METHODS OF PSYCHOLOGY. 185 



Linnaeus instructed his pupils to attend to species and to ignore varie- 

 ties, and this in the end tended to make systematic botany and zoology 

 unfruitful. If the zoologist had limited his work to the discovery of 

 facts that are true for all animals and had ignored the differences 

 between animals, he would have done something analogous to what the 

 psychologist has actually done. 



It may be that individuals can not be grouped into species or even 

 varieties, but animals and plants are separated into species in accord- 

 ance with the noticeable differences between them, and there are as 

 many degrees of just noticeable difference between men as between 

 related species. We have in any case the different species of the ani- 

 mal series and the different races of men for psychological study; it 

 may be that instincts and mental traits have specific or racial signifi- 

 cance for the zoologist or anthropologist. We have the infant, the 

 child, the adolescent and the aged; we have the two sexes; we have the 

 geniuses, the feeble-minded, the criminals and the insane — complex 

 groups to be sure, but open to psychological investigation. It may be 

 that mental imagery or types of character will give workable groups. 

 But even if mental traits and their manifestations are continuous, we 

 can study the continuum. The study of distribution and correlation 

 appears to open up subjects of great interest and having important 

 practical applications. 



The question of the practical applications, of psychology is the last 

 which I shall touch. There are those who hold that there is some- 

 thing particularly noble in art for art's sake or in science divorced 

 from any possible application. We are told of the mathematician who 

 boasted that his science was a virgin that had never been prostituted 

 by being put to any use. It is doubtless true that science justifies 

 itself if it satisfies mental needs. It may also be true that pure sci- 

 ence should precede the applications of science. But of this I am not 

 sure; it appears to me 'that the conditions are most healthful when 

 science and its applications proceed hand in hand, as is now the case 

 in engineering, electricity, chemistry, medicine, etc. If I did not be- 

 lieve that psychology affected conduct and could be applied in useful 

 ways, I should regard my occupation as nearer to that of the profes- 

 sional chess-player or sword swallower than to that of the engineer or 

 scientific physician. 



It seems quite obvious that such knowledge as each of us has of his 

 own perceptions, mental processes and motor responses and of the reac- 

 tions and activities of others, is being continually used, more continu- 

 ally indeed than any other knowledge whatever. This knowledge is 

 partly organized into reflexes and instincts; it is in part acquired by 

 each individual. Control of the physical world is secondary to the 

 control of ourselves and of our fellow men. The child must observe 



VOL. LXVI. — 31. 



