COURTSHIP AND SEX 229 



ductive organs throughout the body, hke chemical keys 

 searching for certain locks which they can open or close. 

 It is not that the gonadial hormones furnish anything towards 

 the production of a structure like a comb or a wattle, they 

 stimulate pre-formed tendencies of growth. They are 

 of the same order as other morphogenetic internal secretions, 

 such as that of the thyroid gland ; and it must be understood 

 that the potency of the gonadial internal secretions is not 

 confined to the secondary sex-characters, it is seen also in 

 relation to habit of body and to general behaviour. 



The influence exerted by the gonads on the secondary 

 sex-characters differs in different groups of animals. In 

 insects the development of the secondary sex-characters is 

 independent of the gonads. In male crabs the removal of 

 the testes profoundly changes the composition of the blood 

 and induces the putting on of feminine characters, which is 

 just the reverse of what happens in birds. In Mammals 

 the removal of the testes produces an effect very similar to 

 that seen on the male bird, but the castrated female mammal 

 undergoes little change in her secondary sex- characters. 



Goodale's Experiments. — An important series of experi- 

 ments on Rouen ducks, brown Leghorns, and other birds 

 has been made by H. D. Goodale, 1916. 



The plumage of the castrated male is little changed ; but 

 that of the female extensively, as regards shape, size, colour, 

 and colour-patterns of feathers. 



The capon's comb and wattles remain rather infantile ; 

 in the castrated hen the comb becomes large and male-like 

 in some, but remains small in others. 



The capons have well- developed spurs, and in the 

 castrated hens which put on male plumage there were well- 

 developed spurs. In cases where the assumption of male 

 plumage was partial or temporary (owing to partial removal 

 of the ovary), the spurs which had begun to grow continued 

 doing so after the plumage had reverted. This suggests that 

 the dependence of the spurs upon the internal secretion 

 is relatively slight. The inhibition exerted in the female 

 upon the development of the spurs is so slight that once they 



