Darwinism Verified. 17 



not accurately define ; but there is no doubt that 

 they go very far indeed, when taken in connec- 

 tion with the facts of what is called " correlation 

 of growth." An organism is not a mere aggre- 

 gation of parts, of which one can be altered with- 

 out affecting the others. Increase in the size 

 and weight of a deer's horns entails an increase 

 in the size of the cervical vertebrae and muscles, 

 and indirectly modifies the shoulders and fore- 

 limbs ; while all these changes, by altering the 

 animal's centre of gravity, cause compensating 

 changes in the rest of the body. Increased thick- 

 ness 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 correla- 

 tions are inexplicable. Thus, to lengthen the 

 beak of a pigeon is to increase the size of his 

 feet, hairless dogs have their teeth imperfect, and 

 white tomcats with blue eyes are almost invari- 

 ably 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, oper- 

 ating on even the most superficial variations, is 



