vii ON THE INFERTILITY OF CROSSES 161 



breeding by securing several hybrids from quite distinct 

 stocks to start with, and by having two or more sets of experi- 

 ments carried on at once, so that crosses between the hybrids 

 produced may be occasionally made. Till this is done no 

 experiments, such as those hitherto tried, can be held to prove 

 that hybrids are in all cases infertile inter se. 



It has, however, been denied by Mr. A. H. Huth, in his 

 interesting work on The Marriage of Near Kin, that any 

 amount of breeding in-and-in is in itself hurtful ; and he quotes 

 the evidence of numerous breeders whose choicest stocks have 

 always been so bred, as well as cases like the Porto Santo 

 rabbits, the goats of Juan Fernandez, and other cases in which 

 animals allowed to run wild have increased prodigiously and 

 continued in perfect health and vigour, although all derived 

 from a single pair. But in all these cases there has been 

 rigid selection by which the weak or the infertile have been 

 eliminated, and with such selection there is no doubt that the 

 ill effects of close interbreeding can be prevented for a long 

 time ; but this by no means proves that no ill effects are pro- 

 duced. Mr. Huth himself quotes M. Allie, M. Aube, Stephens, 

 Giblett, Sir John Sebright, Youatt, Druce, Lord "Weston, and 

 other eminent breeders, as finding from experience that close 

 interbreeding does produce bad effects ; and it cannot be 

 supposed that there would be such a consensus of opinion 

 on this point if the evil were altogether imaginary. Mr. 

 Huth argues, that the evil results which do occur do not 

 depend on the close interbreeding itself, but on the tendency 

 it has to perpetuate any constitutional weakness or other 

 hereditary taints ; and he attempts to prove this by the argu- 

 ment that " if crosses act by virtue of being a cross, and not 

 by virtue of removing an hereditary taint, then the greater the 

 difference between the two animals crossed the more beneficial 

 will that act be." He then shows that, the wider the difference 

 the less is the benefit, and concludes that a cross, as such, has 

 no beneficial effect. A parallel argument would be, that change 

 of air, as from inland to the sea-coast, or from a low to an 

 elevated site, is not beneficial in itself, because, if so, a change 

 to the tropics or to the polar regions should be more beneficial. 

 In both these cases it may well be that no benefit would 

 accrue to a person in perfect health ; but then there is no 



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