396 THE DESCENT OF MAN. 



green tints of tree frogs and the obscure mottled shades of 

 many terrestrial species. The most conspicuously-colored 

 toad which lever saw, the Phryniscus nigricans,^ had the 

 whole upper surface of the body as black as ink, with the 

 soles of the feet and parts of the abdomen spotted with the 

 brightest vermilion. It crawled about the bare sandy or 

 open grassy plains of La Plata under a scorching sun, and 

 could not fail to catch the eye of every passing creature. 

 These colors are probably beneficial by making this animal 

 known to all birds of prey as a nauseous mouthful. 



In Nicaragua there is a little frog " dressed in a bright 

 livery of red and blue " which does not conceal itself like 

 most other species, but hops about during the daytime, and 

 Mr. Belt saysf that as soon as he saw its happy sense of 

 security he felt sure that it was uneatable. After several 

 trials he succeeded in tempting a young duck to snatch up 

 a young one, but it was instantly rejected; and the duck 

 '* went about jerking its head as if trying to throw off some 

 unpleasant taste." 



With respect to sexual differences of color Dr. Giinther 

 does not know of any striking instance either with frogs 

 or toads; yet he can often distinguish the male from the 

 female by the tints of the former being a little more intense. 

 Nor does he know of any striking difference in external 

 structure between the sexes, excepting the prominences which 

 become developed during the breeding-season on the front 

 legs of the male, by which he is enabled to hold the female. X 

 It is surprising that these animals have not acquired more 

 strongly-marked sexual characters; for though cold-blooded 

 their passions are strong. Dr. Giinther informs me that he 

 has several times found an unfortunate female toad dead 

 and smothered from having been so closely embraced by 

 three or four males. Frogs have been observed by Prof. 

 Hoffman in Giessen fighting all day long during the breed- 

 season, and with so much violence that one had its body 

 ripped open. 



* " Zoology of the Vovage of the ' Beagle,' " 1843. Bell, ibid, p. 

 49. 



t " The Naturalist in Nicaragua," 1874, p. 321. 



:}: The male alone of tlie Bufo ikimmensis {\>t. Anderson, " Proc. 

 Zoolog. Soc," 1871, p. 204) has two plate like callosities on the 

 thorax and certain rugosities on the fingers, which perhaps subserve 

 the same end as the above mentioned prominences. 



