80 DIVERGENCE OF CHARACTER. [CHAP. IV. 



is that the number of species throughout the world has not become 

 immeasurably great. 



We have seen that the species which are most numerous in 

 individuals have the best chance of producing favourable varia- 

 tions within any given period. We have evidence of this, in the 

 facts stated in the second chapter, showing that it is the common 

 and diffused or dominant species which offer the greatest number 

 of recorded varieties. Hence, rare species will be less quickly 

 modified or improved within any given period ; they will conse- 

 quently be beaten in the race for life by the modified and im- 

 proved descendants of the commoner species. 



From these several considerations I think it inevitably follows, 

 that as new species in the course of time are formed through 

 natural selection, others will become rarer and rarer, and finally 

 extinct. The forms which stand in closest competition with those 

 undergoing modification and improvement, will naturally suffer 

 most. And we have seen in the chapter on the Struggle for 

 Existence that it is the most closely-allied forms, varieties of the 

 same species, and species of the same genus or of related genera, 

 which, from having nearly the same structure, constitution, 

 and habits, generally come into the severest competition with each 

 other ; consequently, each new variety or species, during the pro- 

 gress of its formation, will generally press hardest on its nearest 

 kindred, and tend to exterminate them. We see the same process 

 of extermination amongst our domesticated productions, through 

 the selection of improved forms by man. Many curious instances 

 could be given showing how quickly new breeds of cattle, sheep, 

 and other animals, and varieties of flowers, take the place of older 

 and inferior kinds. In Yorkshire, it is historically known that 

 the ancient black cattle were displaced by the long-horns, and that 

 these " were swept away by the short-horns " (I quote the words 

 of an agricultural writer) " as if by some murderous pestilence," 



Divergence of Character. 



The principle, which I have designated by this term, is of high 

 importance, and explains, as I believe, several important facts. 

 In the first place, varieties, even strongly-marked ones, though 

 having somewhat of the character of species as is shown by the 

 hopeless doubts in many cases how to rank them yet certainly 

 differ far less from each other than do good and distinct species. 

 Nevertheless, according to my view, varieties are species in the 

 process of formation, or are, as I have called them, incipient species. 

 How, then, does the lesser difference between varieties become aug- 

 mented into the greater difference between species ? That this does 

 habitually happen, we must infer from most of the innumerable 

 species throughout nature presenting well-marked differences; 



