Glacial Epoch. 43 



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 animals and 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 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. The sustaining capacity of a region is 

 limited; hence such a thing as overcrowding, 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 

 must perish overcrowding inevitably leading to death. The 

 mortality in any one year may not have been great, but during 

 the untold ages covered by the movements of the continental ice 

 the aggregate destruction of life must have been stupendous. 



Immediately upon the close of the Glacial epoch life began 

 to reclaim the regions from which it had been so long shut 

 out. This overflow released the tension under which the ani- 

 mals and plants had been struggling for ages and rendered 

 the contest for existence less severe. Overproduction had at 

 last found an outlet, and life became possible to a constantly 

 increasing number of individuals. Normal reproduction was 

 sufficiently rapid to supply occupants for the regions made 

 habitable by the slow recession of the ice, and the advance of 

 both plants and animals kept pace, doubtless, with its pro- 

 gressive increase. But the species that survived to return were 

 only in part those driven out. Many had been overtaken by 

 the cold or had perished in the journey southward ; others were 

 driven into inhospitable regions where the environment was not 

 suited to their needs ; others still succumbed in the struggle 

 resulting from overcrowding, and some that outlived the first 

 great period of glaciation perished during the second. Gilbert 

 tells us that a detailed study of the ancient lake beds of the 



* In a former communication attention was called to the circumstance 

 that the presence or absence of such arctic-alpine colonies on high vol- 

 canic mountains may be of use to the geologist as affording evidence of 

 the age of the volcanic activity resulting in the upheaval of the mountain, 

 the absence of Arctic or Boreal forms indicating postglacial origin. (N. 

 Am. Fauna> No. 3, September, 1890, p. 21.) 



