8 Darwin, and after Darwin. 



to merely useless change — in particular, "the direct 

 action of external conditions, and variations which 

 seem to us in our ignorance to arise spontaneously. ' 



5. Hence, that the " principle of utility," far from 

 being of universal occurrence in the sphere of animate 

 nature, is only of what may be termed highly general 

 occurrence ; and, therefore, that certain other advocates 

 of the theory of natural selection were mistaken in 

 representing the universality of this principle as 

 following by way of necessary consequence from that 

 theory. 



6. Cross-sterility in species cannot possibly be due 

 to natural selection ; but everywhere arises as a result 

 of some physiological change having exclusive refer- 

 ence to the sexual system — a change which is 

 probably everywhere due to the same cause, although 

 what this cause could be Darwin was confessedly 

 unable to suggest. 



Such, then, was the theory of evolution as held by 

 Darwin, so far as the points at present before us are 

 concerned. And, it may now be added, that the 

 longer he lived, and the more he pondered these 

 points, the less exclusive was the role which he as- 

 signed to natural selection, and the more importance 

 did he attribute to the supplementary factors above 

 named. This admits of being easily demonstrated 

 by comparing successive editions of his works ; a 

 method adopted by Mr. Herbert Spencer in his 

 essay on the Factors of Organic Evolution. 



My object in thus clearly defining Darwin's attitude 

 regarding these sundry points is twofold. 



In the first place, with regard to merely historical 

 accuracy, it appears to me undesirable that naturalists 



