188 Mr. J. Croll on the Physical Cause of Ocean-currents. 



in north and south during the glacial epoch explains a great 

 many facts in connexion with the distribution of plants and ani- 

 mals which have always been regarded as exceedingly puzzling. 



There are certain species of plants which occur alike in the 

 temperate regions of the southern and northern hemispheres. 

 At the equator these same temperate forms are found on elevated 

 mountains, but not on the lowlands. How, then, did these tem- 

 perate forms manage to cross the equator from the northern 

 temperate regions to the southern, and vice versa ? Mr. Darwin's 

 solution of the problem is (in his own words) as follows : — 



' ' As the cold became more and more intense, we know that 

 Arctic forms invaded the temperate regions ; and from the facts 

 just given, there can hardly be a doubt that some of the more 

 vigorous, dominant, and widest-spreading temperate forms in- 

 vaded the equatorial lowlands. The inhabitants of these hot 

 lowlands would at the same time have migrated to the tropical 

 and subtropicalregions of the south; for the southern hemisphere 

 was at this period warmer. On the decline of the Glacial period, 

 as both hemispheres gradually recovered their former tempera- 

 tures, the northern temperate forms living on the lowlands 

 under the equator would have been driven to their former homes 

 or have been destroyed, being replaced by the equatorial forms 

 returning from the south. Some, however, of the northern tem- 

 perate forms would almost certainly have ascended any adjoining 

 high land, where, if sufficiently lofty, they would have long sur- 

 vived like the Arctic forms on the mountains of Europe." 



st In the regular course of events the southern hemisphere 

 would in its turn be subjected to a severe glacial period, with 

 the northern hemisphere rendered warmer ; and then the south- 

 ern temperate forms would invade the equatorial lowlands. The 

 northern forms which had before been left on the mountains 

 would now descend and mingle with the southern forms. These 

 latter, when the warmth returned, would return to their former 

 homes, leaving some few species on the mountains, and carrying 

 southward with them some of the northern temperate forms 

 which had descended from their mountain fastnesses. ' Thus 

 we should have some few species identically the same in the 

 northern and southern temperate zones and on the mountains 

 of the intermediate tropical regions." (P. 339, sixth edition.) 



Additional light is cast on this subject by the results already 

 stated in regard to the enormous extent to which the tempera- 

 ture of the equator is affected by ocean-currents. Were there 

 no transference of heat from equatorial to temperate and polar 

 regions, the temperature of the equator, as has been remarked, 

 would probably be about 55 degrees warmer than at present. In 

 such a case no plant existing on the face of the globe could live 



