62 THE GENETIC AND THE OPERATIVE EVIDENCE 



PART III. 

 THE GENETIC AND THE OPERATIVE EVIDENCE. 



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 informa- 

 tion 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 com- 

 plex 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 Cun- 

 ningham (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 re- 

 straints 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. 



The genetic evidence shows, in the case of cock-feathering versus 

 hen-feathering in birds, that only one or two Mendelian factor differ- 



