NATURAL SELECTION 



habit. It is also evident that most changes would affect, 

 either favourably or adversely, the powers of prolonging 

 existence. An antelope with shorter or weaker legs must 

 necessarily suffer more from the attacks of the feline carni- 

 vora ; the passenger pigeon with less powerful wings would 

 sooner or later be affected in its powers of procuring a regular 

 supply of food ; and in both cases the result must necessarily 

 be a diminution of the population of the modified species. 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. These results must follow as surely as old age, in- 

 temperance, or scarcity of food produce an increased mortality. 

 In both cases there may be many individual exceptions : but 

 on the average the rule will invariably be found to hold good. 

 All varieties will therefore fall into two classes those which 

 under the same conditions would never reach the population 

 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 fresh 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 complete extermination, it is evident that, of all the 

 individuals composing the species, those forming the least 

 numerous and most feebly organised 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 circumstances would 

 rapidly increase in numbers and occupy the place of the 

 extinct species and variety. 



Superior Varieties will ultimately Extirpate the original Species 



The variety would now have replaced the species, of which 

 it would be a more perfectly developed and more highly 

 organised form. It would be in all respects better adapted 



