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PAPILIO III., IV., V. 



tineut, there was at length time for a second brood to mature the same season, 

 and the species gradually became digoneutic. Still later, for the same reasons, 

 in the more southern districts, a third generation could mature. And with the 

 summer generation, at some stage, came in the black form, which is restricted 

 even now to the districts in which a summer generation is possible. The cause 

 may have been in some way climatal, or this variety may at first have been 

 accidental, and once in existence, favored by circumstances, was able to perpet- 

 uate its type through its descendants till it has become common, sometimes almost 

 to the exclusion of the yellow and original form. 



'• The origin of the black form we can only explain by supposing that, at one 

 time, when Turnus already occupied a territory as extensive as it holds to-day, 

 some unknown influence caused the black female forni to appear as a distinct 

 variety, and that, owing to some circumstance, it thereby gained an advantage 

 over its rival, which caused it finally to supplant the other, and to spread over a 

 large extent of country. This supplanting process must have begun with one 

 individual, or a veiy few individuals. There is no case known where a whole 

 species became aberrant, and the supposition that the black form appeared sim- 

 iiltaneously among hundreds or thousands of individuals may be rejected as 



untenable One or a few black females here stand opposed to myriads 



of yellow ones, and have finally jjroved victorious over them This vic- 

 tory can be explained in no otlxer way than through the supposition of the use- 

 fulness of the black color." Dr. Weismann inclines to consider it a case of sexual 

 selection, the superiority of the blacks having been gained by their attractiveness 

 to the males. However this may be in general, it may be stated that the yellow 

 females taken l>y me, at Coalburgh, have as surely been fertilized as the blacks, 

 and have as readily laid eggs ; and on the wing the males may be seen coquet- 

 ting with the yellow a.s freeh' as with the blacks. There would seem to be no 

 want of attractiveness in such individual instances. 



I have experimented to see if it were possible that the butterflies emerging 

 from chrysalis in midsummer might show a stronger tendency to melanism than 

 those emerging in the spring, from over-wintering chrysalids, but have found no 

 evidence that the heat of summer or cold of winter exert influence on the re- 

 sulting forms of the female. In June, 1875, I obtained eggs by confining several 

 black females upon the limbs of a tulip tree, and there resulted therefrom, in A.\\- 

 gust following, 9 ^^j 2 black '. Part of the chrysalids passed the winter, and in 

 the spring there emerged 9 '^, 5 black '. 



In the spring of 1872, there emerged from chrysalids of the previous year, the 

 eggs having been laid by black females, 15 '^, 7 black ', 2 yellow '. 



In the spring of 1877, from eggs laid by black, 21 '^, 7 black *. 



