INDUSTRIAL PROGRESS DURING THE YEAR 1875. cxcv 



varieties appearing at different parts of summer, either the 

 beofinninir or close, and that this variation does not occur in 

 the larva state, but is probably due to the different dura- 

 tions of the pupa state. He thinks they are the same in 

 effect with climatic varieties, and that they were induced 

 originally by changes in the climate of Europe during the 

 coming and going of the glacial period. The essay also 

 throws light on the origin of climatic varieties. 



The effect of the glacial period on the distribution of in- 

 sects in North America is discussed by Mr. Grote. The tops 

 of the White Mountains and the Rocky Mountains in Colo- 

 rado offer us particular kinds of insects, living in an isolated 

 manner at the present day, and confined to their respective 

 localities. In order to find insects like them we have to ex- 

 plore the coast of Labrador, and the northern portions of the 

 North American continent in regions offering analogous con- 

 ditions of climate to those existing on the summits of these 

 mountains. The genera CEneis (Chionobas) and Brenthis 

 among the butterflies, and Anarta and Agrotis among the 

 moths, are represented by the same or similar species in all 

 of the above-mentioned localities. In the case of the White 

 Mountain butterfly, CEneis semidea, we have a form sustain- 

 ing itself in a very limited alpine area on the top of Mount 

 Washington. 



The question comes up, with regard to the White Mount- 

 ain butterfly, as to the manner in which this species of 

 CEneis attained its present restricted geographical area. 

 Mr. Grote answers it by the action attendant on the decline 

 of the glacial period. Many of the features of the advance 

 of the glacial sheet were repeated in the reverse order on 

 the subsidence of the main ice-sheet or glacial sea. The 

 local glaciers appeared again, separate from the main bodies 

 of ice, and filled the valleys and mountain ravines, thus run- 

 ning at variance with the main body of the glacier, being 

 determined by local topography. A reversal of the temper- 

 ature shortened the winters and lengthened the summers. 

 Ice-loving insects, such as the White Mountain butterfly, 

 hung on the outskirts of the main ice-sheet, where they found 

 their fitting conditions of temperature and food. " The main 

 ice-sheet had pushed them insensibly before it ; and during 

 the continuance of the glacial period the geographical dis- 



