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The Descent of Man

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several genera <strong>of</strong> the family the vocal organs<br />

differ considerably in structure, and their development<br />

in all cases may be attributed to sexual<br />

selection.<br />

REPTILES.<br />

CHELONIA.<br />

Tortoises and turtles do not <strong>of</strong>fer well-marked<br />

sexual differences. In some species, the tail <strong>of</strong><br />

the male is longer than that <strong>of</strong> the female. In<br />

some, the plastron or lower surface <strong>of</strong> the shell<br />

<strong>of</strong> the male is slightly concave in relation to the<br />

back <strong>of</strong> the female. <strong>The</strong> male <strong>of</strong> the mud-turtle<br />

<strong>of</strong> the United States (Chrysemys picta) has<br />

claws on its front feet twice as long as those <strong>of</strong><br />

the female; and these are used when the sexes<br />

unite. (51. Mr. C.J. Maynard, '<strong>The</strong> American<br />

Naturalist,' Dec. 1869, p. 555.) With the huge<br />

tortoise <strong>of</strong> the Galapagos Islands (Testudo nigra)<br />

the males are said to grow to a larger size

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