10 MIXjSrOWS GAMBUSIA AFFINIS AND CYPRINODON VARIEGATUS. 



specimens the specialized form of the fin becomes evident when the 

 fish is only 13 mm. in length and less than 3 months old; in otl\ers it 

 is not apparent at the age of 5 months or at a length of 17 mm. For 

 example, a lot of 43 young born in May, 1914, the smallest of which 

 was 17 mm. in length, was examined on October 15, 1914, and was 

 thought to comprise females only; but on June 3, 1915, 6 of the 39 

 fish surviving were easily recognized as males. It may, therefore, 

 be stated that the modification of the anal fin into an intromittent 

 organ may take place when the fish reaches a length of 13 mm., or at 

 any later stage until it attains its maximum normal growth of about 

 25 mm. 



The proportion of males to females in this species has been dis- 

 cussed by various writers. In collections the males are generally much 

 in the minority. It has been argued that this is due to the small 

 size of the males, which permits them to pass through the meshes of 

 nets and thus to escape capture. However, when the writer has 

 collected the minnows with mosquito netting of a mesh so small that 

 not even the tiniest male may pass through, the disparity in the 

 numbers of the sexes has remained evident. Among the lots grown 

 in the aquarium, the inequality is quite as great as it seems to be in 

 nature. For example, on June 2, 1915, 60 of the young of the pre- 

 vious season had survived, and of these only 7 were males. The 

 indications are that in the broods of 1915 the sexes are just as une- 

 qually represented, although, as shown above, the sexes can not be 

 positively determined at this time (October, 1915). Owing to the 

 rather heavy loss during the early stages of life in the aquarium, the 

 results as stated above may not afford a reliable criterion, although 

 there is no apparent reason why aquarium life should not be as well 

 suited to the male as to the female. Among the adults there is 

 much fighting between the sexes and the males often suffer severely, 

 but among the young these disastrous conflicts have not been observed. 

 It seems entirely probable that the normal ratio of males to females 

 is about 1 to 8 or 9. 



The extreme prohficness of the species has already been the subject 

 of comment. It is particularly interesting to know that the early 

 broods of the season reach sexual maturity" and some of the fish 

 begin to breed before they are four months old. During both seasons 

 that the young have been observed and grown in the aquarium the 

 oldest and largest females among the broods have delivered their 

 first young during September. At this time the females are only 

 about 23 mm. in length, and the first brood consists of only two or 

 three young. In the fall of 1914 two of the largest females hatched 



o Seal (1911, p. 95) observed that the young of Gambusia holbrooUi and Heterandria formosa began to 

 breed during the season in which they were born. 



