STUDIES IN THE EXPERIMENTAL ANALYSIS OF SEX. 587 
conception lies in the tact that we would have to assume that 
some of the heterozygous individuals ( c? ? ) would have to 
function as males and others as females iu order to secure 
a continuous output of pure males and females. For if all 
the heterozygous individuals were functionally either males 
or females exclusively, they could only produce one form of 
pure zygote iu combination with the gametes of the pure sex. 
The supposition tliat the heterozygote may appear as either 
male or female in the same species of animal is by no means 
impossible, since it only leaves us face to face with the same 
problem, which is at the root of the whole sex question, 
namely. What is the physiological state which brings about 
the suppression or development of the male or female char- 
acters in animals which undoubtedly possess the potentiality 
of both? It is held by a number of observers, for instance 
Mr. ^Valter Heape (9), that the proportional output of the 
Sexes is iuHueuced to a very large extent by external condi- 
tions of feeding, temperature, breeding-seasons, etc., and it 
is quite possible that these influences are sufticient to give a 
bias to the heterozygous embryo to appear as either male or 
female. 
However this may be, the assumption that three types of 
individuals may exist with distinct gametic output would 
account for the marked disproportion in number of the sexes, 
which is known to occur in the offspiing of certain families, 
both among animals (especially butterflies) and human beings. 
There is anotlier class of facts which offers an interesting 
but difficult field for interpretation upon Mendelian lines, viz. 
the sex of parthenogenetically jjroduced ofi^spring. In the 
case of such animals we get every variety of product, the 
parthenogenetically produced young being either only males 
or only females or a mixed brood of both sexes. But there 
are certain common features in these cases. Thus, in the 
case of the bee, in Cladocera, and in certain Eotifera the ferti- 
lised eggs always give rise to females, the males being only 
produced parthenogenetically. The females iu some of these 
cases, e.g.the Cladocera, are proved beyond doubt to be 
