SEX-RATIOS IN GAMBUSIA HOLBROOKI. 



195 



males have an index-value of less than 2.500, while on the other 

 hand, only 4.3 per cent, of the females show an index-value in 

 excess of 2.500. The finding's in the populations graphed are 

 paralleled by the results of other series of measurements. 



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From this we see that the indexes for adult male Gambusia 

 are such that by means of them the male can easily be distin- 

 guished from the females, in the lack of better criteria, even 

 with rather old " sterile males " in which the gonopod is not well 



