In most polymorphic genera some of the species have fixed and definite characters.

Genera which are polymorphic in one country seem to be, with a few exceptions, polymorphic in other countries, and likewise, judging from Brachiopod shells, at former periods of time.

These facts are very perplexing, for they seem to show that this kind of variability is independent of the conditions of life.

I am inclined to suspect that we see, at least in some of these polymorphic genera, variations which are of no service or disservice to the species, and which consequently have not been seized on and rendered definite by natural selection, as hereafter to be explained.
Individuals of the same species often present, as is known to every one, great differences of structure, independently of variation, as in the two sexes of various animals, in the two or three castes of sterile females or workers among insects, and in the immature and larval states of many of the lower animals.

There are, also, cases of dimorphism and trimorphism, both with animals and plants.