270 PROGRESS OF SCIENCE IN THE CENTURY. 



the worms, there are burial beetles, excavating 

 beneath the corpse of bird or mouse, weevils and 

 wireworms destroying the roots of plants — and scores 

 of other more or less subterranean animals. Then 

 the impression of the living earth begins to grow 

 upon us. Moreover to the business of animals we 

 have to add that of plants, — the curving movements 

 of rootlets, the spreading growth of underground 

 stems, and the sprouting of seeds. 



Real, however, as all this visible activity is, it is 

 not that on account of which we have ventured to 

 speak of the living earth. The phrase is even more 

 thoroughly justified by work which is done by the 

 Bacteria of the soil, and the recognition of this — 

 dating from Pasteur — may be fairly called one of 

 the characteristic achievements of the nineteenth 

 century. It has led to a vivid realisation of the great 

 fact of the circulation of matter. 



THE PKOBLEM OF PETROGRAPHY. 



Microscopic Analysis, — Just as the biologist an- 

 alyses the body of an animal into organs, tissues, 

 and cells, and ends with a study of the complex 

 organic substances therein contained, so the geologist 

 distingTiishes different kinds of rocks — limestone, 

 basalt, granite, and so on, proceeds to describe the 

 fine structure of each, and ends with a determination 

 of the chemical composition of the several constitu- 

 ents. In a general way, petrology is to geology 

 what histology is to anatomy, — an analysis of micro- 

 scopic structure; and just as the study of histology 

 inevitably leads to the study of histogenesis — that 

 is, how the different tissues are developed — so petrol- 

 ogy will only be completed when the origin as well 



