STUDIES IN THE EXPERIMENTAL ANALYSIS OP SEX. 601 
who I’egards my views as illogical, self-contradictory, and 
inconsistent with the state of modern biology ; indeed, he 
lays so many charges to my account that modesty compels me 
to suspect that some of them may be true ; but I can hardly 
think that the difference between our respective views is 
proportional to tlie severity of his indictment. The only 
diffe rence of importance which I can discover is that, whereas 
he believes that the internal secretion controlling the develop- 
ment of the secondai-y sexual characters is always produced 
by the differentiated gonad, I do not believe that this theory 
covers all the essential facts, but that we must assume some 
common factor at the back of both primary and secondary 
characters which may act to a certain extent on either 
separately, in an independent manner. On the other hand, I 
have never denied the direct influence of the primary character 
on the secondary to a limited extent, as it appears to me to 
be clearly proved by a very large body of facts. 
In conclusion, Mr. Cunningham agrees with me in believing 
that the explanation of the development of the secondary 
sexual characters, and of their correlation with the primary, 
depends on the presence of a substance, sexual formative 
substance or internal secretion, circulating in the body, which 
in some manner activates the cells of various organs and parts 
of the body and causes them to develop and to become 
differentiated according to sex, and I think that he would 
agree with me that a great deal of experiment and observa- 
tion is necessary before we can decide with any certainty as 
to the nature of this substance or substances. 
That we are dealing with the presence of an internal 
secretion is strongly suggested by the analogy of the internal 
secretions produced by other organs, such as the thyroid and 
other ductless glands. In the development of the sexual 
characters we perceive distant parts of the body being 
affected in a parallel manner at the same time, while a removal 
of part of the system may profoundly modify other distant 
parts. This inter-connection can only be accounted for in 
one of two ways — either by the supposition that it is due to 
