EAKTH CHAKGES AND EVOLUTION I'JO 



ThuSj in alternate belts or more extended areas, our con- 

 tinents have been, step by step, built up throughout the ages, 

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

 valley, of upland plateaus 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 eccentricity of the 

 earth's orbit and the precession of the equinoxes, loading 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. ^ 



Long Persistence of the Motive Power thus caused 



It is in tliis 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 uniformly, 

 and with no serious break of continuity, during the whole vast 

 periods of geological time. These forces have always been 

 curiously balanced, and have been brought into action alter- 

 nately in opposite directions, so as to maintain, over a large 

 portion of the globe, land surfaces of infinitely varied forms, 

 which, though in a state of continuous flux, yet never reached 

 a stationary condition. Everywhere the land is being low- 

 ered by denudation towards the sea-level, and part by part is 



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

 the causes and effects of glacial periods. 



