390 OUTLINES OF EVOLUTIONARY BIOLOGY 



supply, &c., must diminish in numbers, and, in extreme cases, 

 become altogether extinct." 



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" Most or perhaps all the variations from the typieal form of 

 a species must have some definite effect, however slight, on the 

 habits or capacities of the individuals. Even a change of colour 

 might, by rendering them more or less distinguishable, affect 

 their safety ; a greater or less development of hair might modify 

 their habits. . . . An antelope with shorter or weaker legs must 

 necessarily suffer more from the attacks of the feline carnivora. 

 ... If, on the other hand, any species should produce a variety 

 having slightly increased powers of preserving existence, that 

 variety must inevitably in time acquire a superiority in numbers. 

 ... All varieties will therefore fall into two classes those 

 which under the same conditions would never reach the popula- 

 tion of the parent species, and those which would in time obtain 

 and keep a numerical superiority. Now, let some alteration 

 of physical conditions occur in the district a long period of 

 drought, a destruction of vegetation by locusts, the irruption of 

 some new carnivorous animal seeking ' pastures new ' any 

 change in fact tending to render existence more difficult to the 

 species in question, and tasking its utmost powers to avoid com- 

 plete extermination ; it is evident that, of all the individuals 

 composing the species, those forming the least numerous and 

 most feebly organized variety would suffer first, and, were the 

 pressure severe, must soon become extinct. The same causes 

 continuing in action, the parent species would next suffer, 

 would gradually diminish in numbers, and with a recurrence 

 of similar unfavourable conditions might also become 

 extinct. The superior variety would then alone remain, and 

 on a return to favourable conditions would rapidly increase 

 in numbers and occupy the place of the extinct species and 

 variety. 



" The variety would now have replaced the species, of which it 

 would be a more perfectly developed and more highly organized 

 form. It would be in all respects better adapted to secure its 

 safety, and to prolong its individual existence and that of the 

 race. Such a variety could not return to the original form ; for 

 that form is an inferior one, and could never compete with it 

 for existence. . . . But this new, improved, and populous race 

 might itself, in course of time, give rise to new varieties, 

 exhibiting several diverging modifications of form, any of which, 

 tending to increase the facilities for preserving existence, must, 

 by the same general law, in their turn become predominant. 

 Here, then, we have progression and continued divergence deduced 

 from the general laws which regulate the existence of animals in 



