396 



SAMUEL F. HILDEBRAND. 



not normally occur in midsummer with a proper relative number 

 of males present." These authors probably overlooked the fact 

 that copulation is not necessary between broods in Gamlnisia, nor 

 in certain other Poeciliids, as it has been shown by Zolotnisky 

 (1901), Philippi (1908) and Hildebrand (1917) that several 

 broods of young may be produced by females after they are sepa- 

 rated from males. For example, Hildebrand (1917) segregated 

 some female Gainbusia early in the spring, providing them with 

 individual aquaria. One female, at least, produced five broods of 

 young that summer, without further copulation. If copulation is 

 not necessary during the summer, as appears to be the case, then 

 certainly the scarcity of males cannot account for the cessation of 

 reproduction during midsummer. Furthermore, the writer has 

 held both sexes of Gainbusia together in aquaria for a number of 

 years and he has not noticed that the reproductive period is length- 

 ened thereby. It appears to be logical, therefore, to conclude 

 that the great reduction in the proportionate number of gravid fe- 

 males at the end of August is not due to the scarcity of adult 

 males. On the contrary, it is the opinion of the writer that the 

 animals by that time have expended all of the energy on reproduc- 

 tion they can afford and breeding ends, regardless of weather 

 conditions. 



TABLE I. 

 THE SEX IN ADULT Gambiism. 



SEX RATIO IN IMMATURE Gainbusia. 



It already has been pointed out that the sex ratio in young fish 

 has received comparatively little attention. This is rather sur- 

 prising in view of the many accounts that have been published 

 dealing with the sex ratio of adult Pceciliids. Perhaps this inter- 

 esting problem did not receive more attention for the want of 



