4 OUTLINE OF PLANT GEOGRAPHY 



tion, especially in the southern hemisphere, even within the 

 tropics. 



Various theories to account for the climatic conditions in 

 geologic time have been propounded. A recent one * elaborated 

 at some length assumes that prior to the Pleistocene, the climate 

 of the earth was controlled by its own heat, the effects of solar 

 radiation being excluded by a thick cloud envelope. Under such 

 conditions there would be no zonal climates, but changes in climate 

 would be due, mainly, to the relative distribution of land and 

 water. The temperature of the oceans was much higher and more 

 uniform than at present, and glaciation might occur anywhere, 

 provided there was a continental area of sufficient extent to permit 

 a loss of heat by radiation enough to reach the freezing point. 



The presence of glaciers by no means implies an extremely 

 cold climate. Today one may see growing in close proximity 

 to extensive glaciers luxuriant forests, as in southern Alaska, or 

 even plants of tropical origin, as in southern New Zealand, where 

 in lat. 44°, tree-ferns, and trees and shrubs of Malayan affinities 

 may be found close to the great Franz Josef glaciers which descend 

 to less than 1000 feet above the sea. 



The extension of many tropical genera into the temperate parts 

 of New Zealand is very remarkable, and this fact makes one 

 cautious about concluding that the presence of tropical genera 

 in a fossil state necessarily implies a tropical, or even sub-tropical 

 climate. 



Succession of Plants in Geological Time 



The first organisms to appear upon the earth were doubtless 

 extremely simple in structure. Of living forms, the bacteria may 

 offer some suggestions as to the character of the first living things. 

 That bacteria existed at an extremely remote period is certain, 

 as their activities are essential for the existence of all other or- 

 ganisms. Their extremely small size, however, makes a positive 

 demonstration of their presence in a fossil state, a difficult matter. 

 It is interesting to find, however, that the earliest known positive 

 remains of plants belong to the blue-green algae, forms which 

 probably are related to bacteria. 2 Wolcott has described from 



1 Manson, M., The Evolution of Climates, Baltimore, 1922. 



2 Knowlton, F. H., loc cit., p. 506. 



