180 THE WORLD OF LIFE 



are examples of such rapid increase. But as soon as the is- 

 land is fully stocked, a number equal, or nearly so, to the 

 annual increase must die off every year, and these will inevi- 

 tably be the least fitted to survive. Hence natural selection 

 at once begins to act, and as the conditions, even in two 

 adjacent islands, are never quite the same, and as with such 

 a large population slight variations in many directions will 

 be very numerous, some modification to a more perfectly 

 adapted form will necessarily follow. Here comes the point 

 which both critics failed to notice, that the modification of 

 the species into a better-adapted one must have occurred in 

 the island ; and as it is universally admitted that intercrossing 

 between the incipient species and the parent stock would be a 

 serious check to adaptation ; and further, that varieties of the 

 higher animals prefer to mate with their like, then any varia- 

 tion of colour in those better adapted will be advantageous, 

 will lead to more rapid change, and will thus come to charac- 

 terise the new form as distinguished from that of the less- 

 adapted parental form. 



It is clear, therefore, that species which are now peculiar 

 to some island or other restricted locality, even when thev 

 are quite unlike anything else now living around them, must 

 have become differentiated from some parent stock just in the 

 same way as all other species have become differentiated. 

 During all the initial stages, w^hich may have occupied scores 

 or hundreds of generations, some outward sign of the struc- 

 tural change that was taking place was an essential part of 

 the process, as a means of checking interbreeding with the less- 

 modified parental form, which might linger on till the process 

 was almost completed. Now, the distinctive recognition-mark 

 seems to us to have no use ; but as the original form from the 

 adjacent island A may still occasionally visit or be driven to 

 the island B, it would now be treated as a stranger, and thus 

 prevent the better-adapted form being deteriorated by inter- 

 breeding with the less-adapted immigTant. 



