26 



SRXUAL SELECTION. 



[Pakt II. 



fact the vocal organs of the males are more highly de- 

 veloped than those of the females. In some genera the 

 males alotoe are provided with sacs wliich open into the 

 larynx." For instance, in the edible frog {Rana esculen- 

 ta) " the sacs are peculiar to the male.^, and become, when 

 filled with air in tlio act of croaking, large globular blad- 



Fio. .32.— Mcgalophrys TOontana. The two left-band flares, the male; the two 

 right-hand figures, the female. 



ders, standing out one on each side of the head, near the 

 corners of the mouth." The croak of the male is thus 

 rendered exceedingly powerful ; while that of the female 

 is only a slight groaning noise." The vocal organs differ 

 considerably in structure in the several genera of the 

 family ; and their development in all cases may be attrib- 

 uted to sexual selection. 



Reptiles. 

 Chelonia. — Tortoises and turtles do not ' offer well- 

 marked sexual differences. In some species, the tail of 



*■> .1. Hishop, in ' Toild's Cyclop, of An;it. and Thys.' vol. iv. p. 1503. 

 « Bell, ibid. pp. 112-114. 



