156 THE AMERICAN MIDLAND NATURALIST. 
time of spawning :—all these have been suggested in explana- 
tion of atypical sex-ratios found in catches of adult fish. If 
we disregard for a moment these factors, there are two al- 
ternative explanations that appear a priori most probable: 
(a) a possible differential death-rate of the sexes during the 
juvenile and adult period, and (b) an atypical distribution of 
sex-determining chromosomes during the maturation divisions 
of the germ-cells. As a result of such an unequal distribution 
of sex-chromosomes, a preponderance of one sex over another 
could be conceivably be produced.* For simplicity’s sake we 
exclude here from consideration possible causation of atypical 
sex-ratios through the operation of a differential chance-of- 
fertilization of the two kinds of gametes in fertilization. 
Cytological investigation of maturation-divisions of the 
germ-cells of bony fishes has failéd to throw light on their 
chromosomal constitutions; genetic study of the Poeciliids 
Aplocheilus latipes (AIDA, 1921) and Lebistes reticulatus 
has, however, demonstrated them to be of the XX, XY type. * 
A corollary of this finding would be the practical numerical 
equality of the sexes at birth (assuming no differential chance- 
of-fertilization or differential mortality of gametes.) This ~ 
corollary has been established for a number of species of 
fishes: cytological examination of the gonads of litters of 
certain fishes (Cymatogaster, EIGENMANN, 1896; Spinaz, 
PUNNETT, 1904; and Gambusia, GEISER, 1922) have shown 
the proportions of the sexes at birth to be practically equal. 
It remains to be ascertained whether in fishes as a group, 
the sexes are not nearly equal in numbers at birth, the atypi- 
cal ratios later found being due to the operation of a differ- 
ential death-rate. 
While ransacking the voluminous literature of the fishes 
the present writer found recorded numerous data which ap- 
pear to indicate in fishes a differential death-rate of the sexes 
similar to that found in man. As much of this evidence is 
published in out-of-the-way places, or in serials not readily 
accessible, it has seemed well to bring it together in a general 
paper. Together with this evidence for a differential mor- 
tality in the fishes, the writer has also found in the literature 
strong indications that the.same thing holds for other animals, 
