THEOBT OF NATURAL SELECTION. 213 



follow that the necessary parts should always vary in the 

 right direction and to the right degree. With the differ- 

 ent species of our domesticated animals we know that the 

 parts vary in a diiferent manner and degree, and that some 

 species are much more variable than others. Even if the 

 fitting variations did arise, it does not follow that natural 

 selection would be able to act on them and produce a 

 structure which apparently would be beneficial to the spe- 

 cies. For instance, if the number of individuals existing 

 in a country is determined chiefly through destruction by 

 beasts of prey — by external or internal parasites, etc. — as 

 seems often to bo the case, then natural selection will be 

 able to do little, or will be greatly retarded, in modifying 

 any particular structure for obtaining food. Lastly, nat- 

 ural selection is a slow process, and the same favorable 

 conditions must long endure in order that any marked 

 effect should thus be produced. Except by assigning such 

 general and vague reasons, we cannot explain why, in 

 many quarters of the world, hoofed quadrupeds have not 

 acquired much elongated necks or other means for brows- 

 irig on the higher branches of trees. 



Objections of the same nature as the foregoing have been 

 advanced by many writers. In each case various causes, 

 besides the general ones just indicated, have probably in- 

 terfered with the acquisition through natural selection of 

 structures, which it is thought would be beneficial to cer- 

 tain species. One writer asks, why has not the ostrich 

 acquired the power of flight? But a moment's reflection 

 will show what an enormous supply of food would be nec- 

 essary to give to this bird of the desert force to move its 

 huge body through the air. Oceanic islands are inhabited by 

 bats and seals, but by no terrestrial mammals; yet as some 

 of these bats are peculiar species, they must have long in- 

 habited their present homes. Therefore Sir C. Lyell asks, 

 and assigns certain reasons in answer, why have not seals 

 and bats given birth on such islands to forms fitted to live 

 on the land? But seals would necessarily be first converted 

 into terrestrial carnivorous animals of considerable size, 

 and bats into terrestrial insectivorous animals; for the 

 former there would be no prey; for the bats ground-insects 

 would serve as food, but these would already be largely preyed 

 on by the reptiles or birds, which first colonize and abound 



