598 ■ Wm. E. Kellicott 



grass-frog (Rana temporaria). The observed ratio here in the toad 

 (Bufo) of 658 : 1000, males to females is therefore of the same order 

 as that of other lower vertebrates. At first sight the smaller num- 

 ber of male toads, taken in connection with the fact that their 

 variability is much less than that of the females might seem to indi- 

 cate that their reduced number was the result of a selection more 

 stringent among them than among the females. The data them- 

 selves however give no indication of this and that this is probably 

 not the case is suggested by data from Pfliiger ('81). He showed 

 that the relative numbers of males and females in Rana fusca were 

 not sensibly different in the natural population and among indi- 

 viduals resulting from artificial fertilization. His figures give for 

 806 young individuals a ratio of 357 : 1000, males to females, and 

 for older forms 374 : 1000. His results therefore agree with those 

 of Griesheim which his experiments were designed to check. 

 Among adults he found the ratio 963 : 1000, indicating that the 

 greater death-rate was among the females, but the number counted 

 here was too few to give positive results. 



In a very general way it seems true that there is a relation be- 

 tween the ratio of males to females and a condition of monog- 

 amy and polygamy. Among the higher forms where an approx- 

 imation at any rate to monogamy is the general rule the sexes are 

 about equal numerically, with a tendency toward excess of males. 

 While among the lower forms where one male more usually may 

 fertilize the ova of several females, especially if fertilization is 

 external, there is a decided preponderance of females. There 

 are frequent exceptions of course but the general relation seems 

 to exist. Much more information is needed concerning the ratio 

 between the sexes, especially in the lower vertebrates and also 

 regarding their mating habits. 



While there are no definite data which afford a satisfactory expla- 

 nation of this sex disproportion, it may be worth while to sug- 

 gest that it would result from a reproductive (genetic) selection 

 of those individuals which were more largely female producing. 

 The usual overproduction of spermatozoa together with the fact 

 that the ova of several females are fertilized by a single male 

 would give a decided advantage to species in which females pre- 



