OBJECTIONS TO THE THEORY OF NATURAL SELECTION 183 



and largely developed, it is almost indispensable that several 

 other parts should be modified and coadapted. Although every 

 part of the body varies slightly, it does not follow that the neces- 

 sary parts should always vary in the right direction and to the 

 right degree. With the different species of our domesticated ani- 

 mals we know that the parts vary in a different manner and de- 

 gree, 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 species. 

 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 be 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, natural 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 browsing on the higher branches of trees. 



Objections of the same nature as the foregoing have been ad- 

 vanced by many writers. In each case various causes, besides the 

 general ones just indicated, have probably interfered with the 

 acquisition through natural selection of structures, which it is 

 thought would be beneficial to certain species. One writer asks, 

 why has not the ostrich acquired the power of flight? But a mo- 

 ment's reflection will show what an enormous supply of food 

 would be necessary 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 inhabited 

 their present homes. Therefore Sir C. Lyell asks, and assigns cer- 

 tain 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 ani- 

 mals; 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 

 on most oceanic islands. Gradations of structure, with each stage 

 beneficial to a changing species, will be favored only under cer- 

 tain peculiar conditions. A strictly terrestrial animal, by occasion- 



