CriAP. IV. EXTINCTION BY NATURAL SELECTION. 109 



decrease and l^eooinc rare. Rarity, as geolo<T:y tells us, is the 

 precursor to e.vtinclion. We can, also, see that any form rep- 

 resented l>y few individuals will, during fluctuations in the sea- 

 sons or in the numlxir of its enemies, run a good chance of 

 utter extinction. But we may go further than this ; for, as 

 new forms are continually and slowly being produced, unless 

 we believe that tlie mnnber of specific forms goes on perpet- 

 ually and almost indetinitely increasing, many inevitably must 

 become extinct. That the number of speciiic forms has not 

 indcHnitely increased, geology tells us plainly ; and we shall 

 presently attempt to show why it is that the number of species 

 throughout the world has not become immeasurably great. 



AW; have seen tliat the species which are most numerous in 

 individuals have the best chance of producing favorable 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 species which offer the greatest number of recorded 

 varieties, or incipitnit species. Hence, rare species will be less 

 quickly moditieil or improved within any given period, and 

 they will consequently be beaten in the race for life by the 

 modified descendants of the commoner species. 



From the sesevcral considerations I think it inevitably fol- 

 lows, 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 Avhich stand in closest compe- 

 tition with those imderg^oing modification and improvement, 

 will naturally suffer most. And we have seen in the chapter 

 nn 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 

 tlie severest competition with each other. Consequently, eacli 

 new variety or species, during the progress of its formation, 

 will generally press hardest on its nearest kindred, and tcTid to 

 exterminate them. We see tlie same process of extermination 

 among our domesticated productions, through the selection of 

 improved forms by man. Many curious instances could l)e 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 Avriter) "as if by some murderous pes- 

 tilence." 



