1913 ENTOMOLOGICAL SOCIETY. 29 



sheet of ice, and that the fauna and flora of the region thus covered were either 

 exterminated or pushed southward by the advancing ice-sheet. On the final retreat 

 of the ice-sheet northward to the Arctic regions, the country was repopulated with 

 life from the south, chiefly from those species which had been driven southward 

 during the period of advancement. First came the Arctic fauna and flora, followed 

 by those of the succeeding life-zones, until the present distribution of life was 

 established. During the northward movement remnants of the arctic and subarctic 

 faunas were left behind wherever the conditions were suitable for their existence, 

 such as on mountain-tops, in cold bogs, etc. 



This view is very plausible, but there is much biological evidence to show that 

 the fauna and flora of the glaciated regions never occupied the country south of the 

 drift or glaciated area, and that the climatic conditions in this area were more 

 favourable for the existence of life during the so-called Ice Age than they are at 

 present.* The existence of glaciers does not depend upon intense cold, but chiefly 

 upon a copious precipitation in the form of snow, which, of course, requires a fairly 

 low temperature in the region where the glacier is formed, but also demands a con- 

 siderable degree of warmth in the surrounding country. The glaciers of Alaska 

 occur chiefly in the warmest part, the southern shore, whereas the cold interior is 

 devoid of them. The extensive glaciers of the Ice Age were probably- due to the 

 presence of the land-bridges connecting North America with Europe on the east and 

 Asia on the west. The Arctic Ocean was thus isolated, and the temperature of the 

 Atlantic and Pacific considerably elevated, thereby modifying the climate of the 

 Arctic regions on both sides of the continent and bringing about the conditions 

 necessary for the formation of glaciers. 



Species such as the snail Helix kortensis and the wood-louse Oniscus asellus, 

 whose entrance into North America is traceable to the north-eastern land-bridge from 

 Europe, have not yet reached the higher parts of the White Mountains, though they 

 occur in Northern New England, and the origin of the Arctic element in the fauna 

 of the White Mountains is probably of much earlier date than the Glacial Period. 

 There is no doubt, however, that the presence of these relics of the Arctic fauna does 

 indicate that at some period an Arctic climate did prevail over a large part, if not 

 the whole, of Canada, and the Northern States, and that, with the gradual increase 

 of temperature which followed, species adapted to a colder climate were exterminated 

 or driven northward, stragglers remaining behind wherever conditions were favour- 

 able to their existence. These isolated colonies of northern forms occur not only at 

 high altitudes, but also to a smaller extent at quite low levels, e.g., in bogs, where 

 the soil is wet and poorly drained, and thus colder than the surrounding country. 

 Such restricted areas, inhabited by northern species, are termed " boreal islands." 



Let uis now turn our attention to the map of the faunal zones of North 

 America. 



This map was prepared by the U. S. Biological Survey, and is mainly the 

 work of Mr. C. Hart Merriam, an eminent auiihority on the Mammalia, upon the 

 distribution of which the map is chiefly based. Perhaps the first thing on it that 

 is apt to strike our attention is the transverse arrangement of the zones across 

 the continent. This is because the factor of temperature has been regarded as 

 the controlling one in defining the distribution of the Mammalia, it has been re- 

 marked by several zoologists, however, that laws of temperature control do not 

 define transcontinental zones of primary importance zoologically. They emphasize 

 the secondary, not the primary facts of distribution. Thus, in our map, we have 



*See R. F. Scharff, "Distribution and Origin of Life in America," London, 1911. 



