PALEOBOTANY — BERRY. 297 



have proclaimed the conservatism of the vascular cylinder, while 

 others conceive that the parts concerned with spore or seed forma- 

 tion are more conservative. Foliage in general is apt to be con- 

 servative in both form and structure, as witness the similarities in 

 the fronds of the Cycadophytes retained from the Permian to the 

 present, during which their vascular stem structures and especially 

 their fructifications varied through wide limits. Ginkgo leaves 

 have an equally long history, and many angiosperms acquired their 

 distinguishing leaf form during the Upper Cretaceous. 



Another general principle derived from the study of the geological 

 distribution of plants — a principle for the most part unrealized by 

 biologists or meteorologists — is that relating to geological climates 

 of which plants are the most satisfactory known tests. The early 

 paleobotanists drew extreme pictures of former vapor-laden atmos- 

 pheres and tepid, torrid climates, but these are found to have no 

 basis. On the other hand, it has become increasingly clear of late 

 3 7 ears that the sort of climate under which the history of man has 

 been passed is an abnormal climate from the standpoint of geological 

 history. From the first appearance of terrestrial plants in the 

 Devonian down to the present there are only two periods at which 

 it is possible to distinguish anything approaching climatic zoneing 

 For example, Devonian plants are known from Ellesmere Land on 

 the north to Australia on the south, and no differences are dis- 

 cernable in the floras from latitude 85 and those contemporaneous 

 in central Europe. This uniformity and cosmopolitan distribution 

 continues throughout the Lower and Upper Carboniferous. In the 

 Permian, however, there are well-marked floral provinces succeeding 

 widespread glacial conditions. Again from the late Triassic 

 throughout the Jurassic and Cretaceous there were no polar, temper- 

 ate, or equatorial zones — the same plants are found within a few de- 

 grees of the poles as within a few degrees of the equator. Even in 

 the earlier Tertiaiy, tropical floras extended halfway across the 

 temperate zones and temperate forests are known from within five 

 degrees of the pole. At the close of the Tertiary the successive 

 glaciations of the Pleistocene changed all this cosmopolitanism, 

 and to-day man is living in a Pleistocene climate, with well-marked 

 climatic zones and seasonal variations. The principle may be stated 

 thus: That for geological time as a whole climates have been more 

 uniform than at present, and marked variations from this uni- 

 formity are occasional and are the concomitants of glacial condi- 

 tions, both apparently the results of a similar but little understood 

 cause. 



RELATION TO OTHER SCIENCES. 



Paleobotany both contributes to and borrows from the other earth 

 sciences. Its relation to botany in the accepted sense of that term 



