180 Nr Part II. Chapter ı. 
The action of the several uplifts and depressions of the earth’s surface was 
most profound upon the Tertiary forest, the history of which has been traced. 
With every submergence, the forest in the submerged area was destroyed, or 
if existing on higher ground was subjected to such extensive changes of level, 
as to highly modify its character and the distribution of the component species. 
Many species were crowded together by the change of level and the wearing 
away.of the strata to which they had become adapted, for “if we suppose the 
earlier Mesozoic uplands to be the seat of the existing dicotyledons, then by 
the lowering of the surface by gradual consumption of the interstream areas, 
these forms must have been brought into conflict with the flora of the low- 
lands and thereby forced into a contest for supremacy ‘). Xerophytes of the 
hillsides and rock exposures are replaced by mesophytes, which thrive in rich 
alluvial soils; mesophytes by the wearing away of the soil and the formation 
of. cliffs by xerophytes, such as exist on rocky outcrops; hydrophytes replace 
 mesophytes, when an area becomes too wet for the tenancy of ordinary plants. 
An arctic flora, consisting of glacial plants, must have existed north 
of the great Tertiary forest flora, for, we have the introduction of this element 
as a prominent one at the beginning of the glacial period, when the ice sheet 
moved south over the North American continent. This arctic flora must have 
been exceedingly circumscribed in the extreme north of the western hemisphere, 
while there is a probability, that it extended somewhat more to the southward 
in the eastern hemisphere. If we compare the number of Tertiary arctic plants 
and those of today, which although widely distributed in far distant lands, yet 
are undoubtedly of arctic derivation, we will according to theoretic reasons find 
a.wide discrepancy in favor of the arctic plants of the present day floras. 
The ancient arctic flora was limited to the number of plants which had 
adapted themselves to the conditions of the polar lands in Tertiary times. 
The number of species was not so great, because the area to which they 
were confined was small and homogeneous and an excessive differentiation 
of new types could not take place. What the constitution of the original 
arctic flora was it is impossible to state. ENGLER believes that Salir reti- 
culata, S. polaris, S. herbacca, Cassiope tetragona, C. lycopodioides, C. hypnoides, 
Diapensia lapponica, Menyanthes trifoliata, Potentilla palustris, Scheuchzeria 
Ppalustris, Eriophorum Polystachyon, Drosera rotundifolia”), Vaccinium Oxycoceus. 
Chiogenes hispidula, Andromeda polifolia, Cassandra, Cyperaceae and many 
others plants of the same species at present found in widely separated loca- 
lities in America, Europe and Asia represent elements of such an ancient 
flora. It also implies that the great northern land masses must have been 
connected for a long time during the Tertiary period, so that migration of 
% 
1) WOODWORTH, J. B.: The Relation between baseleveling and organic Evolution. American 
Geologist XIV: 231 Oct. 1894 ; HARSHBERGER, J. W.: A pbyta-geographic Sketch of extreme 
southeastern. Pennsylvania. Bulletin Torrey Botanical Club XXXI: 129. 
2) TRANSEAU, E. N.: On the geographic Distribution and geological Relations of the bog 
plant Societies of North America. Botanical Gazette XXXVI: 401— 420. ° 
