SEX RATIO IN GAMBUSIA. 399 



only on specimens that had developed no external sex characters 

 by means of which they could be definitely recognized. These 

 data show that no hard and fast line, with respect to size, can be 

 drawn between adult and immature fish, as the external sex char- 

 acters are developed at a much larger size in some individuals 

 than in others. In this group of large immature fish, based on 

 i, 660 specimens, the males, however, are considerably in the 

 minority, as the ratio is I male to 1.48 female. 



A few immature fish of even a larger size than the group dis- 

 cussed in the preceding paragraph were found. For example, 

 among 285 fish, ranging from 25.5 to 30 mm. in length which 

 could not be sexed definitely from external characters, the ratio 

 was i male to 6.12 females. In this connection it may be noted, 

 however, that not a single immature male exceeding a length of 

 28 mm. was found. 



The variation in size and age at which the anal fin in the male 

 becomes differentiated, that is, when it is developed into an intro- 

 mittent organ or " gonopod "' (as designated by Geiser (1924)) 

 already has been pointed out by Hildebrand (1917) and Mast in 

 Barney and Anson (1921). The variation in size at which this 

 organ may develop appears to be even greater than these writers 

 supposed. It is evident from these data also that a considerable 

 number of males are included among the females when all fish of 

 about 21 mm. and over in total length are considered " adult," as 

 the present writer did in sexing fish by the use of external charac- 

 ters, or as Barney and Anson (1921) did when they classed all 

 fish of 15 mm. and over in length, to the base of the caudal, as 

 adults. A considerable number of males, therefore, were classed 

 as females in Table I. However, as stated elsewhere, the error 

 in sexing is not large enough to cause the great minority of males 

 that is shown in Table I. The greatest error in sexing, that is, 

 the largest number of males classed as females, undoubtedly oc- 

 curred during the months when the males were increasing most 

 rapidly in proportionate numbers, as for example in September 

 and October, for it is then that the largest number of young are 

 maturing and many come within the " doubtful ' ; size group. 

 (The principal error is among fish 21 to 23 mm. in length.) Simi- 

 larly, the error in sexing must be smallest when the fewest young 



