164 THE CANADIAN ENTOMOLOGIST. 



THE EFFECT OF THE GLACIAL EPOCH UPON THE DIS- 

 TRIBUTION OF INSECTS IN NORTH AMERICA. 



BY AUG. R. GROTE, A. M. 



(Read before the American Association for the Advancement of Science, at 



Detroit, Aug. roth.) 



From the condition of an hypothesis the glacial epoch has been 

 elevated into that of a theory by the explanations it has afforded to a 

 certain class of geological phenomena. The present paper endeavors to 

 show that certain zoological facts are consistent with the presence, during 

 past times, of a vast progressive field of ice, which, in its movement from 

 north to south, gradually extended over large portions of the North 

 American continent. These facts, in the present instance, are furnished 

 by a study of our Lepidoptera, or certain kinds of butterflies and moths 

 now inhabiting the United States and adjacent territories. Before pro- 

 ceeding with the subject, a brief statement of phenomena, assumed to 

 have attended the advent of the glacial epoch, is necessary. 



At the close of the Tertiary, the temperature of the earth's surface 

 underwent a gradual change by a continuous loss of heat. The winters 

 became longer, the summers shorter. The tops of granitic mountains in 

 the east and west of the North American continent, now in summer time 

 bare of snow and harboring a scanty flora and fauna, became, summer 

 and winter, covered with congealed deposits. In time the mountain 

 snows consolidated into glacial ice, which flowed down the ravines into 

 the valleys. Meanwhile the northern regions of the continent, which 

 may have inaugurated, submitted extendedly to the same phenomena. 

 Glacial ice, first made on elevations, finally formed at, and poured over, 

 lower levels. Glacial streams finally united to form an icy sea, whose 

 frozen waters slowly plowed the surface of the rocks, and whose waves, in 

 their movement from north to south, absorbed the local glacial streams in 

 their course, and extended over all physical barriers into the Southern 

 States and down the valley of the Mississippi. Before this frozen deluge 

 the animals must always have retreated. The existing insects of the 

 Pliocene must, in submitting to the change of climate which accompanied 

 the advance of the glacier, have quitted their haunts with reluctance, and 

 undergone a severe struggle for existence, no matter how gradually they 



