5] EVOLUTIONARY DEVELOPMENT AND PAST HISTORY 15I 



Thus in North America and across Eurasia, conditions were sub- 

 stantially more favourable than at the present time, with Palms 

 extending plentifully into Canada and England. Already the flora 

 had a largely modern aspect, though herbaceous species in general 

 and Monocotyledons in particular appear to have been far less 

 abundant relatively to woody ones than at present. 



There may have been some deterioration of climate in the northern 

 hemisphere in the next age, the Oligocene, but it appears to have 

 been slight, with plentiful large trees prevailing well north. In 

 any case in the southern hemisphere the climate seems to have 

 remained more comparable with that of today than it did in the 

 northern hemisphere. 



The Miocene was a time of widespread volcanic activity in North 

 America, when uplifting of the Cascade Range deprived the area to 

 the east of much of its accustomed rainfall, so that increasing aridity 

 prevailed — and probably lower temperatures, although the climate 

 was still warm over wide areas. Fig. 41 shows a reconstructed scene 

 in Miocene times, with Palms and Cycads growing in what is now a 

 cool-temperate region. 



By the Pliocene, conditions in America east of the Cascade Range 

 had become generally unfavourable for the growth of dense forests 

 and for the preservation of their remains, trees apparently occurring 

 chiefly along the streams. As a result of the general cooling in the 

 North, the vegetation became more like that of today, and actually in 

 North America and eastern Asia the Pliocene deposits contain a 

 large proportion of the species still found living in the same regions. 

 On the other hand, many of the wade-ranging species then occurring 

 in Europe have since disappeared therefrom. 



In spite of all these changes in various parts of the Cainozoic, the 

 same major plant groups appear to have persisted through it to the 

 present day, even if the species and often higher taxa have changed ; 

 particularly striking is the dwindling importance of some of the 

 Conifers. 



The Pleistocene and Recent together make up the Quaternary and 

 are now estimated to involve only the last million or fewer years 

 {cf. p. 129), of w'hich the Recent or post-Pleistocene occupies per- 

 haps one-hundredth part. Remains of Pleistocene vegetation are 

 preserved chiefly in unconsolidated lake and stream deposits, in 

 peat bogs, or in a frozen condition, while postglacial peat deposits are 

 still accumulating. In general only modern species are represented, 

 most of these being still living and familiar ; but in Pleistocene 



