Chapter X 



—183— 



Historical Causes 



of plants, are those of Irmscher (1922, 1929). This investigator 

 carried on very painstaking studies on the distribution of mosses and 

 angiosperms, but before we take these up, we wish to set forth those 

 regularities in the structure of areas which were established by 

 Irmscher during the course of his investigations. 



On the basis of the present and fossil distribution of the indicated 

 group of plants Irmscher concluded that during past geological periods all 

 the territory embraced by the torrid zone of those times was inhabited 

 by a more or less homogeneous tropical vegetation. The north temperate 

 and south temperate zones were likewise each occupied by a homogeneous 

 vegetation, this greater homogeneity being due to the fact that the con- 

 tinents were differently located than now. Scattered fragments of this 

 flora are still to be found on the continents of the southern hemisphere. 

 The climatic changes that occurred must have affected similarly the 

 areas of distribution of all groups of the plant kingdom. This uni- 



FiG. 29. — Same as in Fig. 25, but in the Pliocene. (After Koppen and Wegener). 



formity of cause inducing uniform changes in the distribution of plants 

 suffices to explain the regularities in the structure of areas first clearly 

 established by Irmscher, though Simroth had noted them earlier in 

 part. 



The first of these regularities in the structure of areas is expressed 

 in a certain symmetry, whereby along the meridians passing through 

 Europe and Africa the given genera or species are entirely absent or 

 are represented to only a very insignificant extent, as they often are 

 likewise in Australia. Such breaks in an area may be explained only 

 by cataclysmic or secular processes in former geological epochs that 

 have destroyed its former continuity. 



The second regularity is found in the zonation which characterizes 

 the structure of discontinuous areas. Such zonation finds expression 

 in the fact that the parts of a discontinuous area are located in a 

 horizontal, not meridional line, so that we find them either in one of the 

 temperate zones, or in the tropical zone, or in both temperate zones 

 (bipolar areas). 



