200 MISCELLANEOUS OBJECTIONS TO THE 



have been much more favorable than others for the develop- 

 ment 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 coadapted. 

 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 deter- 

 mined 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 litfle, or 

 will be greatly retarded, in modifying any particular struc- 

 ture for obtaining food. Lastly, natural selection is a slow 

 process, and the same favorable 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 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 inter- 

 fered 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 bafcs 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 j for the former 



