28 SEXUAL SELECTION. Part IL 



are provided with sacs Avliicli open into the larynx/^ 

 For instance, in the edible frog {Rana esculenta) " the 

 ^' sacs are peculiar to the males, and become, when filled 

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

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

 " the corners of the mouth." Th^e croak of the male is 

 thus rendered exceedingly powerful ; whilst 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 attributed to sexual selection. 



Eeptiles. 



Chelonia. — Tortoises and turtles do not offer well- 

 marked sexual differences. In some species, the tail 

 of the male is longer than that of the female. In 

 some, the plastron or lower surface of the shell of the 

 male is slightly concave in relation to the back of the 

 female. The male of the mud-turtle of the United 

 States (Chri/semys jpida) has claws on its front-feet twice 

 as long as those of the female ; and these are used when 

 the sexes unite.^^ With the huge tortoise of the Gala- 

 pagos Islands {Testudo nigra) the males are said to 

 grow to a larger size than the females : during the 

 pairing-season, and at no other time, the male utters a 

 hoarse, bellowing noise, which can be heard at the dis- 

 tance of more than a hundred yards ; the female, on 

 the other hand, never uses her voice.*^ 



Crocodilia. — The sexes apparently do not differ in 



« J. Bishop, in ' To.ld's Cyclop, of Anat. and Pliys.' vol. iv. p. 1503. 

 ^^ Bell, ibid. p. 112-114. 



•»^ Mr. C. J. Maynard, ' The American IS'aturalist,' Dec. 1869, p. 555. 

 '**' See my 'Juurnal of Kesearches during the Voyage of the 

 Beagle," ' 1845, p. 384. 



