THE FINAL ADJUSTMENTS 121 



males" ; and finally we may get proportions inter- 

 mediate between one X and two Xs to the set of As 

 and then intermediate forms or "intersexes" develop. 

 Again, there are strong and weak varieties of the 

 male and female factors, and unbalanced inter- 

 mediate animals can result from some combinations 

 of them. Most of these abnormalities have been 

 found in insects, though they may perhaps occur in 

 other groups where they have not yet been dis- 

 covered. They are nearly all infertile and generally 

 feeble. 



The sex genes probably work by producing special 

 sex substances or hormones, and if a young embryo 

 comes in contact with these substances before its 

 own sex genes have had time to act, the substances 

 may get in first and overcome the genes. Or again, 

 the two influences may act antagonistically and 

 produce intermediate or intersexual animals. Even 

 in adult life the sex hormones can produce very 

 important effects. By various experimental methods 

 it has been possible to bring about the complete 

 reversal of sex in some animals, so that, for instance, 

 a hen which can lay eggs may be changed into 

 a cock which is functional as a male. The inter- 

 action of the hereditary sex factors and those which 

 arise during development obviously can lead to very 

 complicated results, and there is no space here to 

 discuss the matter at all fully. Those who wish 

 to go more deeply into the matter should read 

 Professor Crew's book, The Genetics of Sexuality in 

 Animals, 



