264 GENERALIZATIONS. 



and there still remains a temperature of 5 C. (or 9 F.) to 

 be accounted for. 



As the action of an Asiatic gulf-stream could not extend to 

 Iceland, Greenland, and North America, the high temperature 

 of those regions in Miocene times requires explanation. It ap- 

 pears probable that a general source of heat existed in the Mio- 

 cene period, influencing the whole northern hemisphere. In an 

 earlier part of this volume the opinion has been expressed (p. 225) 

 that, in the Tertiary epoch, a great continent united Europe 

 with North America ; and if this was the case, that extent of 

 land must be taken into account. 



A considerable influence must have been exerted by such an 

 Atlantic continent on the climate of Europe, but not sufficient 

 to explain the difference of 5 C. (or 9 F.), already men- 

 tioned. If the continent of Atlantis had extended across from 

 Europe to America, the refrigerating influence of an icy sea 

 could not have sent its icebergs into the Atlantic Ocean ; and if 

 the Atlantis had extended southwards to the tropical zone, the 

 warm Gulf-stream would only have reached the south-west coast 

 of France. The Atlantic continent should have raised the sum- 

 mer temperature of the northern hemisphere, and especially that 

 of Iceland, comprised within its limits ; and it ought to have low- 

 ered the temperature of the winter ; but the hypothesis of this 

 Atlantic continent leaves unexplained the elevated temperature 

 of Greenland, Spitzbergen, and the north of America. 



It is impossible, in the present state of science, to determine 

 satisfactorily the circumstances from which the increase of heat 

 in the Arctic zone at the Miocene period was derived, especially 

 with reference to the Miocene flora. Nothing but hypotheses 

 can be presented on this subject, among the most plausible of 

 which are the following : 1 . Change of climate arising from the 

 diminution of heat belonging to the earth ; 2. Modification of 

 the sun itself; 3. Change of position of the earth with regard to 

 to the sun; and, 4. Irregularity of temperature in ethereal 

 space. 



Before entering upon an examination of these different hypo- 

 theses, Prof. Heer reminds the reader that, from the Palaeozoic 

 period to that of the Cretaceous formation, the organic world of 

 the different geological ages, so far as it is at present known, 



