SEX-RATIOS IN GAMBUSIA HOLBROOKI. 203 



times as many males as females. This fact lends support to 

 Carbonnier's ('66) contention (Darwin, '75, p. 335) that the 

 males suffer from their small size since they are liable to be de- 

 voured by females of their own species. 



It is thus apparent that in Ganibusia there is a differential 

 death-rate, and that its operation explains the excess of females 

 found in adult populations. 



Elsewhere the writer (Geiser, '230, '24) has shown that such 

 a differential death-rate obtains in various fishes, Crustacea, in- 

 sects, for man, and for mammals other than man. Among the 

 fishes besides those already mentioned it has been quite clearly 

 demonstrated in the European Plaice, the Canadian Plaice, Witch, 

 British Salmon. Smelt, and Dogfish, and the Japanese Sweet- 

 fish or Ayu. The reader is referred to these papers for the evi- 

 dence, which conclusively demonstrates that in many diverse 

 groups of animals the male is less viable than the female. 



IV. CONCLUSIONS. 



1. Field collections of Gambusia almost invariably possess 

 a great preponderance of females. 



2. These sex-ratios vary with the different seasons of the 

 year. 



3. Studies on the spermatogenesis of Gambusia fail to re- 

 veal any unusual distribution of the chromosomes which would 

 explain the atypical sex-ratios found. 



4. Experiments with Gambusia raised in aquaria show the 

 proportions of sexes at birth to be approximately equal. 



5. The males have a higher deatri-rate than the females, thus 

 causing the atypical sex-ratios found in the adult populations. 



ACKNOWLEDGMENTS. 



The writer wishes to gratefully acknowledge the kindly help 

 and criticism of Professor S. O. Mast, under whose direction 

 the present work was done. He also wishes to express his thanks 

 to Mr. S. F. Hildebrand, of the U. S. Bureau of Fisheries, who 

 at various times gave freely of time and material. 



