404 THE DISTRIBUTION OF ANIMALS AND PLANTS 



parts, in which the long continuance of daylight in summer 

 seems in warm periods to have been peculiarly favourable to 

 the introduction of new vegetable and animal forms, and to the 

 giving to them that vigour necessary for active colonization, 

 have originated the greater number of the inhabitants of the 

 land. 



Regarded as portions of the earth's crust, the continents are 

 areas in which the lateral thrust, caused by the secular con 

 traction of the interior of the earth and unequal settlement of 

 the crust, has ridged up and folded the rocks, producing 

 mountain chains. This process began in the earliest geological 

 periods, and has been repeated at long intervals, the original 

 hnes of folding guiding those formed in each new thrust pro- 

 ceeding from the broad oceanic areas. Along the ridges thus 

 produced, and in the narrower spaces between them, the 

 greater part of the sediment carried by water was laid down, 

 thus producing plateaus in connection with the mountain- 

 chains, while the weight of new sediments and the removal of 

 matter from other areas by denudation, have been constantly 

 producing local depression and elevation. The tendency of 

 the ocean to be thrown toward the poles by the retardation of 

 the earth's rotation, alternating with great collapses of the 

 crust at the equator proceeding from the same cause, along 

 with the secular cooling, have produced alternate submer- 

 gence and emergence of these plateaus. This has been 

 further complicated by the constant tendency of the Arctic and 

 Antarctic currents, aided by ice, to drift solid materials, set free 

 by the vast denuding action of frost, from the polar to the 

 temperate regions, and by the further tendency of animal life 

 to heap up calcareous accumulations under the warm waters of 

 the tropical regions. All these changes, as already stated, have 

 conspired to modify the directions of the great oceanic cur- 

 rents, and to produce vicissitudes of climate under which 

 animals and plants have been subjected in geological time to 



