84 RESULTS OF THE ACTION OP [CHAP. IV. 



or flesh aitme, 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 per- 

 fectly 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-developed orders. In the Australian mammals, we see 

 the process of diversification in an early and incomplete stage of 

 development. 



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 com- 

 pressed, 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 extinc- 

 tion, tends to act. 



The accompanying diagr^* 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 

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

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

 standing 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 the most 

 widely diffused, vary more than do the rare and restricted species. 

 Let (A) be a common, widely-diffused, and varying species, belong- 

 ing to a genus large in its own country. The branching and 

 diverging dotted lines of unequal lengths proceeding from (A) 

 may represent its varying offspring. The variations are supposed 

 to be extremely slight, but of the most diversified nature ; they 

 are not supposed all to appear simultaneously, but often after 

 long intervals of time; nor are they all supposed to endure for 

 equal periods. Only those variations which are in some way 

 profitable will be preserved or naturally selected. And here the 



* See page 90. 



