2Q2 S. W. GEISER. 



Unanalyzed deleterious influences in the environment also 

 bring about a lethal selection to which the males succumb more 

 readily than the females. The writer showed (Geiser, } 2ia), 

 for example, that male Gambusia were less resistant than females 

 to disturbances incidental to shipment, during both cold and warm 

 weather. Thus, in cold-weather shipments the male death-rate 

 was one and one half times the female death-rate, and in warm 

 weather shipments, two and one half times the female death-rate. 

 This latter result, also, was obtained when the females were 

 heavily gravid. In aquarium catastrophes, such as epidemics 

 of Icthyriopthirius- and .S'a/'ro/e'gma-infestations, the males suf- 

 fer much more severely than the females. This experience of 

 the writer is confirmed by that of European aquarists generally. 

 In catastrophes of unknown cause, the same holds. Thus, to 

 record one example out of many recorded in my note book : 

 " On 16 November, 1921, in aquarium containing 94$, 2oJ\ 16 

 dead? and 14 dead J 1 were found. No cause was ascertained. 

 No fungi. Life-conditions apparently excellent. Death-rate for 

 males 800, and for females 148.8- per thousand, i.e., 5 3/8 to 



i.' 



The greater ability of the females to survive is moreover evi- 

 denced in the proportions of males and females which in the 

 writer's aquaria survive the winter. Thus, e.g., in October, 

 1921, approximately 100 Gambusia, fairly equally divided as to 

 sex, were left outdoors in a somewhat protected concrete aquarium 

 to pass the winter: on April 10, 1922 all the survivors, 32 in 

 number, were recovered. Of these, only one was a male. It is 

 consequently evident that the females were more resistant to the 

 weeding-out process than the males, for only 40 per cent, of the 

 females, as compared with nearly 100 per cent, of the males, had 

 died. 



There is still another way in which the numbers of males in 

 a given lot of Gambusia are reduced. The males are smaller and 

 hence are more liable to be devoured by small predaceous fish 

 than the much larger females. Gravid female Gambusia in 

 aquaria, also, attack and frequently kill the males. Records kept 

 of the sex of dead fish taken from the aquaria show over ten 



