DIFFERENTIATION, HEREDITY, SEX 317 



cause both are related to some primary differentiation. Still 

 others hold to the idea that the idiochromosomes actually 

 determine by their presence or absence, the nature of the 

 reactions of development, so that finally organisms with female 

 or male characteristics are formed. The most adequately justi- 

 fied and most conservative view seems to be that the nature 

 of the interrelations of the components of the whole chromo- 

 some group, among themselves and to the cytoplasm, is modi- 

 fied by the presence or absence of certain elements so that in 

 one case the primary and secondary female characters develop, 

 in the other case the male characters. 



Returning now to the general subject of this chapter, namely, 

 the factors determining the course of development and the 

 process of heredity, we come to another extremely important 

 subject. We have thus far emphasized the importance of the 

 internal factors of development. But we have defined develop- 

 ment as a series of reactions between internal and external 

 factors. The omission hitherto of specific reference to the 

 external conditions of development is not because these are of 

 lesser general importance. Alterations in the conditions of 

 gravity, pressure, temperature, light, moisture, and chemical 

 composition of the surrounding medium may, each or all affect 

 the course of development, either in a general or in a specific 

 way. A great deal is known of the results of modifying such 

 conditions and a rather full discussion of these effects would be 

 in order, were the results susceptible of more definite and more 

 uniformly applicable statement. For normal development, 

 normal environing conditions are necessary. However, slight 

 variations in external conditions rarely produce effects com- 

 parable with those following slight variations in the internal 

 conditions. That is to say, slight variations in external condi- 

 tions are "normal." When the modification of external condi- 

 tions is sufficiently marked to produce visible effects upon 

 development, these are frequently so marked as to be regarded 

 as distinct abnormalities, and the organism so affected is rarely 

 able to complete its development to maturity. 



