INFLUENCES DETERMINING SEX. 329 



ism tends to remain stationary as long as no change is needed, and 

 to vary when variation is demanded. 



That this is the true view is shown, I think, by the contrast be- 

 tween domesticated animals and captive animals. The fact that an 

 animal has become domesticated shows that it finds in captivity a fa- 

 vorable environment, and Daring says that domesticated animals are 

 unusually fertile, and that they produce an excess of females. Animals 

 which are kept as captives in menageries and gardens have, as a rule, 

 no fitness for domestication, and their conditions of life are unfavor- 

 able. Geoffrey Saint-Hilaire says that individuals born in menageries 

 are usually male, while skins sent to museums are usually female, and 

 that the attempt to domesticate a wild animal increases the number of 

 male births. Diiring states that captive birds of prey and carnivorous 

 mammals are very infertile, and that the young are nearly always males. 



The wild races of Oceania and America have been suddenly 

 brought into contact with the civilization which has been, in Europe, 

 the slow growth of thousands of years. Food and climate have not 

 changed, but a new element has been introduced into their environ- 

 ment. The New-Zealanders are very infertile, and nearly all the chil- 

 dren are boys, and the census of 1872 for the Sandwich Islands gave a 

 ration of 125 male births to 100 female births. 



I believe that we may see, in these instances, the last effort of 

 Nature to save the race from extinction, by securing a favorable varia- 

 tion. 



It is no more than right, however, to point out that During himself 

 gives a different explanation, and attributes the excess of male births 

 under unfavorable conditions to the need for preventing close inter- 

 breeding. He shows that close interbreeding causes sterility, small 

 size, and lack of general vigor and vitality ; and he also shows that 

 these effects are most marked when the other conditions of life are un- 

 favorable, and that no evil effect follows close interbreeding when 

 food is very abundant and the environment in general conducive to 

 prosperity. As the evil effects of interbreeding are most marked when 

 the environment is unfavorable, and as male births are then in excess, 

 he believes that the excessive production of males is an adaptation, 

 which has been gradually acquired for the purpose of preventing close 

 interbreeding at the time when it is injurious. 



I believe that a little examination will show that this explanation 

 is imperfect, although true in a certain sense. As natural selection 

 can not act in such a way as to establish an injurious property, the 

 evil effects of interbreeding can not be primary. The thing which is 

 advantageous and which has been secured by natural selection is cross- 

 ing, or the sexual union of organisms which are not closely related. 



As the object of crossing is to secure variability, it is most neces- 

 sary when variation is needed, that is, when the conditions of life are 

 unfavorable. 



