168 MISCELLANEOUS OBJECTIONS TO THE [CHAP. 



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 favourable con- 

 ditions 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 

 advanced 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 1 ? 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 

 from tree to tree, like the so-called flying squirrels, for the sake of 

 escaping from their enemies, or for avoiding falls ; but when the 

 power of true flight had once been acquired, it would never be 

 reconverted back, at least for the above purposes, into the less 

 efficient power of gliding through the air. Bats might, indeed, 

 like many birds, have had their wings greatly reduced in size, or 

 completely lost, through disuse; but in this case it would be 

 necessary that they should first have acquired the power of 



