90 THE ORIGIN OF SPECIES 



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 sub- 

 ject 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 gen- 

 eral economy of any land, the more widely and perfectly the ani- 

 mals 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 organization but little 

 diversified, could hardly compete with a set more perfectly diver- 

 sified 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-developed 

 orders. In the Australian mammals, we see the process of diver- 

 sification in an early and incomplete stage of improvement. 



THE PROBABLE EFFECTS OF THE ACTION OF NATURAL SELECTION 

 THROUGH DIVERGENCE OF CHARACTER AND EXTINCTION, ON THE 

 DESCENDANTS OF A COMMON ANCESTOR 



After the foregoing discussion, which has been much compressed, 

 we may assume that the modified descendants of any one species 

 will succeed 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 selection and of extinction, tends to act. 



The accompanying diagram will aid us in understanding this 

 rather perplexing subject. Let A to L represent the species of a 

 genus large in its own country; these species are supposed to re- 

 semble each other in unequal degrees, as is so generally the case 

 in nature, and as is represented in the diagram by the letters stand- 

 ing at unequal distances. I have said a large genus, because, as we 

 saw in the second chapter, on an average more species vary in large 

 genera than in small genera; and the varying species of the large 

 genera present a greater number of varieties. We have, also, seen 

 that the species, which are the commonest and most widely dif- 

 fused, vary more than do the rare and restricted species. Let (A) 

 be a common, widely-diffused, and varying species, belonging to a 

 genus large in its own country. The branching and diverging dotted 

 lin^s of unequal lengths proceeding from (A) may represent its 

 varying offspring. The variations are supposed to be extremely 



