106 Annals of the Carnegie Museum. 



The Sex Ratio. 



The general predominance of females in collections of viviparous 

 Poeciliids has been repeatedly noticed by students of these fishes. The 

 bright colors of the males and structure of the anal fin make the dis- 

 crimination of the sexes in mature examples an easy matter. Garman 

 (1895, p. 7), after observation of numbers of specimens, was impelled 

 to say, " By common experience collectors find males to be less nu- 

 merous than females. The striking appearance of the male will no 

 doubt be claimed as evidence of selection, because of a possible benefit 

 in enabling the female more readily to find him; it may also be utilized 

 in explaining the discrepancy in numbers, since it must be efTectual 

 in making him an object of more prominence and a more frequent prey 

 than the other sex for enemies of the species." 



Most writers are inclined to attribute the discrepancy in numbers of 

 the two sexes to the much smaller size of the male, which causes it to 

 be overlooked by collectors and also enables it to escape through the 

 meshes of seines, which readily hold the bulkier females. From a 

 study of the Haseman collections I observed that in nearly every species 

 the number of females is more than double that of the males. 



I have carefully examined all of the specimens in a pint of Lebistes 

 reticulatiis taken under Dr. Eigenmann's direction in the Barbadoes. 

 This collection was taken with a very finely meshed net and represents 

 the conditions actually found in nature, since it includes specimens of 

 minute water-insects, plant-rootlets, and other things. Many fishes, 

 not larger than five millimeters, have been included and it is quite 

 evident that few went through the seine. This lot comprises 2,070 

 specimens, of which 520 are males, 630 are females, and 920 are too 

 small to be determined. The latter average about eight or ten milli- 

 meters; when that stage has been passed it becomes easier to distinguish 

 the sexes. Males then exhibit the characteristic markings, the anal 

 fin becomes advanced and modified, and full maturity is reached when 

 the male is about fifteen millimeters in length. Females are much 

 larger and may reach a length of from twenty-five to thirty milli- 

 meters. It is c}uite certain that this count of males includes only 

 members of that sex, while a few of the smaller specimens regarded 

 as females may really have been immature males. It will thus be 

 seen that the sex ratio, when an adequate collection is at hand, does 

 not materially differ from that found in other fishes. Mr. Edward 



