EXTERNAL INFLUENCES AS CAUSES OF VARIATION 293 



determine. The first two causes mentioned, both of which are cer- 

 tainly at work, sufficiently explain most phenomena, but whether 

 there is an additional fraction due to inheritance, it is most im- 

 portant for us to know. 



It can be but a fraction at best, and being in exact line with 

 selection and with the direct action of the conditions of life, it is 

 exceedingly difficult of identification and of separation from the 

 larger causes. If inheritance is to be included, however minute 

 the fraction in a single generation, its effect is cumulative, and in 

 time it would become the most powerful and irresistible of all 

 causes influencing type. Its further consideration must be deferred 

 to a later chapter, but the result of its activity, if it has any, is 

 in its influence upon type. 



Summary. Though the impulse to development lies within, 

 the opportunities for that development and the forces controlling 

 subsequent activities are to be found in the conditions of life 

 surrounding the organism. 



These are generally insufficient to afford full development of 

 all the possibilities with which the organism is endowed by hered- 

 ity. Accordingly the individual does not express in its own per- 

 sonality the full extent of its heritage, and individuals generally 

 are to be regarded as having realized something less, rather than 

 something more, than their birthright. 



Living matter, like non-living matter, sustains definite relations 

 to external materials and forces, and the chemical elements of 

 which it is composed are not freed from their ordinary reactions 

 to other elements or to chemical or physical energies. And so it 

 is that living matter is subject to both constructive and destructive 

 combinations, and to definite reactions toward gravity, light, tem- 

 perature, electricity, and to chemical and physical forces generally. 



Herein lies the modifying effect of surrounding conditions upon 

 the development and activities of living matter. Endowed from 

 within with definite properties and capacities, their realization 

 depends very much upon outside materials and forces which pro- 

 vide the conditions under which the definitely organized matter 

 is compelled to discharge its activities, and we do well to become 

 somewhat familiar with the nature and extent of the limitations 

 thus imposed. 



