THE TROUT JUMP. 27 



here and there by the little spot of darkness that 'forms 

 to -them the visible symbol of an eatable insect. One 

 of the great dangers, indeed, which surround the path of 

 scientific psychology is that of being too exclusively 

 human. Here more than anywhere else in science the 

 old Greek doctrine that man is the measure of all things 

 seems especially to beset us on every side. Our own 

 consciousness being the only consciousness which we 

 can experimentally examine, we are peculiarly liable to 

 accept its component elements as being the component 

 elements of all other consciousness whatsoever. It is 

 very hard some philosophers have even told us it is 

 impossible to construct a comparative psychology, as 

 we can construct a comparative osteology or a compara- 

 tive philology. All the other minds about which we can 

 obtain even the second-hand information given us by 

 language are still human minds ; and for the animal 

 consciousness generally we are reduced to very in- 

 ferential and doubtful data. 



Yet even here a good deal can be done by careful 

 sifting of facts, if only we know what facts to sift. The 

 general principle of nihil est in intellects, stands us in 

 good stead when once we have been able to discover 

 what was before in sensu ; and this we can often do 

 provided we take the trouble to follow out all the hints 

 supplied us by the nervous system and by the habits 

 or peculiarities of animals. In some fishes, for instance, 

 there is every indication of the preponderance of smell 

 over sight as an intellectual and guiding sense. In the 

 sharks and rays the membrane of the nose is enormously 

 developed ; the olfactory nerve is by far the largest and 

 most important in the body ; the central organs directly 



