i8o PROBLEMS OF EVOLUTION 



species. Natural Selection is the regulating principle that 

 guides the course of evolution ; isolation is an indispensable 

 henchman. And since evolution can make no headway without 

 it, we might expect to find the means of isolation developed to 

 the very utmost. We must be careful, however, ourselves to 

 keep clear of the teleological fallacy into which Romanes seems 

 to have fallen. We must not say that species resort to every 

 possible expedient in order to keep themselves apart, as if 

 divergent evolution were an aim in view. Putting the matter 

 more correctly, we must distinguish two cases. In the first a 

 group, marked by characters that give it an advantage, survives, 

 because protected by isolation : it does not lose its advantages 

 through the swamping effects of intercrossing with individuals 

 not similarly endowed. In the second case there is, perhaps 

 owing to geographical isolation, a separation into groups which 

 differ from each other very little in habits or in structure so far 

 as observation can detect. When any of the allied groups come 

 into contact, the geographical isolation, for whatever reason, 

 ceasing, fusion may follow. On the other hand, the clannish 

 spirit that is often of service quite apart from the question of 

 isolation, may prevent it, and if so, recognition marks and cries 

 will under the new conditions gain in importance. There may 

 also have arisen, if not sterility, yet comparative infertility 

 between the species in question and its allies. Those individuals 

 that fall away, therefore those that have not the distinctive 

 characters well marked or who fail to recognise them in others 

 and, therefore, mate with "foreigners" will, probably, leave 

 fewer offspring. Once a species is formed, all tendency to 

 disintegration is likely to be checked. 



Recogni- All wild animals seem to have some means of recognising 

 tion marks Q^gj-g o f t h e sam e species. We find many groups characterised 

 by distinctive coloration that must make recognition easy and 

 many by special cries that answer a similar purpose. On the 

 subject of recognition marks a great deal has been written by 

 Dr Russel Wallace, and if their importance is under-estimated 

 by many naturalists, it can only be through a failure to realise 

 the conditions to realise that species have survived as species 



