1 6 DARWINISM VERIFIED. [i. 



fixity at all. Change the surrounding circumstances, 

 and the average character of the species must change. 

 Slight peculiarities that once insured survival will now 

 insure destruction, and tendencies to vary that once 

 would have been nipped short will now be encouraged 

 and exaggerated. In this way the strong tendency, 

 hereditary in all mammals, toward the growth of hair 

 on the surface, was greatly exaggerated in the Siberian 

 mammoth, while checked in his brethren, the elephants 

 of India and Africa. In this way a peculiar curve in 

 the contour of butterflies' wings, which is persistently 

 killed out in India and Java, is with equal persistency 

 selected for preservation in Celebes. How far such 

 alterations in the direction of natural selection may 

 work deep-seated changes in the structure of an 

 organism, one cannot accurately define ; but there is 

 no doubt that they go very far indeed, when taken in 

 connection with the facts of what is called " corre- 

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

 gation of parts, of which one can be altered without 

 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 com- 

 pensating changes in the rest of the body. Increased 



