The genetic and operative evidence shows that there has been included under the general term “secondary sexual characters” a complex of cases that are the outcome of diverse physiological processes. Sex-linked and sex-limited characters have often been confused; some characters depend on the gonad; some of these involve the ovary, others the testes. Still other characters fall under none of these groups, but are the direct product of the male or female genetic constitution. It is not surprising, therefore, that theories proposed on the information derived from certain of these data are controverted by information derived from other data. The theory of sexual selection, in its attempt to bring all the facts under one point of view, has not escaped these difficulties, even although it may be said that neither natural selection nor sexual selection is concerned with the origin or even the kind of variations with which it works. Nevertheless, the latter theory, by ignoring the origin or the physiological process concerned in the production of secondary sexual characters, may make assumptions that are difficult to harmonize with the facts in the case, and we shall find several instances of this sort. For example, if the hen had selected the cock for his fine plumage (which, as we have seen, depends in part on autosomal genes producing their effect without the cooperation of the testes), she would be expected to endow herself with the same adornments (if her selection worked), unless her ovary were already producing some substance inimical to those that she is “calling forth” by selection of the male. The problem is evidently, then, more complex than appears on the surface, and is not so simple as it seemed when these essential facts were unknown or ignored. In the case of other theories, such as those of Wallace and of Cunningham (that appeal more directly to the causes that are producing the variation out of which the secondary sexual characters are built up), the absence of information, physiological or genetic, has only too often given these writers the opportunity to speculate without the restraints which a more recent knowledge of the facts has imposed on us. It is obvious from what we have learned that we shall have to proceed with more caution in disentangling the evidence before we can hope to “explain” it. Despite the meagerness of our present information, enough has been found out to indicate that we must be content for a while with tentative and partial explanations even in the best-known cases, and we must, I think, be prepared to admit that no one theory may be able to account for all of the secondary sexual differences that exist between the sexes.