in,[1996] and the offspring of self-fertilized plants do not always show any loss of vigour in the first generations. Man cannot, in this respect, be subjected to experiments like those tried in the case of other animals, and habitual intermarriage of the very nearest relations is, as we have seen, exceedingly rare. Mr. Adam argues that there is no proof of the physical deterioration of those divisions of mankind amongst whom incestuous unions are known more or less to have prevailed—as the Egyptians and Persians.[1997] But among these nations marriage certainly did not always take place between closely related persons; and breeders of domestic animals inform us that the mixing-in even of a drop of unrelated blood is sufficient almost to neutralize the injurious effects of long continued close interbreeding. Again, Mr. Huth asserts that, though the Ptolemies habitually married their sisters, nieces, and cousins, they were neither sterile nor particularly short-lived.[1998] Mr. Galton, on the contrary, sees in Ptolemaic experience a proof that close intermarriage is followed by sterility.[1999] In ten marriages between brothers and sisters, uncles and nieces, or between first-cousins, the average number of children was not quite two, and three of the unions were entirely sterile.[2000]