VIII 



COUPLING 



The outstanding features in sex are the numerical 

 equality and the physical inequality of males and 

 females. With certain unusual methods of reproduc- 

 tion, one sex is more numerous than the other, but, 

 ordinarily, the two are numerically equal. 



Males and females usually differ from each other in 

 a number of characters, and the differences are such 

 that there is little difficulty in distinguishing the sexes 

 of familiar species at a glance. Sometimes the cha- 

 racters of one sex diverge decidedly from those of the 

 other, and in certain varieties the sexes differ not only 

 in size, markings, and pattern but even in general colour. 

 Thus there are characters linked or coupled with one 

 sex or the other ; and, since the sexes are always equal 

 in number and, at the same time, carry different 

 characters, it is possible that a theory which will explain 

 one of these phenomena will, at the same time, help to 

 explain the other. 



The numerical equality of males and females suggests 

 an inference. It will be remembered that, when a 

 pure kind is mated with its hybrid, the progeny consist 

 of equal numbers of the pure kind and the hybrid. 

 Is it possible that either the male or the female character 

 is a hybrid ? This hypothesis would account for the 



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