5 o THE DARWINIAN THEORY 



hence that our climate was once much colder than it 

 is at the present time. If, on the other hand, we 

 turn to Greenland, which is now in the glacial con- 

 dition, we find beneath the ice, beds containing fossil 

 plants, showing the former existence in Greenland of 

 such plants as the chestnut, oak, plane, beech, and 

 poplar ; nay even the magnolia, vine, walnut, and 

 plum ; proving the former existence not only of a 

 moderate, but of a warm climate. 



Geology shows us that the boundaries of land and 

 sea are not constant ; for instance, that Britain and 

 France were once united, and that the sea is en- 

 croaching on the land on one side, while the land is 

 encroaching on the sea on the other. The crust of 

 the earth is made up chiefly of rocks deposited under 

 water ; therefore where there is now dry land, there 

 must once have been open sea. Geology further shows 

 that these changes have not been of a sudden cata- 

 clysmal character, but gradual ones, changes which 

 are actually in progress at the present day, and 

 which must always have been going on since the 

 earth began. 



The last link in the chain is now complete. 

 Owing to incessant geological change in environ- 

 ment, variations in structure, previously useless or 

 harmful, become advantageous, and their possessors 

 thereby triumph and survive, and hand down their 

 advantages to their descendants ; thereby in course 

 of time causing structural modifications of greater or 

 less extent in the race. All Nature is in a condition 

 of more or less unstable equilibrium. The action of 

 environment is indirect, and changed conditions of 



