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 
