THEORY OF NATURAL SELECTION 233 



more favourable than others for the development of so large 

 a quadruped as the giraffe. 



In order that an animal should acquire some structure 

 specially and largely developed, it is almost indispensable 

 that several other parts should be modified and co-adapted. 

 Although every part of the body varies slightly, it does not 

 follow that the necessary parts should always vary in the 

 right direction and to the right degree. With the different 

 species of our domesticated animals we know that the parts 

 vary in a different manner and degree; and that some species 

 are much more variable than others. Even if the fitting vari- 

 ations 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 deter- 

 mined chiefly through destruction by beasts of prey, — by ex- 

 ternal 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 ob- 

 taining food. Lastly, natural selection is a slow process, and 

 the same favourable 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- 

 ing 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, be- 

 sides the general ones just indicated, have probably inter- 

 fered with the acquisition through natural selection of struc- 

 tures, 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 moment'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 certain rea- 

 sons in answer, why have not seals and bats given birth on 



