SEXUAL CHARACTERS IX BIRDS. 677 



why some birds present great extra-sexual cliffereBce and others 

 little or none. The conclusions arrived at in this paper do not 

 appear to be explicable on this theory ; there seems to be no 

 reason, for instance, why birds especially subject to the attack of 

 enemies should be more katabolic than birds not especially so 

 subject. 



The Mewhlian Theories. — Althougli the followers of Mendel 

 have carried out very important investigations with regard to 

 secondary sexual cliaracters, as far as I am aware, they have 

 made no attempt to account for the distribution of these charac- 

 ters in the animal kingdom. 



The Hormone Theory (' Archiv f iir Entwicklungsraechanik,' 

 1908), J. T. Cunningham. — According to this theory, secondary 

 sexual chai-acters become established in the following manner : — 

 As a result of certain mechanical and functional stimuli, conlined, 

 for instance, to the male, certain organs of the male take on new 

 characters : these are acquired characters. During their pro- 

 duction a specific hormone is produced, which stimulates the 

 corresponding determinant in the gametes and alters them in 

 such fashion that, during development, they reproduce the 

 acquired character ; but it also alters them in another diiection — 

 it implants in them an ability to reproduce the acquired character 

 only in the presence of the male sexual hormone, In the absence 

 of the male sexual hormone, the character cannot develop : for 

 instance, if a stag be castrated the antlers cannot develop on 

 account of the absence of the male hormone — the character 

 i-emains dormant. Obviously the material here dealt with 

 applies only to the first part of the theory, that dealing with 

 mechanical and functional stimuli. 



In the case of birds the functional stimuli, which call forth the 

 bright plumes of the males, are those associated with the display 

 of these ornaments during courtship, "the tail of the peacock 

 became brilliant and hyjoertrophied because it had been con- 

 stantly erected." The stimulus calling upon the male to display 

 seems, therefore, to be the deciding factor. Thus in order to 

 support this theory, it would be necessary to show that the males 

 of birds subject to enemies suffer more from such stimuli than 

 do those birds relativelj' not liable to be attacked. 



Finally, it is necessary to point to the importance of the 

 correlation luider consideration with respect to the possibility of 

 it being able to account for the presence of secondary sexual 

 characters. 



The activities of animals fall under three headings ; — 



(1) Those associated with the procuring of food. 



(2) ,, ,, escape from enemies. 



(3) ,, ,, reproduction. 



It has usually been thought that the sexes bear the same 

 relation to the environment as regards the procuring of food and 

 the escape from enemies ; and therefore to account for secoiulary 



