ON SOMATIC SEX-CHARACTERS 71 



physical agents. Mechanical irritation, especially 

 of the interrupted kind, repeated blows or friction 

 causes hypertrophy of the epidermis and of super- 

 ficial bone. I have stated this argument and the 

 evidence for it in some detail in my volume on 

 Sexual Dimorphism. It is one of the most striking 

 facts in support of this argument that the hyper- 

 trophied plumage which constitutes the somatic 

 sexual character of the male in so many birds is 

 habitually erected by muscular action for the purpose 

 of display in the sexual excitement of courtship. 

 I doubt if there is a single instance in which the male 

 bird takes up a position to present his ornamental 

 plumage to the sight of the female without a special 

 erection and movement of the feathers themselves. 

 Such a stimulation must affect the living epidermic 

 cells of the feather papilla. Even supposing that 

 the feather is not growing at the time, it is probable, 

 if not certain, that the stimulation will affect the 

 papilla at the base of the feather follicle, so as to 

 cause increased growth of the succeeding feather. 

 But we have no reason to believe that erection in 

 display occurs only when the growth of the feathers 

 is completed, still less that it did so always at the 

 beginning of the evolution. 



The antlers of stags are the best case in favour 

 of the Lamarckian view of the evolution of somatic 

 sexual characters. The shedding of the skin 

 ('velvet') followed by the death of the bone, and 

 its ultimate separation from the skull, are so closely 

 similar to the pathological processes occurring in 

 the injury of superficial bones, that it is impossible 

 to believe that the resemblance is only apparent 

 and deceptive. In an individual man or mammal, if 



