INEQUALITIES OF SEDIMENTATION 361 



autumn ice at points like Fond du Lac and Fort Rae on Great 

 Slave Lake, we will see how at certain such points in the lakes 

 and in the bogs which lay near the routes of migration the bodies 

 of numberless caribou may have been entombed while other lakes 

 and bogs remote from the north and south routes of migration 

 might be entirely free from such remains. 



Some species of animals which do not possess the seasonal 

 migration instinct are subject to waves of migration which recur 

 at irregular intervals often separated by two or more decades. 

 Such waves appear to be caused by an excessive increase in the 

 numbers of a species within its normal habitat. The rapid-breed- 

 ing Siberian lemming is subject to these migration waves. Dr. 

 Brehm^ vividly describes the way in which impending famine 

 starts an exodus from an overpopulated tundra. 



Famine threatens, perhaps actually sets in. The anxious animals crowd 

 together and begin their march, hundreds join with hundreds, thousands with 

 other thousands, the troops become swarms, the swarms armies. They 

 travel in a definite direction at first following old tracks, but soon striking out 

 new ones; in unending files — defying all computation — they hasten onwards; 

 over the cliffs they plunge into the water. Thousands fall victims to want 

 and hunger; the army behind streams on over their corpses; hundreds of 

 thousands are drowned in the water or are shattered at the foot of the cliffs; 

 the remainder speed on; other hundreds and thousands fall victims to the 

 voracity of Arctic and red foxes, wolves and gluttons, rough-legged buzzards 

 and ravens, owls and skuas which have followed them; the survivors pay no 

 heed.^' 



The famine crazed hordes of migrating individuals which may 

 result when a species greatly outbreeds the food possibilities of 

 its habitat have probably furnished important contributions to the 

 fossil bone beds of previous epochs as they are doing to those of 

 the present. 



Phenomenal increase in the animalcule life of fresh and salt 

 water sometimes results in the pollution of the water and destruction 

 of all the life in it. The oldest recorded example of this kind is 

 probably the bibHcal one of the Egyptian plagues when the water 

 of the Nile was as it were "turned to blood and all the fish died. "^ 



^ A. E. Brehm, From the North Pole to the Equator, p. 79. 



' R. F. Scharff, The History of the European Fauna (1899), pp. 139-40. 



3 Exodus 7:20, 21. 



