E, V. Wulflf —188— Historical Plant Geography 



There are similar discontinuous areas of mosses in the southern 

 hemisphere, embracing territories in all or some of the following re- 

 gions: Australia, New Zealand, and the southern extremities of Africa 

 and South America. There are estimated to be 4 genera, 3 sections, 

 and 18 species of mosses with areas embracing all the afore -mentioned 

 regions, 2 genera and 16 species found only in South America and 

 South Africa, 5 genera, i section, and 9 species in South Africa and 

 Australasia, and 2 families, 14 genera, 8 subgenera and sections, and 

 64 species in South America and Australasia. The total number of 

 taxonomic units having such southern discontinuous areas amounts to 

 142 (3 families, 33 genera, 12 subgenera and sections, and 94 species). 

 It were just such discontinuous areas among other groups of plants that 

 compelled Hooker to believe that there must have existed a great 

 Antarctic continent uniting in one land-mass the continents and islands 

 of the southern hemisphere. 



The fact that mosses, to an even greater extent than angiosperms, 

 are characterized by discontinuous areas may be ascribed to their 

 greater antiquity and greater ability to withstand changes in climatic 

 conditions, which made it possible for them to survive the lower tem- 

 perature at the close of the Tertiary and beginning of the Quaternary 

 period in many places where the flowering plants were doomed to 

 destruction. 



Cycadaceae. — The origin of the Cycadaceae in the light of Wegener's 

 theory has been studied by Koch (1925). The Cycadaceae are one of 

 those ancient families whose origin must be referred to the Permian or 

 Triassic period, i.e., precisely to that time when, according to Wegener, 

 the rifting apart of the continents began. Consequently, a study of the 

 history of their origin and distribution is of special interest. 



The Cycadaceae, according to Koch (pp. 68-69), arose directly from 

 the Carboniferous flora in the equatorial zone of that time, i.e., in 

 central and eastern North America, central Europe, and the adjoining 

 part of Asia lying at the same latitude, in so far as these latter regions 

 were not covered by the great inland seas. During the succeeding 

 geological periods the Cycadaceae spread to the north and south, em- 

 bracing, on the one hand, extensive regions in northern Europe (at 

 that time subtropical) and in Siberia, Spitzbergen, and Greenland and, 

 on the other, the Mediterranean Region and north Africa. That they 

 acquired such a wide distribution is attested by the numerous fossil 

 occurrences of cycads, belonging to at least 34 genera and 278 species. 

 This flourishing of the Cycadaceae reached its apogee at the beginning 

 of the Cretaceous period, after which they begin to decline, conifers 

 and angiosperms taking their place. 



The history of the distribution of the Cycadaceae followed, according 

 to Koch (pp. 69-71), this course: One group of Zamiaceae, probably as 

 early as the Triassic period, spread from central Europe to "Nord- 

 atlantis", whence it later extended its range southward as far as Cen- 

 tral America and the northern part of South America. Of the four 

 genera belonging to this group two, Dioon and Ceratozatnia, eventually 

 restricted their range to Mexico; the genus Zamia, however, is still 

 represented by not less than 30 species in northern Brazil, Peru, 

 Nicaragua (Granada Dep.), Guatemala, the Antilles, and Florida. The 



