E. V. Wulff ■ —50— Historical Plant Geography 



their enormous size. Such areas often occupied several continents, 

 separated now by the waters of great oceans. Likewise a number of 

 species (sometimes known to us only in a fossil state) of ancient genera, 

 particularly of pteridophytes and gymnosperms but also of many 

 angiosperms, had at one time exceptionally large areas. The remnants 

 of these formerly extensive areas retained in the present-day flora 

 constitute a proof that, as regards these genera and species, there took 

 place a contraction of their areas. But this by no means signifies that 

 such a contraction of area is an invariable rule. On the contrary, new 

 species or species with a wide range of adaptability, finding conditions 

 favorable for them, sometimes extend their area with startling rapidity, 

 within a few decades becoming practically cosmopolitan. 



To illustrate the process of formation and evolution of the areas of 

 species, let us take the Angiosperms. There arose in definite centers 

 in the Cretaceous period — and in the case of many genera probably 

 even earlier, in the Jurassic period — genera of this group of plants, 

 represented at that time by as yet only slightly differentiated species 

 (or generic types, as Engler calls them), which attained exceptionally 

 extensive areas of distribution. Nevertheless, we cannot assume that 

 the flora of the entire globe was at that time homogeneous, as some 

 authors have assumed. Climatic zones have always existed on the 

 globe and have always served as barriers to the unlimited distribution 

 of species. If, despite these barriers, species attained such enormous 

 areas, this was due to the different arrangement of the climatic zones, 

 for precisely in the Jurassic and Cretaceous periods and beginning of 

 the Tertiary period the tropical and subtropical zones spread widely 

 over the continents, embracing all of Europe up to its present arctic 

 limits and considerable portions of Asia and America. Hence, it is 

 comprehensible — assuming, in addition, a connection at that time be- 

 tween the continents of America and Eurasia — how the indicated 

 genera could attain precisely on these parts of the continents such an 

 extensive distribution. This circumstance, and also the seeming ra- 

 pidity with which these plants spread, apparently facilitated the mass 

 extinction of the Gymnosperms and Pteridophytes as a result of chang- 

 ing climatic conditions, which enabled the new representatives of the 

 plant kingdom to extend their area of distribution without competition 

 from the former -inhabitants of the earth's surface. 



The homogeneity of the climate over extensive portions of the con- 

 tinents explains likewise the slight extent to which these ancient 

 species were differentiated. With the shifting to the south in the 

 Northern Hemisphere of the climatic zones, with the sharp demarcation 

 of tropical, temperate, and arctic chmates, with the ever-decreasing 

 humidity and progressive development of arid and semi-arid condi- 

 tions, there developed in the second half of the Tertiary period in those 

 regions where there had formerly existed homogeneous ecological con- 

 ditions a sharp climatic zonation. Simultaneously there occurred a 

 breaking up of these generic types, as a result of divergence, first into 

 large groups of species, now known as collective species or coenospecies, 

 still occupying quite extensive areas, and then into a series of small, 

 vicarious species. This process of differentiation is still in progress, 

 as is shown by the fact that the more our methods of research become 



