E. V. Wulff —152— Historical Plant Geography 



lus, Populus, Juglans, Comptonia and Trapa, and almost devoid of evergreen elements show- 

 ing a southern character; or the southern plants, if present, are rare and doubtful. There 

 are no palms, nor are there cinnamons or figs of a tropical type such as are so conspicuous 



in the older Tertiary flora of Europe In proceeding farther east we find the same 



monotonous Tertiary flora passing across the Pacific into Alaska . . . However, whilst 

 in Siberia proper any traces of a former much warmer cUmate and an associated flora are 

 lacldng, some traces of these phenomena are found in Turkestan and in Japan". 



Nevertheless, there is no doubt that climatic conditions in Siberia, 

 even as far north as the Arctic Ocean, were during the Tertiary period 

 considerably milder than at present, which made possible the existence 

 of deciduous species resembling types at present found in eastern Asia 

 and the eastern part of the United States. At the same time, begin- 

 ning with the Paleocene stage, in western Europe, the Ukraine, and the 

 central part of Russia in Europe there was distributed an evergreen 

 tropical and subtropical flora, characterized by such evergreen plants 

 as the Nipa and Sabal palms and species of Cinnamomutn, Ocotea 

 (Oreodaphne), and other genera of a tropical type. This flora, accord- 

 ing to Kryshtofovich (1932), resembles most closely the Indo- 

 Malayan type of paleotropic flora, whereas the later Tertiary flora of 

 these regions assumes an aspect similar to the present-day flora of 

 eastern Asia and the eastern part of the United States. 



Still later, at the very close of the Tertiary or the beginning of the 

 Quaternary period, due to the further lowering of temperature, the 

 covering of a large part of Europe and northern Siberia with glaciers, 

 and the considerable decrease in precipitation in the Mediterranean 

 Basin and also in eastern Europe and in northern and central Asia, the 

 species composition of the floras of the regions we have been discussing 

 completely changed. The vegetation of the tropical zone, on the other 

 hand, developed almost undisturbed, due to the fact that habitat con- 

 ditions there remained almost unchanged. Similarly, the vegetation 

 of southeastern Asia, particularly Japan and southwestern China, and 

 of northern Mexico and the southeastern and southwestern sections of 

 the United States has remained practically unchanged since the Ter- 

 tiary period, not having undergone those sharp climatic changes that 

 the vegetation of Europe and the northern part of North America has 

 repeatedly had to undergo. 



To explain these great climatic changes many theories have been 

 advanced. It is not our purpose here to go deeply into this problem; 

 for us it suffices merely to point out that the vegetation of the earth 

 has, at various times during the earth's history, been subjected to con- 

 siderable changes, corresponding to changes in cHmatic zonation, and 

 that these changes have been reflected not only in the species compo- 

 sition of floras but also in the numerical distribution of species. 



The remarkable evolution of the plant world has not been a uni- 

 form and gradual process. On the contrary, everything indicates that 

 the sudden changes in habitat conditions, caused by violent upheavals 

 of the earth's crust, that have occurred time and again during the 

 earth's long history have given a marked impetus to the evolution of 

 forms, have induced sudden mutational changes leading to the abrupt 

 creation of new forms, to processes of accelerated species-formation, 

 and also to migrations of floras. If the earth had remained all the time 



