bladders, 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.
(50. Bell, ibid. pp. 112-114.) In the several genera of the family the
vocal organs differ considerably in structure, and their development in
all cases may be attributed to sexual selection.
REPTILES.
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. (51. Mr. C.J. Maynard, The American Naturalist,
Dec. 1869, p. 555.) With the huge tortoise of the Galapagos 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 distance of
more than a hundred yards; the female, on the other hand, never uses
her voice. (52. See my Journal of Researches during the Voyage of the
Beagle, 1845, p. 384.)
With the Testudo elegans of India, it is said that the combats of the
males may be heard at some distance, from the noise they produce in
butting against each other. (53. Dr. Gunther, Reptiles of British
India, 1864, p. 7.)
CROCODILIA.
The sexes apparently do not differ in colour; nor do I know that the
males fight together, though this is probable, for some kinds make a
prodigious display before the females. Bartram (54. Travels through
Carolina, etc., 1791, p. 128.) describes the male alligator as
striving to win the female by splashing and roaring in the midst of a
lagoon, swollen to an extent ready to burst, with its head and tail
lifted up, he springs or twirls round on the surface of the water, like
an Indian chief rehearsing his feats of war. During the season of
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