Page:Darwinism by Alfred Wallace 1889.djvu/184

162 such thing as "perfect health" in man, and probably no such thing as absolute freedom from constitutional taint in animals. The experiments of Mr. Darwin, showing the great and immediate good effects of a cross between distinct strains in plants, cannot be explained away; neither can the innumerable arrangements to secure cross-fertilisation by insects, the real use and purport of which will be discussed in our eleventh chapter. On the whole, then, the evidence at our command proves that, whatever may be its ultimate cause, close interbreeding does usually produce bad results; and it is only by the most rigid selection, whether natural or artificial, that the danger can be altogether obviated.

One or two more cases of fertile hybrids may be given before we pass on to the corresponding experiments in plants. Professor Alfred Newton received from a friend a pair of hybrid ducks, bred from a common duck (Anas boschas), and a pintail (Dafila acuta). From these he obtained four ducklings, but these latter, when grown up, proved infertile, and did not breed again. In this case we have the results of close interbreeding, with too great a difference between the original species, combining to produce infertility, yet the fact of a hybrid from such a pair producing healthy offspring is itself noteworthy.

Still more extraordinary is the following statement of Mr. Low: "It has been long known to shepherds, though questioned by naturalists, that the progeny of the cross between the sheep and goat is fertile. Breeds of this mixed race are numerous in the north of Europe." Nothing appears to be known of such hybrids either in Scandinavia or in Italy; but Professor Giglioli of Florence has kindly given me some useful references to works in which they are described. The following extract from his letter is very interesting: "I need not tell you that there being such hybrids is now generally accepted as a fact. Buffon (Supplements, tom. iii. p. 7, 1756) obtained one such hybrid in 1751 and eight in 1752. Sanson (La Culture, vol. vi. p. 372, 1865) mentions a case observed in the Vosges, France. Geoff. St. Hilaire (Hist. Nat. Gén. des reg. org., vol. iii. p.