62 EXTINCTION OF VARIOUS GENERA OF ANIMALS. 



already been mentioned, in particular the occurrence of 

 erratic blocks, the presumed existence of icebergs, glaciers, 

 and fields of ice, as well as from the arctic character of many 

 fossil shells, it is conceived that at a comparatively recent 

 period, probably towards the close of the tertiary epoch, the 

 tropical temperature of the ancient earth was succeeded by 

 a climate of greater cold than now prevails in the same 

 regions at the present day. 



EXTINCTION or VARIOUS GENERA OF ANIMALS. This 

 fact will be more fully exposed in our chapter on Palaeon- 

 tology. It may be stated in general, that when the physical 

 conditions of climate, &c., were favourable to certain forms 

 of animal life, they existed ; when the conditions changed, 

 they became extinct, and their places were then filled by 

 others better adapted to the new conditions of the globe ; 

 one species or genus became extinct after another, its place 

 being occupied by a new form ; so that after a certain period, 

 the fauna of a country was gradually but completely changed. 



In proportion as we recede in our investigation from the 

 present into the past, do we meet with species and genera 

 that no longer exist ; and in like manner as we trace animal 

 forms from the ancient organisms of the Silurian strata up to 

 those of the tertiary period, do we encounter types that 

 blend with those of the human epoch. 



OPERATION OF MODERN CAUSES. The consideration of 

 this subject involves the statement of so many facts, and the 

 study of such varied phenomena, that we must refer the 

 student to the admirable philosophical principles of Sir C. 

 Lyell,* for an able exposition of the causes now in operation 

 by which the earth's surface is slowly changed. The fol- 

 lowing resume may be useful, by directing his attention to a 

 careful study of this most interesting and instructive branch 

 of physical investigation : 



1. The natural world is the theatre of incessant move- 

 ments, which are governed by laws that are fixed and 

 immutable. 



2. The changes that have occurred in the earth's surface 

 in past time are essentially the same as those now at present 

 in progress. 



* Principles of Geology, vol. i. 2nd edit. 



