The Making of Species 



still more so in 4, etc., etc. Thus it was possible, 

 as Mr Gulick says, roughly to estimate the 

 amount of divergence between the occupants of 

 any two given valleys by measuring the number 

 of miles between them. . . . The variations 

 which affect scores of species, and themselves 

 eventually run into fully specific distinctions, are 

 all more or less finely graduated as they pass 

 from one isolated region to the next ; and they 

 have reference to changes of form or colour, 

 which in no one case presents any appearance 

 of utility." 



Hitherto three different attempts have been 

 made to explain this and allied phenomena : 



1. That it is the result of isolation. 



2. That it is the result of natural selection. 



3. That it is the result of the action of the 



environment on the organism. 



Let us consider these in inverse order. 



In the case of some organisms, more especially 

 plants, invertebrates, and fish, the environment 

 does exert a direct influence on their colouration. 

 But, as we have seen, the changes in colour, etc., 

 thus induced appear never to be transmitted to 

 the offspring of the organisms so affected. They 

 disappear when the offspring are removed to 

 other surroundings. 



On the other hand, local races or species 

 as, for example, the white-cheeked variety of 

 sparrow found in India usually retain their 



376 



