' THE ORIGIN OF SPECIES' 105 



hand in hand. The more perfect the new variety, the 

 more absolutely will it kill off the intermediate forms. 

 The second great difficulty lies in the question of the 

 origin of instinct, which, as Darwin shows, by careful 

 inductive instances, may have arisen by the slow and 

 gradual accumulation of numerous slight yet profitable 

 variations. 



I have dwelt at some length upon those portions of 

 the ' Origin of Species ' which deal in detail with the 

 theory of natural selection, the chief contribution which 

 Darwin made to the evolutionary movement, because it 

 is impossible otherwise fully to understand the great 

 gulf which separates his evolutionism from the earlier 

 evolutionism of Lamarck and his followers. But it is 

 impracticable here to give any idea of the immense wealth 

 of example and illustration which Darwin brought to 

 the elucidation of every part of his complex problem. 

 In order to gain a full conception of this side of his 

 nature, we must turn to the original treatise itself, and 

 still more to the subsequent volumes in which the 

 ground-work of observations and experiments on which 

 he based his theory was more fully detailed for the 

 specialist public. 



The remainder of Darwin's epoch-making work deals, 

 strictly speaking, rather with the general theory of 

 ' descent with modification ' than with the special doc- 

 trine of natural selection. It restates and reinforces, 

 by the light of the new additional concept, and with 

 fuller facts and later knowledge, the four great argu- 

 ments already known in favour of organic evolution as 

 a whole, the argument from Geological Succession, the 

 argument from Geographical Distribution, the argument 



