Publisher's Synopsis
Excerpt from Sexual Dimorphism in the Animal Kingdom: A Theory of the Evolution of Secondary Sexual
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.
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