Mr Barrett-Hamilton, Secondary Sexual Characters, etc. 279 



A suggestion as to a possible mode of Origin of some of 

 the Secondary Sexual Characters in Animals as afforded by 

 Observations on certain Salmonids. By G. E. H. Barrett- 

 Hamilton. 



[Head 19 February 1900.] 



The changes of colour or form which the males of so many- 

 vertebrates undergo in anticipation of, and often immediately 

 before, the breeding-season are so well known that it is needless 

 here to cite any examples of them. Not less striking than their 

 wide prevalence amongst every class of vertebrates, is the exceed- 

 ingly varied form in which these changes make their appearance. 

 Not only may either the whole or only a part, and that by no 

 means always the same part, of the body be affected, but in some 

 cases changes of both colour and form may occur in the same 

 individual, as is found in many species of the genus Salmo. The 

 breeding-season past, all trace of the nuptial colouring or out- 

 growths is lost. 



Phenomena that seem to be analogous are presented by the 

 permanent, and not merely temporary, assumption of sexual 

 characters by the males of many animals. Here we have not 

 only brighter coloration, but superior armature and, more 

 frequently than all, increase of size. But, as in the case of the 

 above, there may be also special local developments of particular 

 organs, such as the nasal glands and appendages of certain 

 Ghiroptera, excess of hair, forming a shaggy mane, in several other 

 mammals, and differences of vocal power especially noticeable in 

 many birds. 



As a rule those who have written on such phenomena leave 

 the question of their possible origin alone ; and, seeking only to 

 understand their present uses, have been content to assign to 

 them a combative or aesthetic cause, the latter often in default 

 of any other apparent reason. Thus Darwin was content to trust 

 to Sexual (which is in part the effect of Natural) Selection as 

 the guiding and perfecting factor in their development, while 

 Mr Wallace, without being clear as to the radical difference 

 between their origin and subsequent perfection, believes that the 

 greater vigour of the males over the females finds an external 

 vent in the manner described. No attempt, so far as I know, 

 has been made to trace their primary physiological meaning, 

 which, nevertheless, may have been widely different from their 

 present or secondary use. 



If Darwin's explanation be of universal application it seems 

 strange that the sexes should be alike and often dully coloured 



