THE PLEISTOCENE OR GLACIAL PERIOD. 493 



latitude 43° 15' north, but how much farther north they extended 

 is only indefinitely implied by their apparent abundance and their 

 congenial associations in this latitude. Penhallow suggests an exten- 

 sion 200 miles farther to the northward. 1 The ice had previously 

 reached about latitude 39° in Kentucky, and 37° 30' in Illinois. How 

 much south of the ice limit the pawpaw and osage orange were driven 

 by the cold zone bordering the ice-sheet, is at present rather a matter 

 of theoretical estimate than of direct evidence, and is differently placed 

 by different students of the subject, since at present quite divergent 

 views are entertained respecting the climatic conditions that sur- 

 rounded the ice-sheets. The best suggestion drawn from the existing 

 evidence is found in the southward migration of the larch or tamarac 

 (Larix). At present, its southern limit is not far from the northern 

 limit of the pawpaw and osage orange. It overlaps the former a little, 

 and falls short of the latter. A fossil Larix has been recovered at 

 Dahlonega, Ga., latitude 34° 30', or about 480 miles south of its pres- 

 ent limit, and 300 miles south of the glacial margin. Its extreme 

 southern migration is undetermined, and may not improbably be 

 appreciably farther south. It is not unlikely that the northern limits 

 of the pawpaw and osage orange were forced as far south as was the 

 southern limit of the larch, thus preserving about their present rela- 

 tions. This would involve a total migration of at least 600 miles, and 

 not improbably 800 miles or more. 



The fact that nearly all the plants of the Toronto beds belong to 

 existing species, while most of the beetles belong to extinct species, 

 is highly suggestive relative to contemporary differences in the stages 

 of evolution of associated organisms, and relative to varying rates of 

 evolution. It is in harmony with other evidence that the insects 

 were still in a state of rapid evolution, while the plants had more 

 nearly reached a static stage. 



Other interglacial epochs. — In the Aftonian formation there is 

 evidence at many points of an ample growth of vegetation, recorded 

 in peat and muck beds, in humus-bearing soils, and in twigs, limbs, 

 trunks, and even stumps of trees. No great variety of life has, how- 

 ever, as yet been identified; more, perhaps, because the beds are not 

 fortunately situated for investigation, than from any probable dearth 



1 Notes on Tertiary Plants, loc cit., p. 69. 



