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 between domesticated animals and captive animals. The fact that an animal has become domesticated shows that it finds in captivity a favorable environment, and Düring 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 unfavorable. Geoffroy 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. Düring 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 environment. The New-Zealanders are very infertile, and nearly all the children 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 variation.
It is no more than right, however, to point out that Düring himself gives a different explanation, and attributes the excess of male births under unfavorable conditions to the need for preventing close interbreeding. 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 unfavorable, 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 crossing, or the sexual union of organisms which are not closely related.
As the object of crossing is to secure variability, it is most necessary when variation is needed, that is, when the conditions of life are unfavorable.
