28 
SEXUAL SELECTION. 
Part IL 
are provided with sacs which 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.” The 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 (Chrysemys picta) 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 
j)airing-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 ‘ Todd’s Cyclop, of Anat. and Phys.’ voL iv. p. 1503. 
Bell, ibid. p. 112-114:. 
Mr. 0. J. Maynard, ‘ The American Naturalist,’ Dec. 1869, p. 555. 
See my ‘Journal of Eesearches during the Voyage of the 
“ Beagle,” ’ 1845, p. 384. 
