264 MENTAL EVOLUTION IN ANIMALS. 



produced by the same unknown causes which produce slight 

 deviation of bodily structure." 



Again, in the " Descent of Man," he repeats substantially 

 the same judgment (pp. 67-8) ; and among his MSS I find 

 the following passage, which I shall quote because it serves to 

 convey his opinion in a still more clear and emphatic manner. 



" Although, as I have attempted to show, there is a strik- 

 ing and close parallelism between habits and instincts ; and 

 although habitual actions and states of mind do become here- 

 ditary, and may then, as far as I can see, most properly be 

 called instinctive ; yet it would be, I believe, the greatest 

 error to look at the great majority of instincts as acquired 

 through habit and become hereditary. I believe that most 

 instincts are the accumulated result, through natural selec- 

 tion, of slight and profitable modifications of other instincts ; 

 which modifications I look at as due to the same causes 

 which produce variations in corporeal structure. Indeed, I 

 suppose that it will hardly be doubted, when an instinctive 

 action is transmitted by inheritance in some slightly modified 

 form, that this must be caused by some slight change in the 

 organization of the brain. (Sir B. Brodie, 'Psychological 

 Enquiries,' 1854, p. 199.) But in the case of the many 

 instincts which, as I believe, have not at all originated in 

 hereditary habit, I do not doubt that they have been 

 strengthened and perfected by habit ; just in the same 

 manner as we may select corporeal structures conducing to 

 fleetness of pace, but likewise improve this quality by train- 

 ing in each Generation." 



From these quotations it is evident that Mr. Darwin 

 clearly recognized both the lapsing of intelligence and natural 

 selection as operating causes in the formation of instinct; 

 but that he regarded natural selection as the more important 

 of the two. Although, however, he does not expressly say 

 so, I cannot doubt — in fact I know — that he fully recognized 

 the importance of intelligence in supplying adaptive as dis- 

 tinguished from fortuitous variations to be seized upon by 

 natural selection. Viewed in this relation, natural selection 

 may be deemed a promoting cause of the lapsing of intelli- 

 gence into instinct, and the two principles working in con- 

 junction must, I think, be more potent than either working 

 alone. But if I were asked which of the two I deem more 

 important, I should say that natural selection must be held to 



