26 TREE ANCESTORS 



A moment's reflection will convince the reader that an ice cap 

 like that of modern Greenland with centers of radiation in Labrador, 

 west of Hudson Bay and in the Cordilleras and extending south- 

 ward to New Jersey and Kentucky was a most profound factor 

 affecting the present flora of North America both as regards the 

 present distribution, the extinction of old, and the evolution of 

 new types. The effect of glaciation was even more marked in 

 Europe than in America for in the former the mountain chains 

 run in an east-west direction and with the Mediterranean, Black 

 and Caspian seas effectually blocked the retreat of the flora to the 

 southward. Hence there are many types of trees that are exclu- 

 sively American and Asian in the existing flora that were common 

 to Europe in pre-Pleistocene and a part of Pleistocene times. Some 

 of these are the magnolia, locust, sassafras, bald cypress, black 

 walnut, butternut, etc. 



The accumulated data are quite insufficient to enable us to trace 

 the waves of plant and animal life that swept back and forth across 

 North America with the fluctuations of the cHmate. We know that 

 the sweet birch and the larch got dovm into Georgia during an 

 advance of the ice sheet and we have found spruce cones in the 

 Pleistocene deposits at Chicago. We find the red bud, osage orange 

 and other warm temperate types as far north as Toronto, Canada, 

 during an interglacial period, and specimens of a fig in fruit has 

 been found in interglacial deposits as far north as the Kootenai 

 Valley in British Columbia. We have such facts as the presence 

 of fossil wood of Pseudotsuga near Kansas City, Missouri, and the 

 wood of the black spruce (Picea mariana) and the cones of the 

 white spruce {Picea canadensis) beneath the drift of the Kansas 

 ice sheet or second glacial advance in Iowa. 



Away from the ice covered regions the climate was probably 

 not severe and if there was any pronounced secular lowering of 

 temperatures it appears to have been offset by increased humidity 

 and the dense stands of timber in the lowlands. We infer this 

 from the fact that in deposits of Pleistocene age in the region to 

 the southward of the ice sheet scarcely ever do the fossil plants 

 indicate any appreciable differences from present day temperatures. 



