RELATIONSHIPS OF METASPERMAE. ^ 601 



archetypal Archegoniatae, it is possible from the evidence of 

 palaeontology to calculate the general period of their emer- 

 gence. In rocks older than those of the Lower Cretaceous re- 

 mains of metaspermic plants are exceedingly rare and doubtful. 

 This indicates an origin somewhere in the Jurassic period, 

 although by some the time of their appearance is placed as far 

 back as the Devonian. During the Lower Cretaceous several 

 highly developed monocotyledonous or archichlamydeous 

 plants must have begun the winning struggle with the less 

 highly organised ferns, club-mosses, cycads and conifers of 

 older geologic time. In the Potomac formation of the Atlan- 

 tic United States, as studied by Fontaine, several remains of 

 metaspermic plants intermingled with those of archaic varieties 

 of ferns and cycads have been discovered. Through the Cre- 

 taceous period the metaspermic plants developed with rapidity, 

 and in the Upper Cretaceous had established themselves as the 

 dominant forms over a considerable area of the earth. The 

 researches of Heer, Lesquereaux and others in the North 

 American continent have revealed the vestiges of an ancient 

 flora, considerably diversified and of a highly modern aspect. 

 During the Cretaceoas period the smaller extent of the North 

 American continent, its isolation and attendant division by the 

 Cretaceous Mediterranean which extended from the present 

 boundary of the Gulf of Mexico through the Rocky mountain 

 region to Alaska, may have had much to do with the rapid de- 

 velopment of metaspermic types. During this time the Cali- 

 fornian and Sierra region formed a separate continent, and on 

 the other side of the sea lay the Atlantic continent^ extending 

 south about to the present region of the Ohio river. Evidently 

 during this time and in succeeding ages, the climatic conditions 

 varied greatly from those of to-day, for in the Cretaceous and 

 later Tertiary rocks of Greenland, Spitzbergen, Nova Zembla, 

 Point Barrow, the Mackenzie islands and of other localities far 

 within the Arctic circle, there are found the remains of a flora 

 characterised by large leaved palms, exogenous plants and 

 even cycads, thus giving a distinctly tropical aspect to the 

 vegetation of circumpolar regions. This tropical character 

 persisted until comparatively recent times, when by the great 

 elevation of the polar regions and by probable changes in 

 oceanic currents the conditions became those of the glacial 

 epoch, since which time there has been a moderation in the 

 temperature of the northern hemisphere, but by no means a 

 ret urn to the Tertiary benignity. 



