266 PROGRESS OF SCIENCE IN THE CENTURY. 



There are others, like Oswald Heer, who have 

 found satisfaction in combining the cosmical and 

 the geographical theories. 



The last, or, since the stock is prolific, perhaps 

 the latest hypothesis as to cause of glacial periods, 

 is that of Professor Chamberlin who maintains 

 that the climatic conditions which brought about 

 ice ages arose from an impoverishment of the quan- 

 tity of carbonic acid in the atmosphere. 



The aim of this section has been to indicate (1) 

 the great change that has occurred in geology since 

 the uniformitarians attempted to interpret glacia- 

 tion apart from glaciers, (2) the gradual develop- 

 ment of glacial geology, from a careful study of 

 existing glaciers and their luorJc to a detection of the 

 range aiid routes of ancient glaciers of much greater 

 size, (3) the importance of the idea of a relatively 

 recent (post-Tertiary) Great Ice Age interrupted 

 hy intervening periods of mildness, and (4) the un- 

 certainty that still obtains as to the cause or causes 

 of this and previous glacial periods. 



THE HAND OF LIFE UPON THE EARTH. 



One of the distinctive results of nineteenth-cen- 

 tury science is the recognition of the important part 

 which living creatures have played in fashioning the 

 features of the earth. Each year's work has of late 

 brought to light some fresh instance of the domi- 

 nance of the hand of life, and we have devoted 

 this section to its illustration. The central names 

 are those of Charles Darwin and Louis Pasteur. 



Plants. — From 1810 when Rennie outlined the 

 history of Scottish peat-bogs to the latest paper on 

 nitrifying Bacteria, the importance of plants in 



