[[p. ccxxvi]] Mr. A. R. Wallace thought the meeting were much indebted to M. Vogt for the eloquent and forcible manner in which he had pointed out the excessive difficulty and complexity of the subject, and the state of ignorance which generally prevails as to what constitutes species. All the facts stated in the paper would, however, go to prove that no two nations could produce fertile offspring, for it might be said that in all instances where fertility existed there had been an influx of new blood. Such problems could not be satisfactorily solved, because it was impossible to make the requisite experiments on men. It might be done with animals, but with men it was a different thing. The only method by which the problem could be solved would be, to introduce into some island women of one race and men of another, and leave them to themselves, taking care that no other races were admitted on the island. But as that could not be done, no evidence could be obtained that was not open to objection. One of the [[p. ccxxvii]] instances alluded to in the paper, as affording evidence against the general fertility of human races, rested on but slight grounds. It was asserted that with the Australians there was great difficulty in producing offspring even at the first cross, and that instances of subsequent fertility are rare. But he had received a communication from a friend, who had recently come from Australia, which contradicted that opinion. He stated that he had known two instances of Australian women having had children by white men and afterwards by men of their own tribe. Numerous cases of the kind, he said, occurred in the bush, in one of which the woman had four children; but the illegitimate children were always destroyed by the chiefs of tribes, which accounted for their scarcity. His friend also mentioned that he had seen half-castes who had children of their own, and his evidence also contradicted the assertion of Count Strzelecki, that Australian women who had lived with Europeans became infertile for their own race. There was the well known case of the Pitcairn islanders, in which the males of one race and females of another race were shipwrecked on the island, and lived together for a long time without communication with other people, and it would be important to know the results.