EXTERNAL INFLUENCES AS CAUSES OF VARIATION 291 



alike they necessarily influence the type of the race, but to 

 what extent cannot be told without some method of subtracting 

 the normal results of simple inheritance. No method of doing 

 this has ever been discovered, and much uncertainty has always 

 prevailed as to what proportion of existing types should be 

 credited to the conditions of life. 



The puzzling problem is greatly simplified if we enlarge the 

 meaning of the word " inheritance " to cover not only the lines 

 of possible development but the full capacity as well. In this 

 view of the case the individual and the race are understood 

 as possessing hereditary capacities for development far beyond 

 that for which opportunities are likely ever to be afforded by 

 environment. This largely removes the conditions of life from 

 among the fundamental causes of variability and relegates them 

 to the realm of passive and permissive, though necessary, requi- 

 sites for the full display of hereditary power. It tends strongly, 

 too, to regard most individuals as something less than would 

 be indicated by their hereditary possibilities, and to consider 

 environment in general as the limiting, not the stimulating, 

 element in development. With this view the writer is inclined 

 to agree, though it necessarily reduces the extent of direct 

 influence of the environment. The student is never to forget 

 that, whatever may be the influence of surrounding conditions 

 upon one form of life, the same influences affect other species 

 differently, thus showing that their characteristic effect is con- 

 ditioned upon the power of the organism to react, a condition 

 that is eminently internal and inheritable. 



Selection as a cause of variation in type. Many individuals 

 are unable to meet the conditions of life with which they find 

 themselves surrounded, and in the attempt become extinct. 

 This is selection, and it is manifest that the prevailing type of 

 the race as it exists at any moment, being made up of selected 

 individuals, is something different from that which was born 

 into the race. It is also manifest that only a limited portion of 

 adults will reproduce, so that selection through the external con- 

 ditions of life exerts a strong influence upon type. 



Selection is therefore one of the most powerful if not the 

 most powerful influences known in the modification of species, 



