90 Results of the Action of Chap. IV. 



a crreatoT^mber of individuals be capable of there supporting 

 UZ Zs A set of animals, with their organ, sation but littb 

 diversified, could hardly compete with a set more perfectly ^diversified 

 in structure. It may be doubted, for instance, whether the Austra- 

 lian 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 mam- 

 mals, 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 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 

 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, belonging 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 importance of the principle of 

 benefit derived from divergence of character comes in ; for this will 

 generally lead to the most different or divergent variations (repre- 



