COLOR AND COLORATION 165 



a larger proportion ot vivid and varied colors in the tropics. Mayer 

 finds, in the widely distributed genus Papilio, that 200 South American 

 species display but 36 colors, while 22 North American species show 17. 

 While the number of species in South America is nine times as great as in 

 North America, the number of colors displayed is only a little more than 

 twice as great; hence Mayer concludes that the richer display of colors in 

 the tropics may be due to the far greater number of species, which gives 

 a better opportunity for color sports to arise; and not to any direct in- 

 fluence of the climate. Furthermore, the number of broods which occur 

 in a year is much greater in the tropics than in the temperate zones, so 

 that the tropical species must possess a correspondingly greater oppor- 

 tunity to vary. 



Albinism and Melanism. These interesting phenomena, wide- 

 spread among the higher animals, are little understood, but have often 

 been attributed to temperature. 



Albinism is exceptional whiteness or paleness of coloration, and is due 

 usually to lack or deficiency of pigment, but in some instances (Pieridae) 

 to the presence of a white pigment. 



The common yellow butterfly, Colias philodice, and its relatives, are 

 frequently albinic. Scudder observed that albinism among butterflies in 

 America appears to be confined to a few Pieridae, and to be restricted to 

 the female sex; is more common in subarctic and subalpine regions than 

 in lower latitudes and altitudes, and only in the former places includes all 

 the females. At low altitudes, however, instead of appearing early in 

 the year as might be expected, the albinic forms appear during the warmer 

 months. 



The experiments made by Gerould on C. philodice show that the 

 number of albinic female offspring from white females crossed with yellow 

 males is in accordance with Mendelian law. Albinism is not entirely 

 confined to the female as Scudder thought, for white males occur, though 

 they are extremely rare. "They may be expected in regions where the 

 white female is especially abundant" (Gerould). 



In Europe there are many albinic species of butterflies, and they are 

 by no means confined to the family Pieridae. 



Melanism is unusual blackness or darkness of coloration. As to how 

 it is produced little is known, though warmth is probably the most po- 

 tent influence, and some attribute it to moisture, as was mentioned. 

 Pictet obtained partial melanism in Vanessa urticce and V. polychloros by 

 subjecting the larvae to moisture. 



In warm latitudes, some females of our Papilio glaucus are blackish 



