THEORIES OF MIGRATION 37 



to-day there are thousands of square miles of forest, 

 prairie, lake, and marsh. Practically all life had been 

 driven from this space, or, in some cases, had been ex- 

 terminated in it. Species that persisted had been 

 forced into the open region to the south of the glacial 

 front. Let us suppose now that after a period the ice, 

 through some change in conditions, begins to recede. 

 The shift is slow. Each year there are regressions in 

 which the ice king seeks to hold his own. Gradually, 

 however, new territory is released, to which vegeta- 

 tion slowly spreads, followed by insects and other 

 invertebrates, with which come the birds. 



The dominant species, the one that is successful in 

 life, produces in greater abundance than the home 

 areas can maintain. The surplus individuals are 

 crowded away from the centre of production, and so 

 are forced out to other regions where conditions are 

 perhaps a little less to their liking at first, but to 

 which conditions they adapt themselves. These we 

 may suppose, in the Pleistocene and later, crowded 

 up into the spaces to the north following the re- 

 treating glaciers. Summer conditions were such that 

 they might breed. As winter approached they found 

 food scarce, and wandered. Their wandering was 

 restricted to three cardinal directions — east, west, 

 and south. If they went north, they perished. To 

 the south conditions were most favorable, so the 

 majority wandered southward. Seasonal change in 



