CHAPTER in. 



THE PHILOSOPHICAL INTERPRETATION OF THE DAR 

 WINIAN HYPOTHESIS. 



The function of natural selection in the origi 

 nation of species of plants and animals has, I trust, 

 been sufficiently described and illustrated in the 

 preceding chapter. We must now go on to in 

 quire into the philosophical significance of the 

 doctrine. And, obviously, the main point can 

 be no other than a precise determination of what 

 it really is that natural selection explains, as well 

 as of what is left unexplained by it, in the origin 

 of species of organic beings. 



Scientific explanation consists in the assign 

 ment of a phenomenon to its causes. These causes 

 must be known natural agencies. It may well be, 

 indeed, that speculative reason is unable to stop 

 at such causes, involving, as they do, an inconceiv 

 able regression in infinitum but it is solely of 

 these secondary causes that science takes account. 

 And when this limitation of its province is con 

 sidered, it must be conceded that science is clearly 



