286 THE WHENCE AND TUB WHITHER OF MAN 



against the law of probabilities ; if the favorable 

 variation occurs in only a part of the series it remains 

 useless until the corresponding variation has taken 

 place in the other terms. And while the variation is 

 thus awaiting its completion, so to speak, it is useless, 

 and cannot be fostered by natural selection. 



Evolution by means of fortuitous variations, com- 

 bined and controlled only through natural selection, 

 seems to me at least impossible ; and this view is, I 

 think, steadily gaining ground. 



Natural selection, while a real and very important 

 factor in evolution, cannot be its sole and exclusive 

 explanation. It presupposes other factors, which we 

 as yet but dimly perceive. And this does not impeach 

 the validity of Mr. Darwin's theory any more than 

 Newton's theory of gravitation is impeached by the 

 fact that it offers no explanation as to why the apple 

 falls or how bodies attract one another. 



For natural selection explains the survival, but not 

 the origin, of the fittest. Given a species or other 

 group composed of more and less fit individuals and 

 the fittest will survive. How does it come about that 

 there are any more and less fit individuals ? This 

 brings us to the consideration of the subject of varia- 

 tion. 



Let us begin with a simple case of change in the 

 adult body. The workman grasps his tools day after 

 day, and his hands become horny. The skin has evi- 

 dently thickened, somewhat as on the soles of the feet. 

 This is no mere mechanical result of pressure alone. 

 Continuous pressure would produce the opposite re- 

 sult. But under the stimulus of intermittent press- 

 ure the capillaries, or smallest blood vessels, furnish 



