THE CANADIAN ENTOMOLOGIST. 171 



tending from Scandinavia, across Siberia to Greenland. During this time 

 we must imagine that no impediment existed to the migrations of animals, 

 and, among them, insects, across what is now Behring's Straits. Were 

 this narrow channel then existing, it could be easily crossed by the flight 

 of almost any Moth, and of itself could make no effective barrier against 

 a constant interchange of species. 



It is probable that the Tertiary, as it witnessed the first appearance of 

 Man, saw also his first wanderings in North America. He, too, came 

 from Asia by way of the North and the Strait. Evolution had performed 

 surprising work in the meanwhile with one branch of the human family, 

 member? of which sailing to the West and landing from Scandinavian or 

 Spanish ships, met, upon American soil, the descendants of a migration 

 from Asia to America in a former geological period, and to the East ! At 

 the close of this Tertiary period of the earth's history, cold and snow and 

 ice set in ; the long winter of the ages made its appearance in the shape 

 of the Glacial Epoch. The circumpolar Moths, whose more humble for- 

 tunes we must be content here alone to follow, were forced gradually 

 southward by the change in climate which gathered its frigid strength in 

 the North. The European, Asiatic and American faunae then became 

 separated, the latter the most completely, and by barriers both of ice and 

 ocean. The American species of Moths which formerly lived by the 

 Arctic Ocean, were gradually forced downwards to the South, year by 

 year, until they reached Mexico or the then elevated portions of the 

 Southern States. When the ice sheet melted and slowly drained away 

 through the valleys and water channels of a continent awakening for the 

 first time to a Spring and released from a Winter of the Years, the Moths, 

 modified as to species in the long conflict with the climate, retraced their 

 way to the North. As marks of this retreat and return, colonies of But- 

 terflies and Moths were left on the mountains to tell of the flood. On 

 the White Mountains we find to-day the White Mountain Butterfly, Oeneis 

 Semidea, and the Arctic Lappet Moth, Laria Rossii.^ At this time the 

 Western Clawed Cut-worm, Copimamestra Occidenta, had become per- 

 manently separated from what is now the European C. Brassicae, and the 

 differences which separate the two to-day as distinct species are the result 



* See a number of papers on this subject, in particular my original communication 

 read before the American Association for the Advancement of Science, August, 1875, 

 and an article entitled "A Colony of Butterflies," originally printed in the American 

 Naturalist, 



