194 THE CAUSATION OF DISEASE. 



an a priori proof of its truth in the fact that instinct has 

 been built up very largely by natural selection ; it must, there- 

 fore, as a guide, be as near perfection as possible. One is apt 

 to think that natural selection acts solely upon the corporeal 

 side of an individual ; whereas, the function of instinct 

 comes equally under its influence, for in the battle of life 

 correct instinct is quite as requisite as correct bodily struc- 

 ture. In order that natural selection shall operate on the 

 instinctive side of the organic being, it is necessary for the 

 instincts to vary, and there is no reason why they should 

 not vary quite as freely as the bodily structure ; indeed, this 

 is obviously an a priori deduction, since instinct is the 

 functional expression of a certain part of the bodily structure 

 — namely, the nervous system. 



Darwin gives several examples of such variations in 

 instincts. Thus, birds vary in their choice of nests ; the 

 magpie is tame in Norway, the crow in Egypt ; and natural 

 selection would not be slow to seize upon any variation 

 favourable to the animal in its struggle for existence. 

 Indeed, man himself has not failed to turn such varia- 

 tions in instinct to his own good account ; for, as Darwin 

 observes, he has actually built up instincts artificially, by 

 artificially selecting such variations as are of service to him. 

 A well-bred pointer will point the first day he is taken out ; 

 and the shepherd's dog, while quite a puppy, shows a tendency 

 to run round, not at, a flock of sheep. If, then, man can, 

 in the course of a few generations, build up an instinct wholly 

 useless to the animal, and solely for his own pleasure, by arti- 

 ficial selection, one can well see how natural selection must tend 

 to preserve any slight variation in instinct which is helpful 

 to the animal in its struggle for existence ; and further, how it 

 has come about that instinct is an accurate guide, regulating 

 the environment of the animal, indeed, with a wisdom that is 

 well-nigh unerring. 



Now, the guiding voice of reason is not, like the voice of 

 instinct, the outcome of natural selection. The evolution of 

 reason itself has doubtless been largely governed by natural 

 selection ; but there is a great distinction between reason as 

 such and the guiding voice of reason — that part of reason, 



