ix RECOGNITION-MARKS 167 



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

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

 ably 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 inter- 

 crossing 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 variation of colour in those better adapted will be 

 advantageous, will lead to more rapid change, and will thus 

 come to characterise 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 they 

 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, which may have occupied scores 

 or hundreds of generations, some outward sign of the 

 structural 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 distinc- 

 tive 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 interbreeding with the 

 less-adapted immigrant. 



Recognition by Butterflies 



This case shows how easy it is to make mistakes or 

 arrive at wrong conclusions, unless we take account of all the 

 details of a problem, and endeavour to follow out the exact 



