34 
ORIGIN OF 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. 
f Transeau, E. N., “ Distribution of Bog Plant Societies,” p. 414. 
