PIPER—-FLORA OF THE STATE OF WASHINGTON. 37 
ORIGIN OF THE UPPER SONORAN FLORA. 
Some considerations regarding the origin of the Upper Sonoran 
species of the Washington flora seem to justify definite conclusions. 
Tn the geographical history of the Columbia Plains, as above outlined, 
there was found, during the existence of Lake John Day in Miocene- 
Tertiary time, a rich flora of subtropical aspect. Subsequent to this 
time and probably not much prior-to the glacial epoch, occurred the 
principal uplift of the Cascade Mountains. This undoubtedly 
‘aused profound changes in climatic conditions, particularly to the 
eastward of this range, accompanied by correspondingly great 
changes in the flora. 
Succeeding the uplifting of the Cascade Mountains came the Gla- 
cial period. During this period immense changes took place in 
western Washington, but there is scarcely a trace of glacial work on 
the plain of the Columbia. Nevertheless, in accord with the lower 
temperatures there was probably a general southward, migration of 
the plants, followed by their return on the retreat of the ice. 
Of the changes which have taken place in the flora since the John 
Day period to the present time there is no record preserved. In 
this enormous lapse of time—perhaps millions of years—a forest 
composed of magnolias, elms, sycamores, etc., apparently much like 
that of the Lower Mssissippi in the present day, has utterly vanished 
and no near relatives remain, either in the same or adjacent territory. 
Inasmuch as somewhat similar subtropical forests existed in Mio- 
cene time in western Washington, the causes which have led to their 
extinction are more profound than such climatic changes as could be 
occasioned by the Cascade uplift and are to be sought rather in the 
general causes which have lowered the temperatures of the earth’s 
surface. 
There remains, then, but one source from which light may be 
thrown on the present constitution of the flora—namely, its relation 
to contiguous floras. 
Such considerations naturally focus first on the physical conditions 
which most likely prevailed at the close of the last great geological 
‘ataclysm, namely, the Glacial period. The most conspicuous result 
of this period of cold is perhaps the large number of Arctic species 
which occur on all the higher peaks of the Cascades or stranded on 
isolated mountains, as the Olympics or the Blues—a fact which ap- 
pears more striking in mountains farther to the southward. How- 
ever slight the effect of the Glacial period may have been in eastern 
Washington, it is quite certain that the temperature was such that no 
plants adapted to Upper Sonoran conditions could survive. They 
were either driven southward, as were the Arctic plants, or else per- 
ished. Following the retreat of the ice, the areas that then became 
