6304 



Tendency of Species 



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. More 

 important changes, such as an increase in the power or dimensions of 

 the limbs or any of the external organs, would more or less affect 

 their mode of procuring food or the range of country which they 

 inhabit. 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 Carnivora; the passenger pigeon with less 

 powerful wings w^ould sooner or later be affected in its powers of pro- 

 curing 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, intemperance, or scarcity of food pro- 

 duce an increased mortality. In both cases there may be many indi- 

 vidual 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 popu- 

 lation 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 complete extermination ; it is evident that, of all the indi- 

 viduals composing the species, those forming the least numerous and 

 most feebly organized variety would suffer first, and, w^ere the pressure 

 severe, must soon become extinct. The same causes continuing in 

 action, the parent species would next suffer, w^ould 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. 



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 to secure its safety, and to pro- 

 long its individual existence and that of the race. Such a variety could 



