86 EXTINCTION BY NATURAL SELECTION. [Chap. IV. 



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 progress 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 s»».vie 

 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. Neverthe- 

 less, 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 augmented 

 into the greater difference between species ? That this does habitu- 

 ally happen, we must infer from most of the innumerable species 

 throughout nature presenting well-marked differences ; whereas 

 varieties, the supposed prototypes and parents of future well-marked 

 species, present slight and ill-defined differences. Mere chance, as 

 we may call it, might cause one variety to differ in some character 

 from its parents, and the offspring of this variety again to differ 

 from its parent in the very same character and in a greater degree ; 

 but this alone would never account for so habitual and large 

 a degree of difference as that between the species of the same 

 genus. 



As has always been my practice, I have sought light on this 



