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 
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