n] FEATURES OF ENVIRONMENT 59 



Summary of the Influence of Environment. 



Having reviewed the main features of the faunas 

 characteristic of forests, deserts and high mountains, 

 each of which may be taken as intensified samples of 

 environment, we may draw some conclusions. 



Since it is obvious that typical desert specialists 

 cannot live in forests, and as high mountains also 

 possess inhabitants of their own, it follows that these 

 various faunas can have attained their present, often 

 widely scattered, discontinuous range only under one 

 of the two following conditions. Either the respective 

 faunas had a multiple origin, and are instances of 

 convergent evolution where similar environmental 

 conditions prevailed ; these cases are of enormous 

 biological importance. Or, if the respective creatures 

 are truly blood-related, above all if they belong to 

 the same genera or species, they must have spread 

 from a common centre. This would imply continuous 

 forests for some, uninterrupted deserts for others, 

 extending perhaps all over the world, not necessarily 

 at the same time, but forest succeeding desert, and 

 arid highlands taking the place of lowland swamps. 

 The mere emergence of a land-bridge would help no 

 monkey across, unless or until it was covered with 

 suitable vegetation. 



If the specialised features of the dwellers in forests 

 and deserts had arisen from preformed latent variations, 



