6 
was once much warmer than at present. In those distant ages, 
various woody representatives of a number of present-day trop- 
ical, sub-tropical or warm temperate families grew much farther 
north than now. Many examples might be given, but space 
for the time being allows citing only a few of the most striking 
cases. Representatives of the present-day sub-tropical or trop- 
ical cycad group then grew in the Black Hills of South Dakota, 
in such large quantities that their fossil remains have led to 
considering certain areas in this region as desirable possibilities 
for a national preserve or ‘‘cycad cemetery.” Other fossil repre- 
sentatives of this group have been found in Alaska. Fossil 
species of the genus Juglans, according to Knowlton (1919), 
occur in Alaska, indicating a range in ancient times far beyond 
the present geographical range of the genus. Fossil representa- 
tives of the genus Magnolia are known from Montana and 
Alaska. Sequoia, Ficus, and Persea are known in fossil remains 
representing practically every general region of the United States. 
Such present-day tropical genera as Mimusops (the balata 
genus), Dalbergia (the rosewood genus), and Artocarpus (the 
bread-fruit genus), judged by the fossil records, once grew in 
regions where they are now grown only as greenhouse plants. 
Mimusops is known from Tennessee, Dalbergia from Staten 
Island, and Artocarpus from fossil remains collected in Oregon, 
Colorado, and possibly Yellowstone National Park. 
When the climate changed and became colder, the families 
represented by such genera as cited above were forced to limit 
themselves to regions that still maintained the temperature to 
which they had become adapted. Many families of those days, 
however, must have been able to readjust themselves to the 
temperature changes in a comparatively short time, or perhaps 
in some cases they were already able to successfully meet these 
colder temperatures. Obviously this characteristic would not 
be demonstrated until the climate changed, any more than 
susceptibility to a given disease would show itself until the 
disease germs were present. From such groups came our tem- 
perate and polar floras. Whole plant families appear to have this 
temperate-climate characteristic, e.g., Rosaceae, Liliaceae, Faga- 
ceae, Betulaceae, Salicaceae. Others appear to be practically 
tropical, e.g., Amaryllidaceae, Bombacaceae, Bromeliaceae, Scita- 
minaceae, Artocarpaceae, Palmaceae, Dioscoraceae. 
