CHAPTER XXVIII 



SECONDARY SEXUAL CHARACTERS 



Introduction 



Hunter was the first, I believe, to use the term secondary 

 sexual characters for those differences in structure between the 

 males and females that are not directly connected with the organs 

 of reproduction. One of the most important facts connected 

 with the occurrence of secondary sexual characters is that they 

 are found almost exclusively in the higher groups of animals. 

 Moreover, they are, on the whole, characteristic of certain large 

 groups, so that it might appear that in these groups there is 

 some inherent tendency for such differences to develop. Thus 

 in the groups of mammals, birds, and insects secondary sexual 

 differences are quite common, while in the worms, mollusks, 

 echinoderms, and coelenterates, the male and the female are 

 closely similar except for the differences in the organs of repro- 

 duction. Why the higher and the lowxr groups should differ 

 in these respects is difficult to explain. Two suggestions have 

 been proposed that may appear to account for the facts. The 

 groups in which these differences are found contain forms that 

 are extremely active, and the males are as a rule more active than 

 the females. Correlated with this difference in activity or ''vi- 

 tality" we may imagine that differences in the development of 

 some of the bodily structures may occur. This is the idea that 

 Wallace has suggested. 



The other view is the one proposed by Darwin. He ascribes 

 the differences to the selection by the females of those males 

 that are more highly ornamented, or have finer voices, or that 

 develop certain odors, etc. This selection by the female implies 

 a highly developed condition of the sense organs, and perhaps of 



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