i.] DARWINISM VERIFIED. 17 



thickness of fur modifies the efficiency of the skin as 

 an excreting organ, and thus reacts upon the lungs, 

 liver, and kidneys. But it is not only in these clearly 

 traceable ways that correlation of growth is manifested. 

 Sometimes the correlations are inexplicable. Thus, 

 to lengthen the beak of a pigeon is to increase the 

 size of his feet, hairless dogs have their teeth imper- 

 fect, and white tomcats with blue eyes are almost 

 invariably deaf. In the present state of physiological 

 knowledge we cannot account for such facts ; but it is 

 enough for the purposes of the Darwinian theory to 

 know that they exist. For, taken all together, they 

 show that natural selection, operating on even the 

 most superficial variations, is quite competent to work 

 deep-seated changes of structure and function. 



When we consider, then, that the circumstances 

 which determine what individuals shall survive are 

 not constant in the long run for any species, though 

 apparently constant for limited periods of time; 

 when we reflect that there is no one of the larger 

 groups of plants and animals such as orders, or 

 families, or even genera which have not been sub- 

 jected again and again to great and complicated 

 changes of environment, it becomes evident that 

 anything like " fixity of species " is utterly out of 

 the question. No such thing is possible or even 



C 



