The fact that so little is known about the mammals during that period when the all-important work of securing mates is going on, and of the subsequent events, is largely due to the difficulties which close observation of this phase of their life-history entails. With the birds matters are far otherwise; their haunts are more accessible; they are far more numerous, and much more easily kept under observation. Consequently, we have a tolerably complete knowledge of the lives of some species, at any rate, during the reproductive period; that is to say, as to the sequence of events from the beginning of the reproductive activities onwards; but the interpretation of what is seen is another matter. No attempt which has yet been made to fathom the psychology of sex has yielded more than a slight insight into what is taking place. Nevertheless, this is an aspect of the subject which has a far more important bearing on the problems of evolution than is generally realized. But these pages are concerned rather with the relations between the sexes, than with the subtle forces which have fashioned and control conduct in this regard. In all that concerns the problems of sex, which is to say of reproduction, birds, speaking generally, display a briefer and more condensed sequence of events than the mammals; and, moreover, many species compel the attention even of the most incurious, to their behaviour at this time, through the development, either of song, or of fantastic displays of their amorous feelings: while others force themselves no less conspicuously under notice by their habit of nesting in large, and often enormous colonies. In the matter of the development of secondary sexual characters birds stand conspicuous among the Vertebrates, and easily eclipse the mammals; among which bright, strongly contrasted, colours are the exception. Among the birds they may almost be said to be the rule. Also, in this category we have to reckon song, and the production of more or less musical sounds by the agency of internal resonators or of specially modified feathers; as well as quaint forms of posturing which may be included under the head of dances. Further, some species have developed formidable weapons of offence. These things are interesting enough in themselves, but they become still more so when we reflect that they formed the corner-stone of Darwin’s theory of “Sexual Selection,” and that Wallace’s criticisms thereof were inspired by evidence from the same source. The interests of this chapter will best be served if the evidence on which this theory was founded be first surveyed: when Darwin’s deductions and the criticism which they have aroused will be the more readily appreciated.