180 MISCELLANEOUS OBJECTIONS TO THE [Chap. VII 



some species are much more variable than otters. 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 ap- 

 parently 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, &c, — 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 selec- 

 tion is a slow process, and the same favourable conditions must 

 long endure in order that any marked effect should thus be pro- 

 duced. Except by assigning such general and vague reasons, we 

 caunot explain why, in many quarters of the world, hoofed quadru- 

 peds 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 advanced 

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

 one3 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 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 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 colonise and abound on most oceanic islands. Gradations 

 of structure, with each stage beneficial to a changing species, will 

 be favoured only under certain peculiar conditions. A strictly 

 terrestrial animal, by occasionally hunting for food in shallow 

 water, then in streams or lakes, might at last be converted into an 

 animal so thoroughly aquatic as to brave the open ocean. But seals 

 would not find on oceanic islands the conditions favourable to their 

 gradual reconversion into a terrestrial form. Bats, as formerly 

 shown, probably acquired their wings by at first gliding through 

 the air frcm tree to tree, like the so-called flying-squirrels, 



