42 MerrJam Geographic Distribution <>f Life. 



tropics, and which explains several of the otherwise inexplic- 

 able problems presented in the study of the past and present 

 distribution of life. 



The snows at the beginning of the Glacial epoch fell upon a 

 continent of great forests forests that gave shelter to multitudes 

 of mammals and birds and other forms of life, a large proportion 

 of which no longer inhabit America, and many of which do not 

 exist in any part of the globe. 



During the period of maximum development the great glacier 

 is believed to have been not less than "8,000 feet in thickness in 

 northern New England, and its southern border crossed New 

 Jersey and Pennsylvania, and thence, curving irregularly south- 

 westerly to southern Illinois and then northwesterly, finally 

 reached the Pacific Ocean in British Columbia. The disastrous 

 effect upon animals and plants of this tremendous body of ice 

 must have reached far south of its actual borders. 



The Glacial epoch is believed to have been made up of at 

 least two principal and a number of minor advances and retreats, 

 separated by long intervals and accompanied doubtless by cor- 

 responding fluctuations in the northern boundaries of the faunal 

 and floral areas immediately to the south ; for it is reasonable to 

 suppose that throughout the period covered by the movements 

 of the ice mantle, and probably in later preglacial times as well, 

 the forms now known as Boreal and Arctic (or their immediate 

 ancestors) inhabited areas characterized by temperatures not 

 very different from those they now require, and that the north- 

 ern limit of each species kept at a certain uniform distance from 

 the ice line. " Plants," says Dr. Gray, " are the thermometers of 

 the ages, by which climatic extremes and climates in general are 

 best measured." 



Important evidence of the correctness of this hypothesis is 

 afforded by the well known presence of colonies or assemblages 

 of arctic species on isolated mountain summits in southern lati- 

 tudes, where the altitude carries them into the low temperature 

 of their homes in the far North. It is obvious that such colonies 

 could not have reached their present positions during existing 

 climatic conditions. But during the return movement of animal 

 and plant life following the retreat of cold at the close of the 

 Glacial epoch, many Boreal species were stranded on mountains, 

 where, by climbing upward as the temperature increased, they 

 were enabled to survive, finding a final resting place with a 



