SEXUAL DIMORPHISM 



listinguish kinships of animals from one another, the special 

 characters by which we classify animals. 



The individuals generated by two parents of the same 

 species are in many cases similar to those parents and to one 

 another, with the exception of those irregular and generally 

 slight differences which we call individual variations. But 

 in other cases we find that the individuals generated by the 

 same two parents differ greatly either from their parents or 

 from one another, so that the species includes two or more 

 well-marked forms or types, intermediate forms being more 

 or less exceptional. The different forms may be sexually 

 perfect, or may reproduce without sexual union, or may be 

 quite sterile. When they are sexually perfect they unite in 

 sexual intercourse and reproduce the same distinct types, 

 which although constantly mingled in the fertilised ova, 

 separate spontaneously in each successive generation, like oil 

 and water after agitation. 



This is the well-known phenomenon of polymorphism. 

 One of the most frequent cases of this is sexual dimorphism, 

 the occurrence of characters peculiar to the males on the one 

 hand, to the females on the other. Another important 

 instance is the distinction of several classes of individuals in 

 social insects, such as workers and queens, or workers, soldiers 

 and fertile forms. 



Thirdly, we have to consider the important fact that the 

 structure of the individual is not constant throughout its 

 existence, but that in many cases there is a more or less com- 

 plicated metamorphosis, the difference between the two states 

 of the same individual at different periods of life being often 

 greater than that between individuals of distinct orders or 

 classes. 



These three categories, which we may entitle diversity, 

 polymorphism, and metamorphosis, include all the pheno- 

 mena of structure in the animal kingdom. The other classes 



