7 -eneee 
GEOGRAPHIC DISTRIBUTION OF LIFE IN NORTH AMERICA. 397 
New England, and its southern border crossed New Jersey and Penn- 
sylvania, and thence, curving irregularly southwesterly 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 corresponding  fluctua- 
tions in the northern boundaries of the faunal and floral areas immedi- 
ately to the south; for if is reasonable to suppose that throughout the 
period covered by the movements of the ice mantle, and probably in 
later pre-glacial 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 northern 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 climatie 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 latitudes, 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 climate 
sufficiently cool for their needs, and here they have existed to the present 
day.* 
Throughout the growth of the great ice mass and its extension from 
the north southward it is clear that the animalsand plants that could 
not keep pace with its advance must have perished, while the steady 
pushing toward the tropics of those that were able to escape to the 
rapidly narrowing Jand in that direction must have resulted in an over- 
crowding of the space available for their needs and a corresponding in- 
crease in the severity of the struggle for existence. The sustaining ca- 
pacity of a region is limited; hence such a thing as over-crowding, in 
the sense of greatly increasing the number of organisms a region can 
support, is an impossibility, for beyond a certain limit all excess of life 
*In a former communication attention was called to the circumstance that the 
presence or absence of such arctic-alpine colonies on high voleanic mountains may be 
of use to the geologist as affording evidence of the age of the volcanic activity result- 
ing in the upheaval of the mountain, the absence of Arctic or Boreal forms indicat- 
ing postglacial origin. (N. Am. Fauna, No, 3, September, 1890, p. 21.) 
