254 THE COLOUKS OF ANIMALS 



but to increase the perceptions of the predatory 

 species of insects and birds, so that there is a con- 

 tinual progression towards a perfectly mimetic form. 

 This progressive improvement in means of defence 

 and of attack may be illustrated in this way. Suppose 

 a number of not very swift hares and a number of 

 slow-running dogs were placed on an island where 

 there was plenty of food for the hares, but none for 

 the dogs except the hares they could catch ; the slowest 

 of the hares would be first killed, the swifter preserved. 

 Then the slowest-running dogs would suffer, and, 

 having less food than the fleeter ones, would have least 

 chance of living, and the swiftest dogs would be pre- 

 served ; thus the fleetness of both dogs and hares would 

 be gradually but surely perfected by natural selection, 

 until the greatest speed was reached that it was possible 

 for them to attain. I have in this supposed example 

 confined myself to the question of speed alone, but, in 

 reality, other means of pursuit and of escape would 

 come into play and be improved. The dogs might 

 increase in cunning, or combine together to work in 

 couples or in packs by the same selective process ; 

 and the hares, on their part, might acquire means of 

 concealment or stratagem to elude their enemies; 

 but, on both sides, the improvement would be pro- 

 gressive until the highest form of excellence was 

 reached. Viewed in this light, the wonderful perfec- 

 tion of mimetic forms is a natural consequence of the 

 selection of the individuals that, on the one side, were 



