188 THE WORLD OF LIFE 



earth's surface — is not stable ; but that very considerable 

 changes in climate, in the contour of the land surface, and 

 even in the minor distribution of land and water, have con- 

 tinually occurred during past ages; and that just in proportion 

 to the evidence for such changes do we find that changes have 

 occurred in the forms of life inhabiting every part of the 

 earth. A short statement of the nature of these two groups of 

 coincident and interdependent changes will therefore be useful 

 here. 



The most general and most arresting facts of world-history, 

 revealed by geology, are, that the superficial crust of the earth 

 consists of various " rocks " (including in this term every kind 

 of inorganic matter of which the crust is composed) deposited 

 in more or less regular " strata " or layers, one above another ; 

 that these strata are sometimes horizontal, more often inclined 

 at various angles to the horizon, and even occasionally vertical; 

 usually continuing at about the same angle or slope for many 

 miles, but often curved or waved, or even crumpled up and 

 contorted in remarkable ways. These various strata consist of 

 many distinct kinds of rock — sandstones, limestones, clayey 

 or slaty rocks, metamorphic or gneissic rocks ; and all of these 

 give distinct evidence of having been deposited in water, both 

 from mechanical texture and the arrangement of their com- 

 ponent particles, and also by frequently having embedded in 

 them the remains of various organisms, those that live in seas 

 or lakes being by far the most abundant and varied. As an 

 example of this abundance we may mention the Barton Cliffs 

 on the Hampshire coast east of Christchurch, where, in a dis- 

 tance of a few miles, over a thousand distinct species of the 

 fossilised shells of molluscs, radiates, and other marine animals 

 have been found. 



But the most suggestive fact from our present point of view 

 is, that almost eveiy mountain-range on the earth presents us 

 examples of such stratified rock-strata, often with abundant 

 fossils of marine animals, at enormous heights above the sea- 

 level. Such are found in the Alps at 8000 feet, in the Andes 



