5] EVOLUTIONARY DEVELOPMENT AND PAST HISTORY I49 



ately following the Permian glaciation show strongly developed 

 ' annual ' rings. 



The early part of the Triassic period, at the beginning of the 

 Mesozoic, tended to have an arid and generally unfavourable climate 

 in the northern hemisphere, the fossil record being, moreover, 

 fragmentary. Pteridosperms, Cycadophytes, Ginkgoales and Coni- 

 fers appear to have been plentiful, as well as numerous Ferns and a 

 lesser number of Lycopods and Horsetails. In Argentina, South 

 Africa, and much of southeastern Asia, the climate was humid at 

 that time, and supported a rich flora. It became drier and probably 

 arid in these regions tow^ards the close of the Triassic, when to the 

 north conditions became much as in the Jurassic. Nevertheless the 

 wide occurrence of similar floras indicated that comparable con- 

 ditions extended over an extremely wide area. Fig. 40 shows an 

 attempted reconstruction of a Triassic landscape, with plentiful 

 Cycadophytes, etc., and some large Horsetails. 



Jurassic floras were apparently developed under warm and moist 

 conditions as indicated by fossil deposits, and were widely distributed. 

 Indeed, their composition appears to have been fairly uniform all 

 over the world, involving (with relatively minor variations) such far- 

 flung lands as continental Europe, Spitsbergen, Greenland, tem- 

 perate North America, Mexico, India, Japan, Australia and New 

 Zealand. They included numerous Cycadophytes, Ginkgoales, and 

 Conifers, besides representatives of most of the modern groups of 

 Pteridophytes, though the giant Lycopods and Horsetails had long 

 since disappeared, as had the Cordaitales. During this period the 

 Pteridosperms declined and, according to some authorities, the first 

 indubitable Angiosperms appeared. 



The Cretaceous period witnessed the latest transformation of the 

 plant world, in which the Angiosperms really came into their own. 

 Conversely, most of the other groups of vascular plants were on the 

 decline. It is interesting to note, however, that the lower cryp- 

 togams had meanwhile often held their own, as many do to this day — 

 presumably because they do not normally compete with Angiosperms, 

 though actually they have not been widely dominant on land since 

 early Palaeozoic times. Thus, of the widespread upper Cretaceous 

 ' Dakota flora ' of North America, more than 90 per cent, of the 

 species were Angiosperms, most of them belonging to familiar woody 

 genera of Dicotyledons, although the presence of a few Palms 

 indicated a fairly warm climate. Cycads were reduced to 2 per cent, 

 and the Conifers w^ere only slightly better represented. Whereas 



