84 ORIGIN OP LIFE IN AMERICA 



on the accuracy of these data, on which the majority of 

 geologists are agreed, biologists have endeavoured to work 

 out the past history of the American fauna in strict accord- 

 ance with the facts these phenomena are thought to reveal. 

 " Throughout the growth of the great ice-mass, and its ex- 

 tension from the north southward," says Dr. Merriam, " it 

 is clear that the animals and plants that could not keep pace 

 with its advance must have perished, while the steady push- 

 ing towards the tropics of those that were able to escape to 

 the rapidly narrowing land in that direction must have 

 resulted in an overcrowding of the space available for their 

 needs and a corresponding increase in the severity of the 

 struggle for existence."* Immediately upon the close of the 

 Glacial Epoch life began to reclaim the regions from which 

 he thinks it had so long been shut out. 



Dr. Allen's views are very similar. " There is evidence," 

 he remarks, " that towards the close of the Tertiary, a marked 

 change in the earth's climate took place, culminating in the 

 Glacial Period, during which the whole northern half of the 

 northern hemisphere became covered with a heavy ice-cap, 

 lasting for possibly thousands of centuries, and extending' 

 its chilling influence nearly to the northern tropic. The rise 

 of the Glacial Period was of course gradual, and the south- 

 ward progress of the great ice- cap drove before it all forms 

 of life capable of any considerable powers of locomotion, while 

 those unable thus to escape must have perished from cold. 

 Finally the ice receded to its present limits and the whole 

 north, under- radically altered climatic conditions, became 

 again available for occupation by the more or less modified 

 descendants of the pre-Glacial exiles. "f 



The bog plant societies so graphically described by Mr. 

 Transeau probably existed, he thinks, along the whole ice 

 front. The bog and tundra types were eventually the first 

 to push into the barren ground left by the retreating ice. J 



Professor Adams takes a more independent attitude. He 

 assumes that repeated glaciation had almost sterilised the 



* Merriam, C. H., " Life in North America," p. 45. 



t Allen, J. A., " Distribution of North American Birds," p. 100. 



X Transeau, E. N., " Distribution of Bog Plant Societies," p. 414. 



