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 areexceedingly 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 
highiy 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 Cretaceous 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 
return to the Tertiary benignity. 
