280 MISCELLANEOUS OBJECTIONS TO THE [Chap. TIL 



which hoofed animals are singularly ill-constructed) im- 

 plies greatly increased bulk of body ; and we know that 

 some areas support singularly few large quadrupeds, for 

 instance S. America, though it is so luxuriant ; whilst 

 S. Africa abounds with them to an unparalleled degree. 

 Why this should be so, we do not know ; nor why the 

 later tertiary periods should have been much more 

 favourable for their existence than the present time. 

 Whatever the causes may have been, we can see that 

 certain districts and times would have been much 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 indispens- 

 able 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 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, &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 selection is a slow pro- 

 cess, and the same favourable conditions must long endure 

 in order that any marked effect should thus be produced 



