600 
GEOFI'EEY SMITH. 
the gonad in the great majority of animals directly inhibits 
the full development of the secondary characters, and it may 
appear that the theory outlined above gives no explanation 
of this fact. I clearly realised this in my first statement of 
the theory, and put forward the suggestion that the -sexual 
formative substance accumulated, especially at maturity, in 
the gonad, and that the removal of the gonad removed a 
large quantity of the substance and so inhibited the growth 
of the secondary sexual characters. I do not feel, however, 
that this explanation is at all adequate, principally for the 
reason that the removal of the gonad in the young immature 
animal has usually a more pronounced effect than its removal 
in the adult. It is therefore more probable that the sexual 
formative substance is in many cases actually worked up and 
qualitatively altered by the gonad, and that its presence in 
this altered state is essential in most cases for the full 
development of all the sexual characters. 
We may indeed hold, with the highest degree of probability, 
that tlie sexual formative substance, both male and female, 
is by no means a single simple substance, but that it consists 
of numei’ous substances continually changing dui-ing develop- 
ment, and both acting and acted on by the various organs of 
the body. A view very similar to this is held by Mr. Walter 
Heape, as the result of his experiments (16). He considers 
that there is present a generative ferment” which is pro- 
duced somewhere in the body and which governs the activity 
of the generative glands, and another substance, “ gonadin,” 
secreted by ovary or testis, which controls the other sexual 
characters, but he is clearly of opinion that in certain cases it 
may be the generative ferment which controls the secondary 
sexual characters, and this would bring his view into close 
agreement with my own. 
The theory which has been outlined above, and which 
differs from other theories chiefly in that it attempts to 
include those cases in which the correlation between primary 
and secondary sexual characters is of an uncertain and per- 
plexing nature, has been attacked by Mr. Cunningham (14) 
