602 PSYCHOLOGY 



but the study of their behavior has already yielded much and 

 promises much more. Although those who make their psycho- 

 logy coterminous with introspection cannot enter far into this 

 field, they still have their own genetic problems. In whatever 

 direction we turn, the harvest is waiting; it is only the reapers 

 who are few. Almost every observation, experiment, or theory 

 of organic evolution offers parallel problems for the psychologist. 

 The development of the individual opens questions more numerous 

 and more important for psychology than does the development 

 of the body for other sciences. Senile, degenerative, and path- 

 ological conditions are all there for psychological investigation. 

 The evolution of society and the interrelations of individuals are 

 being gradually brought within the range of genetic psychology. 

 It is quite possible that the chief scientific progress of the next 

 fifty years will be in this direction. 



The problems of psychology are certainly made endlessly com- 

 plex by the fact that we have to do, not with the development and 

 condition of a single mind or individual, but with innumerable in- 

 dividuals. The traditional psychology has been disposed to ignore 

 individual differences; but in attempting to prescribe conditions 

 for all minds, it becomes schematic and somewhat barren. It is 

 surely wasteful to select those uniformities that are true for all, 

 and to throw away those differences which are equally fit material 

 for scientific treatment. Linnaeus 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 cannot 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 im- 

 agery 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 



