338 EVOLUTION AND GENETICS 



of the experiment was the production of distinctly male and 

 female individuals, still joined by the same forked corm (Fig. 191). 



Conclusions. As we consider the entire succession of organic 

 forms it is evident that the more highly organized a species may 

 be, the less directly dependent it is upon the conditions of environ- 

 ment. The lower organisms and even such complex animals as 

 the insects are directly influenced by temperature but the birds 

 and mammals have surmounted this limitation. Because they 

 are homothermous they can withstand a great range of external 

 temperature without cons})icuous modification of their activities. 

 In man this process culminates, for through his intelligence he 

 exerts control over all phases of his environment. 



To a certain extent this independence of the organism is due 

 to the fact that the complex body itself establishes an environment 

 for its several parts through which the exigencies of life are tem- 

 pered. This internal environment has become a definite part of 

 the heritage, since it is in reality the result of correlation of herit- 

 able structures. Since the chromosomes are the carriers of heredity 

 it seems wholly logical that all inherited characters should be in 

 some way connected with them, and the internal environment 

 may be regarded as a product of chromosomal activity. 



No characters, however, are developed unless the proper con- 

 ditions of environment favor the normal expression of the chromo- 

 somes. An animal without air cannot live; a nucleus without 

 cytoplasm is equally incomplete. Since these conditions are 

 resident in the body in the case of characters which appear auto- 

 matically in every generation it is not a simple matter to determine 

 what they are, but it is logical to believe that when a condition 

 can be discovered, or when an external condition on which it 

 depends can be determined, the behaviour of the genes with which 

 it is associated can be modified. There is no reason to suppose 

 that sex is any different in this respect from other characters. 



It is therefore possible to conclude that sex is determined by 

 the chromosome complex reacting to definite environmental con- 

 ditions which are in most cases determined by the organism 

 itself. The modification of these conditions, whether within or 

 outside of the organism, results in the modification of sexual 

 characters. 



Modification of a sex is, however, subject to definite limitations. 

 In most animals sexual organs are developed only once during 



