222 GLACIAL HISTORY. 



the lignite district being in its turn invaded by a fresh advance 

 of the masses of ice. This solves the apparent contradictions 

 presented by the animals and plants of the drift deposits, species 

 both of the temperate and cold zones which they have preserved 

 for us being explained by the changes of climate which took 

 place during this long period. The remarkable fact that the 

 Swiss flora has retained so few Miocene types of plants is also 

 thus accounted for. It may easily be understood that all the 

 forms of the torrid and warm zones could not be introduced 

 into the present flora of Switzerland, as these- would have dis- 

 appeared even in Pliocene times ; but the Swiss Miocene flora 

 contained a number of species representing those of a temperate 

 climate, which would very probably have remained under altered 

 circumstances in the existing flora if a great interval had not 

 separated the flora of our time from the Miocene. This interval 

 was caused by the Glacial epoch, to which it is owing that the 

 planes, the red maple, the balsam-poplar, the walnut, the tulip- 

 trees, the liquidambars, &c. have so little part in the constitu- 

 tion of the existing Swiss flora, whilst they reappear in America 

 in very nearly allied forms. These species support the Swiss 

 climate admirably, and are seen here as foreign cultivated trees, 

 while their ancestors in Miocene times were among the most 

 abundant Swiss plants. In the uppermost Pliocene deposits 

 some Miocene types are still met with which are now exclusively 

 American, and which were exterminated in Switzerland during 

 the first Glacial period ; so that even at the time of the formation 

 of the lignites all traces of them had disappeared, and the flora 

 had acquired its present Asiatico-European character. 



Prof. Heer is of opinion that with the drift or diluvium a 

 period was reached when man appeared on the stage of life. 

 Only some obscure traces of human beings have been hitherto 

 found in the Swiss drift-deposits. 



The lake-dwellings of Switzerland belong to a much later 

 time, as is shown rh the section of the Wetzikon district near 

 the Lake of Zurich (vol. i. p. 27), where the remains of lacustrine 

 habitations are found over a* series of lake-chalk, drift-debris, 

 lignites or paper-coals, and gravel-beds. 



Facts have been ascertained in France, Belgium, and England 

 which give great probability to the existence of Man contempo- 



