RELATING TO SECONDARY SEXUAL CHARACTERS. 99 



female in birds produces an internal secretion that suppresses in her 

 the ornamentation shown by the male, and in the mammal an internal 

 secretion produced by the testes causes the full development in the male 

 of the secondary sexual characters. If, as seems probable, these secre- 

 tions are some particular kind of substance, the condition that led to 

 their appearance historically need not have been very complex; and 

 if not, the problem appears simplified. It still remains to give some 

 reasonable explanation as to why such substances should continue to 

 be produced if their products the secondary sexual characters 

 possess no "beauty" for the female. Here more work is necessary, 

 but the modern genetic point of view may possibly give an important 

 clue. We are coming to realize more fully that the hereditary genes 

 generally have more than a single effect on the characters of the animal. 

 The secondary sexual characters may, then, be only by-products of 

 genes whose important function lies in some other direction. If, for 

 example, the secretion produced by the cells of the male have an 

 important influence on his output of energy, or strength, or activity, 

 their secondary influence over certain parts of the body would not call 

 for any further explanation on the modern view of natural selection. 

 If the secretions of the ovary of the female bird have some direct rela- 

 tion to her physiological processes that are important in the develop- 

 ment of the oviduct, for instance, it would be a matter of no importance 

 from an evolutionary point of view if that same secretion suppresses 

 in her the development of the high color shown by the male. 



