NATURAL SELECTION 105 



By considering the nature of the plants or animals 

 which have struggled successfully with the indigenes 

 of any country, and have there become naturalised, 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 the other natives ; and we may at least 

 safely infer that diversification of structure, amount- 

 ing to new generic differences, would be profitable to 

 them. 



The advantage of diversification in the inhabitants 

 of the same region is, in fact, the same as that of the 

 physiological division of labour in the organs of the 

 same individual body — a subject so well elucidated by 

 Milne Edwards. No physiologist doubts that a stomach 

 adapted to digest vegetable matter alone, or flesh alone, 

 draws most nutriment from these substances. So in 

 the general economy of any land, the more widely and 

 perfectly the animals and plants are diversified for 

 different habits of life, so will a greater number of 

 individuals be capable of there supporting themselves. 

 A set of animals, with their organisation but little 

 diversified, could hardly compete with a set more 

 perfectly diversified in structure. It may be doubted, 

 for instance, whether the Australian marsupials, which 

 are divided into groups differing but little from each 

 other, and feebly representing, as Mr. Waterhouse and 

 others have remarked, our carnivorous, ruminant, and 

 rodent mammals, could successfully compete with these 

 well-pronounced orders. In the Australian mammals, 

 we see the process of diversification in an early and 

 incomplete stage of development. 



After the foregoing discussion, which ought to have 

 been much amplified, we may, I think, assume that the 

 modified descendants of any one species will succeed by 

 so much the better as they become more diversified in 

 structure, and are thus enabled to encroach on places 

 occupied by other beings. Now let us see how this 

 principle of benefit being derived from divergence of 

 character, combined with the principles of natural selec- 

 tion and of extinction, will tend to act. 



