1903] WEBSTER: — DIFFUSION OF INSECTS aq 



hemisphere in many cases readily become naturaHzed in North America, it is very 

 rare indeed that the reverse is the case. The reason for this is not understood, nor 

 do we know for a certainty how far back in the past these conditions have obtained ; 

 but the general opinion among entomologists is, that the trend of diffusion was from 

 the northwest and not toward it. 



But there came a tremendous climatic change, and the northern ice sheet 

 plowed its way southward, crushing and grinding the rocks into sand, clay, boulders, 

 and pebbles, leveling down the elevations, filling up the channels of rivers and the 

 beds of lakes, at the same time transforming the once tropical country into that of 

 the frigid north. How long this condition continued we do not know; but while it 

 did continue, all intercommunication from northern Asia and Europe by the way 

 of the northwest was necessarily cut off. Presumably the ice sheet began to melt 

 aw^ay along its southern border in the now United States, gradually uncovering the 

 land to the northw^ard. How rapidly this area was again covered by vegetation and 

 reoccupied by animal life w^e do not know ; nor can we state how far beyond the 

 tennination of this ice sheet the flora and fauna had been obliged to retreat. We 

 know that, in our day and in case of the present glaciers, one can almost stand on 

 the edge of the ice and collect insects of the temperate zone ; but this really proves 

 nothing in the case of the huge ice sheet covering millions of square miles of area 

 and being probably hundreds of feet in thickness. But, be these facts as they may, 

 it seems very probable that the country as fast as it was uncovered by the receding 

 ice became occupied by plant and animal life /rom the south. Indeed, it hardly 

 seems possible that the trend of diffusion could have set in from any other direc- 

 tion ; but how much of a basis for this diffusion was left along the south Atlantic 

 and Gulf coasts we do not know. 



Reoccupied by plant and animal life, we have a country as first discovered by 

 the white man, comprising an immense inland plain, the eastern border bulwarked 

 by the Appalachian mountain system, low and broken, it is true, and by the much 

 more stupendous and continuous Cordilleran system to the west, open to the Gulf to 

 the south and to the frigid zone to the northward. This area, now known as the 

 basin of the Mississippi River, because of its being largely drained to the south by 

 that river and its tributaries, was comparatively level, and though heavily timbered 

 to the south and east, was scantily or not at all timbered to the west and north. 

 The drainage to the north, being by the Saskatchewan into Hudson Bay and the 

 Mackenzie into the Arctic Ocean, was, perhaps, a less potent factor in the problem 

 of restocking this country with insects. We have now three natural gateways, so 

 to speak, through which insects not introduced by commerce must make their way 

 into North America. These are by way of northern Asia into Alaska, and thence 

 south and east ; by way of Central America, through Mexico ; and by way of the 



