26 



SEXUAL SELECTION. 



[Part II. 



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

 veloped than those of the females. In some genera the 

 males alone are provided with sacs which open into the 

 larynx. 43 For instance, in the edible frog (Rana esculen^ 

 to) " the sacs are peculiar to the males, and become, when 

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



Fig. 32.— Megalophrys montana. The two left-hand figures, 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. 44 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 



43 J. Bishop, in 'Todd's Cyclop, of Anat. and Phys.' vol. iv. p. 1503. 

 ** Bell, ibid. pp. 112-114. 



