G. R. Wieland — Polar Climate in Time. 41 T 



a great iiiTmber of the important genera occurring in both 

 floras. There are no less than fortj-six of these common to 

 the two, though in some cases the author's individuality is 

 probably alone responsible for slight differences in the names. 

 For example, forms referred to Baiera by one would be 

 referred to Baierojpsis by another, and so with Ctenis and 

 Cte7iicliii7n^ Myrsine and Myrsinojpliyllum^ Oleandra and 

 Oleandridi%im^ Salix and Salioiphylltim^ Thuya, and Thu- 

 jites^ etc."* 



Moreover, Professor Ward finds that the proportion of 

 species of the similar genera bears some relation to the rela- 

 tive size of the two florae, about 1 to 4 (200 species being 

 known from Portugal to about 800 in Maryland). Thus the 

 proportion of Portuguese to American species is respectively 

 for the genus Aralia 2 to 11, for Brachyjjhylliim 5 to 9, for 

 Cladophlehis 12 to 25, for Frenelopsis 2 to 6, for Laurus 3 to 

 8, for Myrica 2 to 11, for Podozamites 7 to 15, for Sphenole- 

 pedium, 3 to 9, etc. Of Magnolia, however, there is only 1 

 species, to 12 in Portugal. Professor Ward concludes that, " On 

 the whole it may be considered that the Lower Cretaceous flora 

 of Portugal is, botanically speaking, a very close repetition of 

 that of America; and in view of the fact that in both coun- 

 tries a number of distinct horizons showing the progressive 

 change in the flora throughout that period have yielded fossil 

 plants in such a way that, if the Portuguese beds were as fully 

 developed as are the American ones, each of these florules 

 might be compared, the subject becomes rather fascinating." 



To note further the parallelism between continental florae of 

 Europe and America from the Cretaceous on, is scarcely 

 required. It may be remarked that previous to the discovery 

 of the rich Potomac flora and of various archetypal dicotyls 

 in certain Jurassic strata it was supposed that the Angiosperms, 

 now the predominant type of vegetation, had their origin in 

 the Middle Cretaceous. The sudden and simultaneous appear- 

 ance at this time in large number of species in Europe and 

 America of the main forest types of to-day with but few fore- 

 runners, however, constitutes such a profound phenomenon of 

 American and Eurasian plant parallelism as is scarcely explica- 

 ble on the basis of lateral distribution. Furthermore, there is 

 indicated a steady decline in temperature and retreat of trop- 

 ical conditions to the southward. The Dakotas and Wyoming 

 still enjoyed Floridiau conditions in the Eocene. From this 

 time on the temperature decline was marked. 



But let us take a glance at the northern record. It is of 

 more than passing interest to note that it so happens that the 

 most varied Upper Devonian flora yet discovered is that of the 



* XVIth Ann. Eep. U. S. Geol. Survey, 1894-95, pp. 469-540. 



Am. Jour. Sci. — Fourth Series, Vol. XVI, No. 96. — December, 1903. 

 29 



