43 2 Experimental Zoology 



cases in which both sexes are brilliantly colored, but the num- 

 ber of cases in which the males greatly excel the females is com- 

 paratively small. This statement holds also for other groups of 

 insects, where, despite the highly developed coloration, the males 

 are not, in most cases, more ornamented than the females. 



In the crustaceans, where highly colored forms are not so com- 

 mon, there are only a few cases of secondary sexual differences, 

 yet in the group of copepods there are some species where the 

 male excels the female in ornamentation even more than does the 

 peacock excel the peahen. 



These examples will serve to illustrate some of the more strik- 

 ing differences between the male and the female individuals. 

 Before discussing further the possible interpretations of these 

 facts, let us examine the evidence showing that a correlation 

 exists between the development of the secondary sexual char- 

 acters and the presence of the essential organs of reproduction. 



Correlation between the Secondary Sexual Characters and the 

 Essential Organs of Reproduction 



It has been long recognized that in vertebrates there exists 

 an intimate connection between the presence of the essential or- 

 gans of reproduction and the full development of the secondary 

 sexual characters. The removal of the testes of the male has 

 been sometimes performed artificially, and sometimes has re- 

 sulted from disease or abnormal development of these organs. 

 The results are well shown in the case of the stag. If very young 

 males are castrated, before the knobs of the antlers have ap- 

 peared, the antlers never develop. If the operation is performed 

 at the time when the antlers have already begun to develop, 

 complete development does not take place ; they remain covered 

 with skin and are not moulted or renewed. They form the 

 so-called peruke antlers. If the stag is castrated when the 

 horns have fully developed, they are precociously thrown off, 

 and are replaced by new antlers, showing a tendency toward per- 

 uke-formation, and these are not again renewed. If the castra- 

 tion of the young animal is on one side only, the horns show a 



