A. D. Darbishire. 
proves on closer investigation to be a case in which the hybrid is 
intermediate between the two parents. And the question whether 
all cases of dominance may not be due to the fact that we have not 
yet succeeded in detecting the differences between the hybrid and 
the parent from which it is apparently indistinguishable, presents 
itself most forcibly to the mind. 
LECTURE VI. 
The Mendelian Inheritance of Sexual Characters. 
EC ENT genetic research has thrown a flood of light on the 
nature and mode of inheritance of sex. In the first place it 
has practically demonstrated the correctness of the view that the 
sex of an animal or plant is already determined in the fertilized 
egg-cell; in the second place, the probability that there is a very 
close similarity between the Mendelian unit-character and the 
factor which determines the sex of an organism puts us in the way 
of discovering the essential nature of one of these two things. 
Let us deal first with the first of these two results of genetic 
research. The theory of sex which until recent years had met with 
general acceptance was that the sex, not only of the fertilized ovum, 
but of the growing embryo, was indeterminate and that it was not 
determined until a fairly late stage in development by factors, the 
nature of which then became the subject of discussion. It was 
held that the developing organism maintained the balance between 
the two sexes, that its sex was, as it were, up to a certain moment 
in development, poised on a knife-edge, and that at this point 
external stimuli of some kind resulted in its falling off on one side 
or the other, i.c., in its becoming a definitive male or female. One 
result of the prevalence of this theory was that all work on the 
determination of sex consisted in testing hypotheses as to the nature 
of the stimuli which resulted in the production of males and of those 
which resulted in the production of females. Schenk’s experiments 
with man and Yung’s with tadpoles are among classical instances 
