10 



PAPILIO III., IV., V. 



are common, for a yellow female to produce a black, though the chances are that 

 every yellow female in such district has had one or more black female ancestors. 

 Therefore, where the black is an extremely rare form, with the chances largely 

 ao-ainst any yellow female having inherited a tendency to melanism, that one 

 should produce two black, and two deep ochraceous examples (for this latter col- 

 oration is of the same nature as the other), is almost as if this variation in the 

 present case arose spontaneously. I have applied to several collectors in Brook- 

 lyn besides Mr. Akhurst, and all agree in the statement that Glauais is an un- 

 common insect in that vicinity. Mr. Akhurst says also that he has occasionally 

 taken such ochraceous individuals in the field. Here at Coalburgh there is ob- 

 servable in the yellow females a deeper coloring than in the males, especially of 

 the fore wings (Plate III., Fig. 1), but I have seen no such example as that from 

 Brooklyn. Formerly, in the collection of Mr. Wood, at Philadelphia, 1 saw a 

 chocolate colored Glaucus, and Mr. Aaron writes me that he has taken one sim- 

 ilarly colored. 



Many explanations of the phenomena of melanism in general have been sug- 

 gested, such as meteorological conditions — excessive moistui'e, deficient sun- 

 shine, impure air ; also vegetation defiled by soot from furnaces, and the like ; 

 none of which are satisfactory when applied to species on this continent, however 

 it may be when they are restricted to limited areas, as in parts of Europe. In 

 North America, the very reverse of these conditions obtains throughout the re- 

 gions in which the melanic species are mostly found. And as a rule, melanism 

 among butterfiies is confined to one sex, and that the female. It is not ahvays 

 so, as appears by the melanic Philodice male figured in this volume, but nearly 

 every known example belongs to the female. In case of the present species, the 

 facts are, that in the warmer regions, where it is polygoneutic, or many-brooded, 

 both yellow and black females exist, in some districts apparently in about equal 

 numbers, in others with a more or less decided predominance of the black, but 

 occasionally, as in the mountain district visited by Mr. Morrison, the yellow pre- 

 dominating almost if not quite to the exclusion of the other ; that at the north 

 the black disappear at the line at which the species becomes monogoneutic, and 

 the yellow form in both sexes flourishes even to the arctic portions of the conti- 

 nent. Dr. Weismann,^ speaking of 2'urnus, expresses the opinion that " the jel- 

 low is the ancient and original form, the black a much yonnger, or more recent 

 form." During the glacial period, when the shortness and coolness of the season 

 permitted but one brood in the year, just as in the boreal regions now, the spe- 

 cies was yellow in both sexes. As the season became longer and climate milder, 

 from the receding of the ice which had covered the larger portion of the con- 



1 Ueber den Einjluss der Isolirung auf die Arthildung, Leipzig, 18V2, p. 95. 



