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 ccelenterates, the male and the female are 
closely similar except for the differences in the organs of repro- 
duction. Why the higher and the lower 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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