CONDITIONS OF ORGANIC EVOLUTION 29 



This assertion, however, may properly be qualified. 

 For it might be that change or evolution could 

 occur, though heredity were absolute, complete and 

 invariable. For every individual undergoes some 

 change in its own person, in the course of its 

 individual life ; not merely the change of normal 

 development from the immature to the mature 

 stasre, but change due to its converse with its sur- 

 roundings. In other words, it undergoes adaptation 

 to its environment. If, now, after undergoing a 

 certain change — say the thickening of the skin of 

 the sole of the foot due to walking — the individual 

 reproduced himself, as changed, then racial evolu- 

 tion would be possible, even though heredity were 

 invariable and complete. 



But, whether or not this power to transmit 

 acquired characters or acquirements really exist, and 

 whatever the measure of its action if it do exist, 

 there is also another condition of evolution, to 

 which it is scarcely possible to attach too much 

 importance, and that is variation. A white mouse 

 gives birth to a mouse, and that is an instance of 

 heredity; but the young mouse is sometimes not 

 white but grey, and that is an instance of variation. 

 Hereafter we shall see the essential importance of 

 this condition in the Darwinian explanation of 

 organic evolution. 



Now variation, like heredity, is a subject of 

 great complexity : the two must be treated to- 

 gether, and indeed are foremost in the attention of 

 biologists at the present day. It is not necessary 

 or desirable to discuss variation here, for indeed 

 it is a part of the subject of heredity, as we are 



