SEXUAL SELECTION- 456 



served by Prof. Hoffman in Giessen fighting ail day long 

 during the breeding season, and with so much violence 

 that one had its body ripped open. 



Frogs and toads offer one interesting sexual difference, 

 namely, in the musical powers possessed by the males; but 

 to speak of music, when applied to the discordant and over- 

 whelming sounds emitted by male bull-frogs and some other 

 Bpecies, seems, according to our taste, a singularly inappro- 

 priate expression. Nevertheless, certain frogs sing in a 

 decidedly pleasing manner. Near Eio Janeiro I used often 

 to sit in the evening to listen to a number of little Hylse, 

 perched on blades of grass close to the water, which sent 

 forth sweet chirping notes in harmony. The various sounds 

 are emitted chiefly by the males during the breeding season, 

 as in the case of the croaking of our common frog.** In 

 accordance with this fact the vocal organs of the males are 

 more highly developed than those of the females. In some 

 genera the males alone are provided with sacs which open 

 into the larynx." For instance, in the edible frog {Hana 

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

 when filled with air in the act of croaking, large globular 

 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, while that of the female 

 is only a slight groaning noise." 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. 



BBPTILES 



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 



^ Bell, "History of British Eeptiles," 1849, p. 93. 



« J. Bishop, in "Todd's Oydop. of Anat. and Phys.," voL Iv. p. 1603. 



H> Bell, ibid. p. lia-114. 



