EVIDENCES OF A DIFFERENTIAL DEATH RATE, ETC. 157 
é. g., erustacea, insects, and other mammals besides man. In 
the following pages the evidence for the existence of such a 
differential death-rate is presented. 
a). THE PLAICE, Pleuronectes platessa L. 
In the common plaice, which occurs abundantly over a wide 
distribution on the shores of western Europe, a considerable 
number of workers have gathered data which demonstrate the 
operation of a differential death-rate of the sexes. FULTON 
(1902, p. 356) first called attention to the fact that while in 
population taken as a whole, the sexes are practically equal 
( a conclusion drawn much more cogently from larger masses 
of cata by HEFFORD, 1909, ATKINSON, 1908, et al.) neverthe- 
less with older populations marked discrepancies from the 
typical sex-ratio occur, due to the “slower growth and earlier 
death of the males.” PETERSEN, GARSTANG & KYLE (1907, 
Table LX VI) showed by age-frequency tabulations of German 
and English investigations (the condition of the otoliths be- 
ing the criteria of age) that the proportions of males to fe- 
males progressively decrease with age. WALLACE (1907) in- 
dependently arrived at the same conclusion respecting the 
plaice of the southern North Sea. ATKINSON (1908)-found 
the same general age-frequency relation in plaice from an 
unfished area, the Barents Sea, thereby disposing of the objec- 
tion that this relation is due only to an “artificial” factor, 
i. €., trawling and fishing in the North Sea. He concludes 
that “it is probable that the rapid diminution in numbers and 
final disappearance of males” in the age-groups is “accentu- 
ated by an earlier mortality of that sex.” Thus, while in the 
4511 young plaice which he measured and studied the males 
were nearly as numerous as the females (males, 47%; fe- 
males, 53%) in the older fish 50-plus m. long the propor- 
tions were: male, 2-minus % ; females, 98-plus %. 
HEFFORD (1909, 1916) and WALLACE (1914) have shown 
with a large mass of carefully analyzed data the same rela- 
tion. The work of HEFFORD (1909) is especially thorough- 
going, and its implications clear. This writer made a statis- 
tical analysis of the records of measurements of plaice caught 
in the period of 1903-1907 by the various international re- 
