INTRODUCTION 33 



there is a connection between them. He found that it was a 

 general rule that characters were inherited in the offspring at 

 the same age at which they appeared in the parents, and that 

 when characters appeared in one sex late in life they were 

 generally inherited only by the same sex. He attributed this 

 to the fact that the sexes do not differ much in constitution 

 until the power of reproduction is gained, while after that 

 the differerence is great. In support of this he cites the well- 

 known contrast between ordinary species of deer and the Eein- 

 deer. In the latter both sexes bear antlers, although those of the 

 female are considerably smaller, and the antlers commence 

 to appear in both sexes within four or five weeks after birth ; 

 in other deer the male alone bears antlers, and they begin to 

 appear at periods varying from nine months to twelve or more 

 after birth. It must be remembered, however, that reindeer 

 as well as other deer shed and renew their antlers every year, 

 and even in the reindeer the full size and branching is not 

 attained till after several years. Darwin, however, concludes 

 that one, though not the sole, cause of characters being 

 exclusively inherited by one sex, is their development at a 

 late age. How far this is a sufficient explanation I shall 

 discuss in the following pages, but it is clear without further 

 discussion that it is not an explanation of the origin of the 

 variations themselves. Darwin does not attribute the latter 

 to any definite causes, merely pointing out that secondary 

 sexual characters are highly variable, and therefore afford 

 plenty of opportunity for selection. 



Later investigators have offered various suggestions as to 

 the origin of the variations in question, for the most part 

 attributing them to " constitutional " differences between the 

 sexes, and basing their views largely on the fact that, as a 

 general rule, it is the males that have been modified, while 

 the females exhibit the original condition. Thus the Ameri- 

 can zoologist, W. K. Brooks (1883), attempted to explain 



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