NATURAL SELECTION 1507 



with simple habits. Take the case of a carnivorous 

 quadruped, of which the number that can be sup- 

 ported in any country has long ago arrived at its full 

 average. If its natural power of increase be allowed 

 to act, it can succeed in increasing (the country not 

 undergoing any change in conditions) only by its 

 varying descendants seizing on places at present oc- 

 cupied by other animals : some of them, for instance, 

 being enabled to feed on new kinds of prey, either 

 dead or alive; some inhabiting new stations, climbing 

 trees, frequenting water, and some perhaps becoming 

 less carnivorous. The more diversified in habits and 

 structure the descendants of our carnivorous animals 

 become, the more places they will be enabled to 

 occupy. What applies to one animal will apply 

 throughout all time to all animals that is, if they 

 vary for otherwise natural selection can effect 

 nothing. 



By considering the nature of the plants or animals 

 which have in any country struggled successfully 

 with the indigenes, and have there become natural- 

 ized, we may gain some crude idea in what manner 

 some of the natives would have to be modified in 

 order to gain an advantage over their compatriots; 

 and we may at least infer that diversification of struc- 

 ture, amounting to new generic differences, would be 

 profitable to them. 



The advantage of diversification of structure in 

 the inhabitants of the same region is, in fact, the same 

 as that of the physiological division of labor in the 

 organs of the same individual body a subject so 

 well elucidated by Milne Edwards. No physiologist 



