Volume X DECEMBER, 1920 No. 4 



NOTE ON AN ALTERNATING PREPONDERANCE 

 OF MALES AND FEMALES IN FISH, AND ITS 

 POSSIBLE SIGNIFICANCE. 



By JULIAN S. HUXLEY. 



In a paper published some years ago Mr E. G. Boulenger (1912) 

 mentioned that the sex-ratio of some " Millions Fish " {Girardinus 

 poeciloides), bred by him in the Zoological Gardens, was approximately 



3?:lc/. 



In private conversation and in letters, Mr Boulenger has recently 

 informed me that this state of affairs continued for nearly a year, the 

 preponderance of females being observed in nearly every brood. During 

 the winter, however, the sex-ratio changed, and there was a pre- 

 ponderance of males, but this preponderance was not so great as that 

 of the females had been. He has unfortunately no record of the actual 

 figures, but is sure that the preponderance was less ; he places the new 

 sex-ratio at about 2 $ : 3 </. This second period lasted only a short time 

 (not more than six weeks), and was succeeded by a third in which 

 approximately equal numbers of males and females were produced. 

 The sex-ratio continued thus (1 ? : 1 </) for the remainder of the time 

 during which the fish were bred at the Gardens — a period of several 

 years. 



Most of the fish had been some time at the Gardens before 

 Mr Boulenger came there, having been received from Barbados about 

 1909. The sex-ratio during that time was not recorded. A few were 

 received fi-om Kew Gardens during his own experiments. All, however, 

 belonged to the same sub-species. The sex-ratio in the wild state is 

 not known, but is presumed to be 1:1. 



The experiments were not carried out with a view to results of 

 genetic interest, but to furnish a supply of Girardinus for various 

 tropical stations, in order to devour mosquito larvae. About 150 adults 

 were kept. The number of young in a brood is from 5 — 25. A new 



Jouru. of Gen. x 19 



