196 THE WORLD OF LIFE 



water, and in several cases footprints of Amphibia or reptiles 

 have been preserved as well as impressions of raindrops, so 

 exactly corresponding with those which may be seen to-day 

 in suitable places, that we cannot suppose the operations of 

 nature to have been more violent then than now. All our 

 great coal deposits of Palaeozoic age indicate long, and often 

 repeated, but very slow depression of large areas of land, 

 with intervening periods of almost perfect stability, during 

 which dense forests had again time to grow, and to build up 

 those vast thicknesses of vegetable matter which, when buried 

 under successive rock-strata, became compressed into coal- 

 seams, usually of several feet in thickness. 



It is an extraordinary fact that in all the great 

 continents, including even South America and Australia, coal- 

 fields are more or less abundant at this period of the earth's 

 history. This is proved by the identity or close similarity 

 of the vegetation and animal life, as well as by the position 

 of the coal-beds, in regard to the strata above and beneath 

 them. It is true that coal is also found in some 

 Secondary and Tertiary strata, but these beds are much less 

 extensive and the coal is rarely of such purity and thickness ; 

 while the later coal-fields are never of such world-wide 

 distribution. It seems certain, therefore, that at this 

 particular epoch there were some specially favourable 

 conditions, affecting the whole earth, which rendered possible 

 a rapid growth of dense vegetation in all situations which 

 were suitable. Such situations appear to have been 

 extensive marshy plains near the sea, probably the deltas or 

 broad alluvial valleys of the chief great rivers ; and the special 

 conditions were, probably, a high and uniform temperature, 

 with abundance of atmospheric moisture, and a larger 

 proportion of carbon-dioxide in the air than there has ever 

 been since. 



We may, in fact, look upon this period as being the 

 necessary precursor of the subsequent rapid development of 

 terrestrial and aerial animal life. A dense and moisture 

 laden atmosphere, obscuring the direct rays of the sun, 

 together with a superabundance of carbonic-acid gas and a 

 corresponding scarcity of free oxygen, would probably have „ 

 prevented the full development of terrestrial life with its 



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