Linnæus instructed his pupils to attend to species and to ignore varieties, 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 accordance 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 animal series and the different races of men for psychological study; it may be that instincts and mental traits have specific or racial significance 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 something 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 science 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 believe that psychology affected conduct and could be applied in useful ways, I should regard my occupation as nearer to that of the professional 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 reactions and activities of others, is being continually used, more continually 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
