150 LIFE ON THE EARTH. 



of the, land and fresh waters, which are identical with 

 those now living in the same neighbourhood, un- 

 mixed with any from latitudes further south, or 

 further north ; so that here we have a decisive test, 

 and must admit in these cases that the climate was 

 nearly the same as now. This applies to the Post- 

 glacial period, in some part of which we place the 

 commencement of the History of Man. 



The Glacial period itself marked over a great 

 part of the north temperate zone in America and 

 Europe, by marine deposits heaped over what had 

 been dry land covering sometimes the lacustrine 

 and peaty layers of that earlier (Preglacial) land, this 

 period was one of considerable refrigeration, within 

 the large areas mentioned, for the shells found in 

 the deposits are of the colder arctic types ; and the 

 deposits are found to be consistent with the idea of 

 icebergs floating in deep water over all but the 

 mountainous tracts, (these being covered with ice and 

 snow, the source of glaciers and icebergs,) and not 

 consistent with any other probable condition of things. 



Thus we have one well-marked period, at least, 

 of considerably greater cold in the northern tempe- 

 rate regions, succeeding many periods of greater 

 warmth in these same regions. It has been supposed 

 that toward the close of the Palaeozoic period, a very 

 early glacial sea was spread round several of our 



