E. V. Wulff — 184— Historical Plant Geography 



The third regularity is found in the fact that the areas of many 

 genera are linked with definite continents, areas embracing both America 

 and Euro-Africa being rare, while there is a considerable number of 

 areas embracing Euro-Africa, Asia, and Australasia. This linkage 

 accords with the present location of the continents but, nevertheless, 

 attests their different location in the past— a circumstance of great 

 importance for further conclusions. 



In order to understand the present area of any given angiosperm, 

 it is essential to compare it with the same area supplemented by the 

 territories where fossil remains of the genus or species or of closely 

 related genera or species have been found. Such fossil remains occur 

 chiefly in the northern hemisphere, particularly in North America and 

 Europe, including their arctic regions. Most fossil forms have a dis- 

 tribution restricted to North America and Europe, temperate and sub- 

 tropical species extending into the North Polar region. Tropical forms, 

 on the other hand, are found chiefly in central and southern United 

 States, in Europe, and in northern Africa. From the data derived 

 from such comparisons of past and present areas we may draw the 

 following very important conclusions: 



1. Many genera with discontinuous areas, now having repre- 

 sentatives only in America and Asia, in past geological periods had 

 them also in Europe, so that the now separated portions of such areas 

 were formerly connected. 



2. Many forms were in past ages found farther north than they are 



at present. 



The latter circumstance, i.e., that the habitats of fossil forms of 

 many species and genera are found in zones which at the present time 

 are unsuitable, as regards temperature conditions, for their growth, 

 combined with the established zonation of their distribution, makes it 

 necessary to assume similar climatic conditions for the habitats of fossil 

 remains that supplement a discontinuous area. If these fossil remains 

 are found outside the climatic zone within which lies the given area, 

 then, taking for granted the constancy of the physiological character- 

 istics of plants, we must presume either that at that time the given 

 climatic zone was broader or that, while retaining the same latitudinal 

 range, it was subjected to a shifting from north to south. A solution 

 to this problem we find in the fact that fossil remains have been 

 found farther north than the habitats of their present-day repre- 

 sentatives only in Europe and, to a lesser extent, in America, but they 

 have never been found in Asia as far north as in the case of these other 

 continents. On the other hand, the fossil remains from the tropics of 

 Asia point to the fact that here climatic conditions have not suffered 

 any great changes since the Tertiary period. Hence, it is necessary to 

 conclude that the changes in climate did not affect equally all parts of 

 any zone parallel to the present equator; thus, as regards our tropical 

 zone, Asia remained untouched by these climatic changes. The only 

 possible assumption capable of explaining this circumstance is that the 

 climatic zones shifted obliquely, due to a change in the location of the 



poles. 



On the basis of the foregoing it is further necessary to assume, m 

 order to explain the former continuity of areas within any given 



