232 DOCTRINE OF EVOLUTION 



be expected, for the contention that the structures and 

 the functions of the several organs constituting any 

 system are inseparable has never been gainsaid. 



Mental variation is real. It needs no scientist to tell 

 us that human beings differ in intellectual qualifications 

 and attainments, and that no two people are exactly 

 similar even though they may be brothers or sisters. 

 The struggle for existence or competition on the basis 

 of mental ability is equally real, and every day we see 

 the prize awarded to the more fit, while those who lose 

 are crowded ever closer to the wall. As in all other 

 fields of endeavor, the goal of success can be attained 

 only by adaptation, which involves an adjustment to 

 all of the conditions of existence — to social and ethical 

 as well as to the more expressly material biological 

 circumstances. 



Heredity of mental qualities has also been demon- 

 strated notably by Galton, Pearson, Woods, and Thorn- 

 dike, who have also shown that the strength of inheri- 

 tance in the case of mental traits is approximately the 

 same as for physical characteristics like stature and eye- 

 color. Just as a worker-bee inherits a specific form 

 of nervous system which cooperates with the other 

 equally determined organic systems, wherefore the 

 animal is forced to perform ^^instinctively" its peculiar 

 specialized tasks, so the mental capacity of a human 

 being is largely determined by congenital factors. 

 Upon these primarily depends his success or failure. 

 It is quite true that environment has a high degree of 

 influence, so great indeed that some speak of a ^^ social 

 heredity" ; they mean by this phrase that the mental 

 equipment of an individual is determined by the 



