Hormones 



related species, in one of which the sexes are 

 alike and in the other dissimilar in appearance. 



Another important point to be borne in mind 

 is the intimate correlation that exists between the 

 reproductive organs and the general appearance 

 of the organism, more especially of the secondary 

 sexual characters. These last, in most cases, do 

 not show themselves until the maturity of the 

 sexual organs. The well-known effects of castra- 

 tion illustrate this connection. Again, females in 

 which the reproductive organs have ceased to be 

 functional often assume male characters. 



It has lately been proved by experiment that, 

 in many cases at any rate, the development of 

 the ornaments, etc., characteristic of the sexes 

 is due to the secretion by the sexual cells of 

 what are known as hormones that is to say, 

 secretions which excite development of the 

 secondary sexual characters. The tendency to 

 produce the external characteristics of the sex 

 to which an organism belongs is inherited, but 

 the actual development thereof is in many cases 

 dependent on the secretion of these hormones. 

 Accordingly, if a male individual be completely 

 castrated it ceases to develop the external 

 characters of its sex. The evidence upon which 

 the doctrine of hormones is based is admirably 

 summarised in the above - quoted paper by 

 Cunningham. Into this evidence we cannot 

 go. It must suffice that the doctrine is quite 



335 



