156 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



poles. All such changes would inevitably lead to alterations in the 

 distributions of plants and of the animals that are dependent upon 

 them, profoundly affecting also their evolutionary development. 

 Thus, whereas in the northern hemisphere the distribution of the 

 climatic zones favoured the development of relatively luxuriant 

 vegetation over wide areas up to nearly the end of the Tertiary, in 

 the southern hemisphere conditions appear to have been much more 

 disturbed. Particularly did the great Permian glaciation, whose 

 principal centre of development apparently lay in South Africa, play 

 havoc with the flora and fauna ; to it is attributed in some degree 

 the poverty to this day of the flora of tropical Africa as compared 

 with the floras of South America and Asia. It is supposed that 

 relatively arid conditions prevailed subsequently over much of the 

 southern hemisphere, which may have been further diversified by the 

 separation of the continents beginning as far back as the Mesozoic. 

 Thus tropical Africa is supposed to have been cut off from the rich 

 vegetation of Eurasia by a wide sea occupying the position of the 

 Sahara during the Cretaceous, and, in later times, by the Sahara 

 Desert as we know it today — both sea and sand being eiTective in 

 barring the migration, it has been suggested, of the majority of 

 species from the North. 



The climatic deterioration that began well back in the Tertiary 

 led ultimately to widespread glaciation in the northern hemisphere. 

 Thus, early in the Quaternary, vast areas of North America and 

 northern Eurasia became enveloped in ice which may be compared, 

 in extent and continuity if not in thickness, with that covering most 

 of Greenland and Antarctica today. This Pleistocene ice reached a 

 maximum extent and receded probably four times, the limits reached 

 in each extension being different. The intervening, ' interglacial ' 

 periods were protracted and relatively favourable to plant life, being 

 apparently for long periods at least as warm as the present day. 

 The ice largely ousted the previously rich floras of the North, the 

 plants being forced to migrate before its advance or, if they could 

 not do so successfully, being exterminated — apart perhaps from some 

 which persisted on mountains or other ice-free refugia, a subject 

 to be discussed in the next section. 



The poverty of the present western and central European flora 

 is commonly ascribed to the fact that the Tertiary components were 

 largely destroyed shortly before or during the Ice Age, by being 

 driven into the sea or against high mountains. Eor post-Pleistocene 

 restocking across the Mediterranean, over the Alps or Pyrenees, or 



