Chapter X — 193 — Historical Causes 



characterized by a subtropical flora at the close of the Tertiary- 

 period. 



Further evidence of the occurrence of great climatic changes is pro- 

 vided by the asymmetry of areas, already mentioned as regards other 

 types of vegetation and likewise characteristic of conifers. Thus, if we 

 glance at the areas of conifers, we find: (i) uniformity in the composi- 

 tion of the coniferous floras of the Paleozoic and Mesozoic eras, often 

 even to the extent of comprising the same species, due to the exception- 

 ally wide distribution of many genera and species {e.g., Podocarpus and 

 Araucaria); (2) similarity between the Mesozoic conifers of North 

 America (particularly those of the Pacific coast) and Europe {e.g., 

 Sequoia); (3) occurrence of such genera as Araucaria, now distributed 

 only in the southern hemisphere, quite far north in the northern hemi- 

 sphere; (4) zonal distribution of areas; (5) discontinuity of the present 

 area of Araucaria with two widely-separated parts in South America 

 and Australia. 



By assuming, as Wegener does, that in the Permian period all the 

 continents formed one great land-mass (Pangaea) and that subse- 

 quently they broke apart and shifted their position in relation to the 

 poles, all the peculiarities in the distribution of conifers become under- 

 standable. 



Angiospermae. — The regularities in the structure of areas of angio- 

 sperms — established, as we have already noted, by Irmscher, on the 

 basis of a study of the distribution of the various famiUes belonging to 

 this group of plants — led him to certain conclusions, which we shall 

 now present in brief. 



The facts established regarding the first stage in the development of 

 the areas of angiosperms — fossil occurrences of tropical genera in 

 Europe and North America, migration of plants from America to the 

 east, the law of zonation — cannot be explained either by the theory of 

 the permanence of the Atlantic Ocean or by the assumption that there 

 existed a trans-Atlantic land-bridge from Greenland to Iceland and 

 Scotland. In order to explain these facts, it is necessary to assume 

 either that there existed a huge continent connecting America with 

 Europe across the Atlantic or, which is the more probable, that these 

 two continents were in direct contact with each other. 



The second stage in the development of areas is very clearly de- 

 marcated from the first stage. If one were to assume the permanence 

 of the oceans, it would be impossible to explain the entirely different 

 configuration of areas in their first and second stages. 



The picture drawn by Wegener of the position of the continents 

 before and after the breaking up of Pangaea could not be in closer 

 accord with these first and second stages in the development of the 

 areas of angiosperms. Wegener's theory of continental drift not only 

 makes plausible a close connection between North America and Europe 

 but also provides a solution of the hitherto inexplicable riddle of the 

 distribution of vegetation in the southern hemisphere. The fact that 

 the floras of South America, Australasia, Africa, and the subantarctic 

 islands have much in common, quite incomprehensible in the light of 

 their present widely-separated locations, is at once elucidated by pre- 

 suming that these territories in the Mesozoic era were united, thus 

 making possible an interchange of vegetation. 



