94 THE LAW OF BIRTHS AND DEATHS 



favourable conditions, but both show that they still 

 remain subject to the law by becoming infertile when 

 the conditions become loo favourable. As plants can 

 be rendered sterile by high cultivation, so animals may 

 be rendered sterile by high breeding. But there is some 

 difference in the facts affecting the last case as between 

 plants and animals. 



In the case of asexually propagated plants the action 

 of the principle is not antagonistic to the effects of selec- 

 tion, whilst those plants which are propagated by seed 

 are not, as a rule, selected for qualities which are peculiarly 

 antagonistic to fertility. But among domesticated animals 

 there is no such thing as asexual propagation, and the 

 animals are often selected for those very qualities which 

 are most antagonistic to fertility, the qualities, in fact, 

 most intimately associated with nervous energy. 



In the case of the race-horse, for example, we see two 

 antagonistic tendencies at work. The most fertile mares 

 will, of course, produce the most young, but the owner 

 will desire to secure foals from the most successful racers, 

 and will make special efforts to do so. The effects of 

 breeding for speed will in some measure be counteracted 

 by the selection of the most fertile mares ; but in the 

 case of asexually propagated plants the principle has full 

 play. So in the one case the result is merely diminished 

 fertility, and in the other almost complete sterility. 



Darwin mentions a number of peculiar and apparently 

 capricious facts, but nothing inconsistent with the principle. 

 While some species are rendered almost completely sterile 

 in captivity, others breed freely under the most unnatural 

 conditions. This latter result is probably due to some 

 peculiarity of constitutional organisation. Thus we know 

 that some individuals are powerfully affected by changes 

 of climate and others are not. One individual may be 



