E. V. Wulff — 186 — Historical Plant Geography 



exchange of forms between Australia and Africa. This was a time when 

 many families and genera had their maximum area. This period in the 

 formation of areas Irmscher regards as the first phase in their de- 

 velopment. 



In the succeeding period these areas were subjected to changes 

 under the pressure of various factors, of which changes in climatic 

 conditions and in the location of the continents were the most im- 

 portant. These changes resulted in the areas becoming discontinuous, 

 acquiring characteristics of symmetry and zonation. At the same time 

 the genera occupying these areas continued to develop, there arose new 

 species and genera, and as a result of their dispersal new boundaries 

 for these areas were created. This constitutes the second phase in the 

 development of areas. A characteristic feature of this phase is the 

 clearly expressed adaptation of areas to the present location of the 

 continents and their separation from one another. During this period 

 the Atlantic Ocean was formed, separating Europe and Africa from 

 America, and Australia and South America likewise became definitely 

 separated. 



Mosses. — Having made a study of discontinuities in the distribution 

 of mosses, Herzog (1926) came to the conclusion that their present 

 distribution is comprehensible only in the light of Wegener's theory of 

 continental drift. Irmscher (1929) Hkewise studied the distribution of 

 mosses from this point of view, and he arrived at the same conclusion 

 as Herzog. As a result of his investigations, Irmscher found that the 

 distribution of mosses does not bear a chance, haphazard character, as 

 would be the case if the transport of their spores by wind played as 

 important a role as has usually been ascribed to it. His study of the 

 distribution of mosses revealed just such discontinuous areas as are 

 found in the case of conifers and angiosperms, this phenomenon being 

 perhaps of even more frequent occurrence among mosses. 



The most characteristic of such discontinuous areas are those whose 

 separate parts are found in America, on the one hand, and in Euro- 

 Africa, on the other. Such a discontinuity we find in the areas of two 

 families {Eustichiaceae and Oedipodiaceae), each having only one genus, 

 and in 36 genera, 8 subgenera and sections, and 144 species belonging 

 to other families — a total of 190 taxonomic units. These areas lie, for 

 the most part, in a single climatic zone; only a few embrace more than 

 one zone. In some cases there is a gap in the area over the entire 

 continent of Africa, while habitats are found on adjacent islands, as, 

 for instance, in the case of the areas of the genera Eustichia, Staberia, 

 and Tortula. The frequency of such very similar discontinuous areas 

 cannot be accidental; it undoubtedly testifies to the fact that the now 

 separate parts of these areas were at one time united. This lends sup- 

 port to the theory of continental drift, according to which America and 

 Euro- Africa were formerly in contact. 



That such a contact existed is likewise confirmed by those dis- 

 continuous areas whose separate parts lie not only in America and 

 Euro-Africa but also in Asia and, in some cases, in Australasia. These 

 areas are directly linked with those that show a break in the Euro- 

 African sector. Thus, the genus Rhegmatodon, having a total of 12 

 species, is represented by 6 species in Central and South America, 



