416 G. R. Wieland — Polar Climate in Time. 



the Botanical Society of America in 1875. In 1883 additional 

 facts were given b}^ Mr. G. Hilton Scribner, who took up the 

 subject from the standpoint of the northern origin of life itself. 

 Moreover, in 1883 Nathorst published a map of fossil Tertiary 

 plant localities in tlie north polar regions and the hypothetical 

 routes of their migration southwards. 



As we have already seen, the point at which the evidence 

 from the side of the vertebrates is most wanting, lies in the 

 entire absence of known vertebrate-bearing horizons within 

 the polar circles, whilst, on the other hand, the American and 

 European record is quite complete in Tertiary time, there 

 being abundant horizons covering this period of rapid mam- 

 malian development. Contrariwise in the case of the plants, 

 the northern record from the Cretaceous on, is one of the most 

 interesting within the ken of the paleobotanist. But as the 

 main development of the early Dicotyls and other plants con- 

 stituting the best liorizon> markers took place in the late Jurassic, 

 at a time when there is a considerable dearth of freshwater 

 beds in close succession, the plant record* also has its serious 

 weak point. The general facts of both records are, however, 

 so entirely complementary that it is a matter of some surprise 

 that they have so little occupied the attention of biologists. 



The fact that, as recently shown by Seward,^ there are strik- 

 ing similarities in the plants of the Inferior Oolite of York- 

 shire, and those of the lower Jurassic of Bornholm (Sweden), 

 Japan, and the Rajmahal series of India, may, so far as present 

 knowledge goes, be considered an early indication of a north to 

 south movement of plants. It is noteworthy that Gingho and 

 Baiera^ so abundant in the Lower Jurassic of England, Japan, 

 and Bornholm, are so far unknown in this horizon in India. 

 Few forms, albeit, have a more striking northern development 

 during Jurassic and Cretaceous time than the Gingkoales. 



Following the period of very generalized tropical conditions, 

 the earliest extensive comparison of European and American 

 florae that can now be made is that between the Jurassic of 

 Portuj.'^al and the Trias of America. Notwithstanding this 

 time hiatus, about two-thirds of the Portuguese genera, as has 

 been pointed out by Ward, are present in the American Trias. 

 In the Lower Cretaceous of Maryland and of Portugal the 

 comparison is a much more striking one. There are, indeed, 

 some species of wide distribution common to both. But as 

 Professor Ward has said, " We should not, of course, expect 

 the species to be common to any great extent, and the com- 

 parison is practically limited to the genera. Looked at from 

 this point of view, we see that the resemblance is indeed close, 



* Occurrence of Dictyozamites in England, witli remarks on European and 

 Eastern Mesozoic Floras, Quart. Jour. Geol. Soc, May, 1903, vol. lix. 



