106 RESULT OF THE ACTION 



compete with a set more perfectly diversified in structure. 

 It may be doubted, for instance, whether the Austrahan 

 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 com- 

 pete with these well-developed orders. In the Australian 

 mammals, we see the process of diversification in an early 

 and incomplete stage of development. 



THE PKOBABLE EFFECTS OF THE ACTION" OF NATURAL 

 SELECTION" THROUGH DIVERGENCE OF CHARACTER 

 AND EXTINCTION, ON THE DESCENDANTS OF A COM- 

 MON 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. ]N^ow 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 sj^ecies 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 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 



