Glacial Epoch. ' 43 



c'liiuato siifficienth^ cool f\)r their needs, and here thcv have 

 existed t() the present day.-'^ 



Throughout the growth ol" the great ice mass and its extension 

 from the north southward it is clear that the animals and plants 

 that could not keep ])ace with its advance must have perished, 

 wliile the steady pushing toward the tropics of those that were 

 ahle to escape to the rapidl}'' 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 oi' organisms a region can sup})ort, 

 is an impossibility, for lieyond a certain limit all excess of life 

 must i^erish — (jvercrowding inevitably leading to death. The 

 mortality in au}?^ one year may not have been great, l)ut 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 i)lants had been struggling for ages and rendered 

 the contest for existence less severe. Overproduction had at 

 last found an outlet, and life l)ecame possible to a constantl}^ 

 increasing number of individuals. Normal reproduction was 

 sufficiently rapid to supply occupants for the regions made 

 haliital)le by the slow recession of the ice, and the advance of 

 both })lant< and animals kept pace, doubtless, with its ]n-o- 

 gressive increase. But the species that survived to return were 

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

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

 ch-iven 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 tliat a detailed studv of the ancient lake beds of the 



* In a former communication attention was called to the circumstan(;e 

 that the presence or absence of such arctic-alpine colonies on lii.yli vol- 

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

 the ao;e 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. o, September, 1890, p. 21.) 



