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THE SEXES AND SEXUAL SELECTION.
THAT all higher animals are represented by distinct male and female forms is one of the most patent facts of obseration, striking enough in many a beast and bird to catch any eye, and familiarly expressed in not a few popular names which contrast the two sexes. In lower animals, the contrast, and indeed the separateness, of the sexes often disappears; yet even naturalists have sometimes mistaken for different species what were afterwards recognized to be but the male and female of a single form.
I. Primary and Secondary Characters.—When we pass from this commonplace of observation and experience to inquire more precisely into the differences between the sexes, we speedily recognize that these are of very different degrees. In some cases no marked differences whatever are recognizable; thus a male starfish or sea-urchin looks exactly like the female, and a careful examination of the essential reproductive organs is