546 GENETICS IN RELATION TO AGRICULTURE 
intersexes of Lymantria according to Goldschmidt with the one exception 
that the quantitative differences between the male and female factors of 
the female zygote necessary for the differentiation of female characters, 
are reduced in the free-martin by internal secretions instead of by varia- 
tions of potency of the male factors in different varieties as in the inter- 
sexual hybrids of Lymantria.”” In attributing the free-martin condition 
to the male hormones Lillie means only to assert that they are the primary 
causes, and not that they are the decisive factors in each member of the 
series of events which result in the intersexual condition. He can, how- 
ever, state confidently on the basis of present results that sex-determi- 
nation in mammals is not irreversible predestination, and that with 
known methods and principles of physiology we can investigate the 
possible range of reversibility. 
It will be observed that neither of these cases invalidates the funda- 
mental hypothesis that the sex-chromosomes are the normal differen- 
tiators in sex-determination. Moreover, the sex-chromosome hypothesis 
has this virtue, that it is based upon observed and firmly established 
differences between the sexes. It is disappointing in that it provides 
so little hope for control of the process, but our dissatisfaction with it 
from this standpoint should not close our minds to its superiority in 
definiteness and experimental evidence to all other theories of sex- 
determination. 
Inheritance of Unusual Sex-ratios.—From time to time reports are 
made of families both in man and other animals which appear to exhibit 
consistently abnormal sex-ratios. Families are reported in which male 
children only have been born for a number of generations, or in which 
only females have been born. Now according to the laws of chance such 
instances may occur occasionally without necessitating in any way the 
adoption of hypotheses subsidiary to that of the existence of a mechanism 
which potentially is calculated to give an approximate equality of the 
two sexes. But sometimes other factors do appear to be at work, and 
these may be mentioned briefly here. 
The existence of sex-linked lethal factors in Drosophila has already 
been pointed out. Presumably these are factors which affect adversely 
the development or operation of some vital organ as a consequence of 
which individuals possessing the factor are doomed from the moment of 
conception to death at some stage in their life history. In some cases 
this occurs relatively late in the life history. Thus Bridges reports the 
discovery of a strain of flies with such a factor in which the morescent 
larve are distinguished by the production of black specks within the 
body cavity. These larvee die when they reach maturity, but in other 
cases death must occur soon after fertilization. Moreover, in some cases 
the doomed individuals may occasionally overcome the defect and 
develop into normal adults. 
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