l6o INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



warm lowlands (at least in the southern hemisphere) experienced 

 ' pluvial ' periods of increased precipitation. Simultaneously there 

 was more extensive glaciation in some regions of the South, notably 

 the uplands of South America and southern Australasia. All these 

 and allied changes were gradual, the advance and retreat of the ice 

 each time occupying many thousands of years, and allowing plants 

 to migrate before its face in accordance with climatic changes. 



Numerous living genera, especially of woody plants, are known 

 once to have been far more widely distributed than they are today 

 (striking examples are shown in Figs. 42 and 43). Many of our 

 species and even vegetational associations are of similar nature to, 

 but geographically more restricted than, those developed as far back 

 as the Miocene. It seems that by this time most of the major 

 land-masses of today had been formed, at least in outline, and that 

 thereafter the deterioration in climate was marked, leading to a 

 considerable reduction of the areas of many species of plants and 

 animals — quite apart from the restriction effected by or through the 

 Pleistocene glaciation. Already in the Pliocene the floras had 

 tended to be markedly impoverished as compared with those of the 

 Miocene, considerably less than 1,000 species of plants being known 

 from the Pliocene in contrast with over 6,000 from the Miocene. 

 The analogy of animals would suggest that over 80 per cent, of 

 Pliocene plant species are still living today — some of them, e.g. the 

 Bog-cypress {Taxodium distichum) and Black Oak (Ouercus nigra) 

 in Alabama, in the selfsame region. In other instances they are 

 not now known to live in a wild state within thousands of miles of 

 their old haunts — as in the case of the Water-chestnut (Trapa), fruits 

 of which are widespread in deposits in North America up to the 

 Pliocene, though it appears to be no longer a native of the New 

 World. An interesting set of examples is afforded by a middle 

 Pliocene flora of western Europe whose closest agreement is to be 

 found with some areas of southeastern Asia ; on the other hand, 

 the flora yielded by the overlying late Pliocene beds finds its closest 

 relationship with the existing flora of central Europe and indicates 

 an already much cooler climate. 



It is supposed that the Miocene and early Pliocene floras were 

 practically circumpolar in distribution and also widespread latitudin- 

 ally. Then the increasingly colder conditions in the North, often 

 accentuated locally by the uprising of mountain ranges which 

 furthermore caused aridity in their rain-shadows, forced the plants 

 to migrate southward. It has been suggested that in this pre- 



