OBJECTIONS TO THE THEORY 307 



cated 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 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 de- 

 termined 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 par- 

 ticular structure for obtaining food. Lastly, natural se- 

 lection is a slow process, and the same favorable condi- 

 tions must long endure in order that any marked effect 

 should thus be produced. Except by assigning such gen- 

 eral and vague reasons, we cannot explain why, in many 

 quarters of the world, hoofed quadrupeds have not ac- 

 quired 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 natu- 

 ral 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 

 I 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 





