174 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 there- 

 fore 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 

 component 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 distance 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 every 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 at 14,000 feet, and in the Himalayas at 

 1 6,000 feet elevation. Innumerable cases of marine fossils at 

 lesser heights are to be found in every part of the world, and 



