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Show 162 DARWINISM CHAP. such thing as "perfect health" in m~n, ~nd pr~bab.ly no. such thin<r as absolute freellom from const1tutwnn.l t:tmt m ammals. The t> experiments of Mr. Darwin, showing. t~1e grea~ a~d immediate o-ood effects of a cross between chstmct strams m pbnts, can;ot be explained away; neither can.the innumerable arrano-ements to secure cross-fertilisation by msects, the real use a~d purport of which will be discussed in our eleventh chapter. On the whole, then, the ev~dence at our com~and proves that, whatever may be its ultmmto cause; c!ose mterbree<. ling 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. Fertile Ifybrids among Animals. One or two more cases of fertile hybrids may be given before we pass on to the corresponding experiments in pla.nts. 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 dm:klings, 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 tho progeny of the cross between the sheep and goat is fertile. Breeds of this mixed race arc numerous in the north of Europe." I Nothing appears to he l:nown of such hybrids either in Scandinavia or in Italy; lmt Professor Giglioli of Florence has kindly given me some u.·efnl 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 aH a fact. Buffon (Supplements, tom. iii. p. 7, 1756) obtained OliO such hybrid in 1751 and eight in 1752. Sanson (La Culture, vol. vi. p. 372, 1865) mentions a case observed in the Vosgm>, France. Geoff. St. Hilaire (lli:it. Nut. Gen. des reg. 01·g., vol. iii. p. 1 Low's Domesticated .Animals, p. 28. VII ON TilE INFERTILITY 0 Jl' CROSSES 163 163) wns the first to mention, I believe, that in different parts of South America the ram is more nsmdly crossed with the she-goat than the sheep with the he-goat. The well-known 'pellones ' of Chile arc produced by the second and third gcn~ration of such hybrids (Gay, 'IIist. de Chile,' vol. i. p. 466, Agncul~u1·e, ~8?2z. Hybrids bred from go~t and s?eep are c~l~ed chabm . m French, and ' cabruno' m pam h. In C. hile' such hybrids arc called 'carnoros lanudos '· their breed- ) m? ~nte1· se appears to be not always successful, and often the ongma~ cross ha. to be recommenced to obtain the proportion of tl.1rce-mghths of he-goat and five-eighths of sheep, or of threemghths of ram and five-eighths of she-goat; such being the reputed best hybrids." \Vith these numerous facts recorded hy competent observers w_e ~an hard~y doubt that races of hybrids between the. c very d1~tmct Sl?CCI~s have been produced, and that such hybrids are fmrly fer~1le ~nter se ,· and the analogous facts already given lead us .to behove tha~ ':hatever amount of infertility may at first exist could be ehmmated by carefnl selection, if the crossed races were bred in large numbers and over a considerable area of country. This case is especially valuable, as showing how careful we .should be in assuming the infertility of hybrids w~en experiments have been made with the progeny of a single pa1r, and 'have been continued only for one or two generations. Among insects one case only appears to have been recorded. The hybrids _of tw? moths (Bombyx cynthia and B. arrindia) were proved m Pans, accordmg to M. Quatrefages, to be fertile inter se for eight generations. Fertility of Hybrids among Plants. .Am~ng plants the cases of fertile hybrids are more numerous, owmg, m part, to tho large scale on which they are grown by gar~leners a~d nurserymen, and to the greater facility with whiCh expenments can be made. Darwin tells us that Kolreuter found ~en_ cases i~ which two ·plants considered by botanists to be chstmct species were quite fertile together, and he therefore ranked them all as varieties of each other. In some cases these were grown for six to ten successive o-enerations but after a time the fertility decreased, as we saw t~ be the ca~e in |