440 Miscellaneous. 



The Effect of the Glacial Epoch upon the Distribution of Insects in 

 North America. By Aug. R. Grote, A.M. 



From the condition of an hypothesis the Glacial period has been 

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

 a certain class of geological phenomena. The present paper endea- 

 vours to show that certain zoological facts are consistent with the 

 presence, during past time, of a vast progressive held 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 proceeding with the subject, 

 a brief statement of the phenomena assumed to have attended the 

 advent of the Glacial period 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-Atnerican continent, 

 now in summer time bare of snow and harbouring 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 the conditions, 

 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 ploughed the surface of the rocks, and, in their move- 

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

 course, and extended over all physical barriers. The Appalachians 

 and Rocky Mountains are supposed to have had local glaciers. The 

 animals must always have retreated before this frozen deluge. The 

 existing insects of the Pliocene, in submitting to the change of 

 climate which accompanied the advance of the glacier, must have 

 quitted their haunts with reluctance, and undergone a severe struggle 

 for existence, no matter how gradually they had been prepared for 

 the encounter. "We may expect that multitudes of specific forms 

 ultimately perished, of whose remains no traces have been preserved. 



After this brief statement of the outlines of the opening of the 

 Glacial period, we turn to some facts offered by a study of certain of 

 our existing species of butterflies and moths. 



The tops of the "White Mountains and the ranges of mountain- 

 elevations in Colorado offer us particular kinds of insects, living in 

 an isolated manner at the present day, and confined to their re- 

 spective localities. In order to find insects like them we have to 

 explore the plains of Labrador and the northern portion of the 

 North- American continent, in regions offering analogous conditions 

 of climate to those existing on the summits of these mountains. 



