184 THE WORLD OF LIFE 



older masses were subjected to intense subterranean heat 

 and compression, which altered their texture, and often 

 crumpled and folded them up in the strangest manner 

 conceivable. Then, perhaps, a long period of elevation 

 brought them up and up, till they were many thousand 

 feet above sea-level ; and, when the superficial covering of 

 newer beds had been all removed by denudation, the folded 

 strata were themselves exposed to further denudation, and 

 all the strange peaks and ravines and rushing cataracts of 

 alpine mountains became revealed to us. 



Thus, in alternate belts or more extended areas, our 

 continents have been, step by step, built up throughout the 

 ages, with repeated alternations of sea and land, of mountain 

 and valley, of upland plateaux and vast inland seas or lakes, 

 the indications of which can be clearly traced throughout 

 the ages. And, along with these purely terrestrial changes, 

 there have been cosmic changes due to the varying eccen- 

 tricity of the earth's orbit and the precession of the equinoxes, 

 leading to alternations of hot, short summers with long, cold 

 winters, and the reverse ; culminating at very distant intervals 

 in warm and equable climates over the whole land surface 

 of the globe ; at other shorter and rarer periods in more 

 or less severe " ice-ages," like that in which the whole north 

 temperate zone was plunged during the Pleistocene period, 

 long after the epoch when man had first appeared upon the 

 earth. 1 



Long Persistence of the Motive Power thus caused 



It is in this long series of physical modifications of the 

 earth's surface, accompanied by changes of climate, partly 

 due to astronomical revolutions, and partly to changes in 

 aerial and oceanic currents dependent on terrestrial causes, 

 that we find a great motive power for the work of organic 

 evolution, the mode of operation of which we now have to 

 consider. 



Before doing so, however, I would call attention to the 

 fact of the very extraordinary complexity and delicacy of 

 the physical forces that have continued to act almost uni- . 



1 See my Island Life, chapters vii., viii., and ix., for a full discussion of the 

 causes and effects of glacial periods. 



