Definitions are always liable to exceptions; and concrete cases are better than abstract terms. Birds, then, perhaps better than any other group, illustrate what is meant by the term “secondary sexual characters,” if only because examples are so constantly at hand. Save among experts, sex among birds cannot be determined except by the differences in plumage, or sometimes in size, which the sexes display. But even here, it is only among species which occupy what we may call a mid-evolutionary phase in which this discrimination is possible. Among “generalized” species, wherein the plumage is of sombre hue, there is no external distinguishing mark between male and female; and the same is true with species which have attained to the maximum of resplendent plumage; as for example many Parrots and Kingfishers, where again both sexes, and at all ages, display the same vivid hues. Thus, in the case of either of the two extremes, the study of behaviour during the breeding season is one of great difficulty and no less uncertainty. Where the sexes are sharply distinguished by differences of coloration, however, as with the Peacock, the matter is otherwise. This bird, from time immemorial the symbol of vanity, illustrates in a singularly effective manner the broad features of what is commonly meant by “courtship” among birds, while it furnishes a no less striking example of the development of “secondary sexual characters.” One might have supposed that birds, under the spell of that irresistible desire for sexual intercourse, would behave differently in regard to their “courtship” according to whether they were monogamous, polygamous, or polyandrous: but while their behaviour during this period of the life-history presents an extraordinary variety, it is only at any rate slightly determined by the plurality or otherwise of mates; and the same rule holds in regard to the brilliancy or otherwise of coloration.