Ill 



ble into partial and complete, the former being confined to the 

 phenomena of albinism and melanism. Melanic antigeny 

 is more common in this country at the south, and albinism 

 at the north ; while, in apparent contradiction to this, albinic 

 females of a partially antigenic species, as Eurymus philodice, 

 never appear in the spring brood, but increase in numbers 

 throughout the hot season. So far as we know (^Cyaniris 

 pseudargiolus is an instance-^), the opposite is the case with 

 melanic females. Structural antigeny is found in the antennee, 

 legs and wings, affecting especially the contour of the wings, 

 or the direction of their veins, and showing itself in peculiar 

 patches of scales or rows of special hairs (of wdiich the discal 

 dash of our smaller skippers is a good example) or in folds 

 of the wings, as seen in other skippers. But one of the most 

 extraordinary cases, extraordinary both in its nature and in 

 its concealment, is found in the presence of peculiar scales, 

 called plumules or androconia, which occur in nearly every 

 group of butterflies, although often entirely hidden by the 

 ordinary scales. In endeavoring to account for them, the 

 theory of sexual selection, put forth by Darwin, appears to 

 fail just where we most need its aid. 



Baron Osten Sacken has given us an important paper on the 

 distribution of Diptera in this country, based, in great measure, 

 upon the large collections made by him in the western territo- 

 ries. He contends for the essential unity of the entire western 

 region of this continent, from the eastern limit of the dry 

 plains to the Pacific Ocean, a region characterized by its 

 extreme dryness in summer. Adducing his examples from all 

 orders, he compares the organic forms of this region with those 

 of the Mediterranean and central Asiatic regions of the old 

 world, a district possessing very similar meteorological condi- 

 tions ; he recalls especially the prevalence in both of Het- 

 eromera among the Coleoptera and of Bombylidae among the 

 Diptera, and remarks that the resemblance between the faunas 

 of these two vast regions is an analogy, not a relationship. 



1 Since the delivery of this address, Mr. Edwards, to whom we owe our knowledge of 

 the melanic form of this insect, and wlio has heretofore stated that the melanism was 

 peculiar to the female, finds that all melanic individuals are males. 



