E. V. Wulff — 212— Historical Plant Geography 



ments stand in favor of the continental. By selecting from any given 

 flora the oceanic elements and subjecting them to study, we can deter- 

 mine the ratio of these elements to the rest of the flora, the approxi- 

 mate rate of decrease or increase in their number within the limits of 

 the given flora, etc. In other words, by establishing which are the 

 oceanic elements, representing the most ancient elements of the earth's 

 flora, we can obtain data of great value in determining the history and 

 origin of any given flora. 



Historical Elements: — Without aiming to give an emmieration of 

 all the groups of species whose distribution is linked with definite his- 

 torical moments, we shall consider here, by way of example, only two 

 of such elements: arctic-Tertiary and boreal-Tertiary, terms that were 

 introduced into ph3i;ogeography by Engler and that have become 

 widely accepted. 



According to Engler, in our present-day arctic region in the Ter- 

 tiary period (he, as Heer, considered it to be during the Miocene stage, 

 but it is now referred to a considerably earlier period) there flourished a 

 flora composed of coniferous and deciduous trees now characteristic of 

 North America and eastern Asia (exclusive of its tropical regions). 

 This group of species he called "arctic-Tertiary". Next to this flora 

 on the south there was found the "boreal-Tertiary" flora, bearing to a 

 considerable degree a tropical character, since it comprised palms and 

 evergreen elements. Still farther south, stretching over the vast ex- 

 tent of territory from southern England to Japan and including, on the 

 south, our present-day Mediterranean region, was the domain of a 

 truly tropical flora, which Engler characterized as consisting of 

 "megatherms". 



Engler himself did not give any precise indications as to the changes 

 that took place in these floras during the latter part of the Tertiary and 

 during the Quaternary period. He only indicated, by lines on his map, 

 their migration from arctic latitudes to the Mediterranean region. 

 Many subsequent authors interpreted this in the following way, which 

 undoubtedly is in accord with Engler's viewpoint: that all three 

 types of flora — the tropical flora of the Mediterranean Region, the 

 boreal-Tertiary, and the arctic-Tertiary— were forced by climatic 

 changes to migrate southward, each flora replacing that lying next to 

 it on the south. 



Paleobotanic data undoubtedly indicate that during the Tertiary 

 period there occurred a succession of floras, tropical and subtropical 

 floras being gradually replaced by temperate and arctic floras, so that 

 the existence of these two historical elements may be considered as 

 practically established. We wish, however, to draw attention to certain 

 drawbacks to the terms applied to these elements. The term "arctic- 

 Tertiary element" does not well express what is intended, since almost 

 to the end of the Tertiary period there was no Arctic zone in Europe. 

 The application of a term employed in modern climatic zonation to the 

 Tertiary allocation of floras makes this designation of the element in- 

 sufficiently precise, all the more since Engler did not restrict the 

 distribution of this flora to the exact boundaries of our present-day 

 arctic region. Similarly, the term "boreal-Tertiary element" does not 



