290 CAUSES OF VARIATION 



SECTION XI EXTERNAL INFLUENCES AS CAUSES OF 

 VARIATION IN TYPE 



The student must distinguish clearly between the influence 

 of external conditions upon an occasional individual and their 

 effect upon the type of the race. There are three possible ways 

 in which the environment may result in a modification of type : 

 (i) by affecting all individuals in the same way; (2) by selec- 

 tion ; (3) by the inheritance of the modifications due to condi- 

 tions of life. It remains to examine each somewhat carefully. 



All individuals affected in the same manner, thus influencing 

 the type directly. The modifying effects of the conditions of life 

 have been quite fully noted. If but few individuals are affected, 

 it is manifest that the type will not be seriously changed ; but 

 if, on the other hand, every individual is affected, and in the 

 same way, then the type is to that extent due to the conditions 

 of life. 



For example, size is directly influenced by the food supply, 

 and increase of size in a race, contemporaneous with a better 

 food supply, may fairly be attributed to the favorable influence 

 of full feed acting upon all individuals alike. 



Size is also inherited, so that the limits of development are 

 due to two influences acting together, inheritance and food 

 supply. It is often exceedingly difficult to determine how much 

 to attribute to the one influence and how much to the other. 



What is true of size in this respect is true of every other char- 

 acter that is in the slightest degree dependent upon environment 

 for its development. Accordingly much uncertainty prevails as 

 to the comparative influence of inheritance and environment. 

 The racial type can be determined only by the study of the indi- 

 viduals constituting the mature population ; but their develop- 

 ment is the result of two sets of causes, the one of heredity, 

 the other of environment, both contributing to the same effect 

 and both continuous through life. 



Under the old view every individual was regarded as the re- 

 sult not only of what was born into it but also of the direct 

 influence of its environment. Individuals constitute the type, 

 and so it is that when the conditions of life affect all individuals 



