160 THE AMERICAN MIDLAND NATURALIST. 
1901-3—in which embryos were collected, the proportions of 
males to females were respectively 35:39, 48:46 and 66:74.) 
However, in adult populations, the females are very much in 
the majority, being twice as numerous as the males. He con- 
cluded, consequently, that the post-natal mortality is greater 
among the males than among the females—a conclusion which 
was pointed independently by a statistical treatment of the 
meristic variations. 
PUNNETT’S study suggests that a differential death-rate of 
the sexes may be responsible for the low percentages of males 
found in certain rays, as reported by FULTON (1903.) 
g.) THE Top-MINNow, Gambusia holbrooki Grd. - 
GEISER (1921) showed a greater death-rate among the 
males in shipments of top-minnows. This obtained in both 
cold weather and warm weather-.shipments. Thus, in cold 
weather the male death-rate was one and one-half times the 
female death-rate; in warm weather, it was two and one-half 
times the female death-rate. The same writer (GEISER, 1922) 
has also noted that.the proportions of the sexes of this fish 
at birth are practically equal. Adult populations, on the other 
hand, show a great preponderance of females, thus indicating 
a higher male death-rate after birth. Further experimental 
studies yet unpublished, demonstrate a differential death-rate 
of the sexes when diverse harmful substances are added to 
the environment. 
II1I.—EVIDENCES FROM OTHER GROUPS OF ANIMALS. 
The writer has not been able to ransack the literature ef 
other groups in a thoroughgoing fashion. There have, how- 
ever, a number of pieces of work and observation appeared 
which appear to indicate unmistakably the operation of a 
lethal selection such as has been shown so well in the fishes. 
These will be briefly referred to in the subsequent paragraphs. 
a.) Mammals other than Man. JEWELL (1921) in his study 
of foetal cattle found a differential death-rate of the sexes 
similar to that found in man, and in fishes. 
b.) Crustacea. PUNNETT. (1904a) has noted that in the 
Shore-Crab, Carcinus moenas, the males are less viable than 
