176 



SEXUAL DIMORPHISM 



are large pouches formed from the skin of the mouth, into 

 which they open. The males croak chiefly in the breeding 

 season, the vocal sacs serve as resonators to the voice, and 

 their formation is evidently due to the distension of the 

 mouth with air in the effort of croaking. The croak may be 

 of use to the male in attracting the female, and the loudest 

 croakers may or may not be, on the whole, most successful in 

 obtaining mates. But the fact that under the influence of 

 sexual excitement the males croak, is the reason why the sacs 

 have been formed in the males and not in the females. In 

 one species, Rhinoderma Darwinii, belonging to the narrow- 

 mouthed family, or Engystomatidre, the male puts his vocal 

 sacs to a strange use. The sacs are much enlarged and 

 internally confluent, so that they form an extensive chamber 

 on the lower side of the body. The chamber is entered by 

 the two apertures at the sides of the tongue, and the eggs laid 

 by the female are taken by the male into his mouth, and 

 passed into this chamber, where the tadpoles are developed. 

 It is difficult to understand how such a peculiar habit arose. 

 But whether the eggs first got into the vocal sacs accidentally 

 when the male was devouring them, or whether he took 

 them into his mouth with the intention of protecting them, 

 the only reasonable explanation of the enlargement of the 

 vocai sacs is that it was due to the distension caused by the 

 eggs and tadpoles. 



In two other cases the female has a peculiar structure 

 whose function is the protection of the eggs during develop- 

 ment. In Nototrema the pouched tree-frog of tropical 

 America (Fam. Hylidae), the skin of the back is invaginated 

 to form a pouch opening backwards. The eggs are pushed 

 into this cavity by the male by means of his hind feet. Here 

 again the pouch may be attributed to mechanical distension, 

 although it is difficult to imagine how on a female, originally 

 with no pouch, the male should have set about forming one. 



