INDEFINITELY FROM THE ORIGINAL TYPE. 35 



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 could inhabit. 

 It is also evident that most changes would affect, 

 either favourably or adversely, the powers of pro- 

 longing 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 would sooner or later be 

 affected in its powers of procuring a regular supply 

 of food ; and in both cases the result must neces- 

 sarily 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 su- 

 periority. Now, let some alteration of physical 

 conditions occur in the district a long period of 

 drought, a destruction of vegetation by locusts, the 



D 2 



