PIPER—FLORA OF THE STATE OF WASHINGTON, 33 
THE ZONAL DISTRIBUTION OF WASHINGTON PLANTS. 
That there are physical causes which profoundly influence the dis- 
tribution of plants no one who has crossed the State of Washington 
from east to west can for a moment question. The contrast between 
the treeless bunchgrass prairies and sage plains of eastern Washing- 
ton and the luxuriant coniferous forests of western Washington is 
too striking to overlook. In this particular case the principal factor 
is one of humidity, the Puget Sound region possessing a notably 
moist climate, while that of the Columbia Basin is markedly arid. 
A similar change of vegetation may be witnessed in the ascent of 
any of our higher mountain peaks. As elevation increases the famil- 
iar lowland plants disappear and different ones present themselves, 
which in turn give way at high altitudes to still others. The mest 
marked of these changes is that where the timber ceases and the 
alpine meadows present their charming carpet of flowers. Here the 
changes are due manifestly not to differences in humidity, but to les- 
sened temperature, a conclusion emphasized by the fact that many of 
these alpine plants are the identical species which occur in arctic 
regions. 
Heat and moisture are undoubtedly the principal physical factors 
upon which the distribution of plants depend. A third factor may 
be important, even determinative, namely, the character of the soil, 
but this is much less potent than the two above named. In addition 
to these physical factors only one other need be considered, the bio- 
logical factor of ancestry. In general, plants inhabit the regions 
where their ancestors thrived. This factor is usually continental in 
its scope; thus cacti and yuccas are confined to America; eucalypti 
to Australia, and lilacs to Asia. But in a similar way this same fac- 
tor operates over small areas, and it is the principal cause why the 
Pacific coast flora as a whole is different from that of the Atlantic. 
The existing plants are different because their immediate ancestors 
were, whatever factors may have determined that. 
It is not to be understood from this that all the plants which for- 
merly flourished in Washington have left descendants there. In Ter- 
tiary times such plants as palms, cinnamon trees, and sequoias grew 
in Washington. Some of these require tropical or subtropical condi- 
tions of heat; others, as the sequoias, probably have given way in 
competition with more aggressive species. Nevertheless the broad 
statement remains true that the present vegetation of the region owes 
its character in large part to similar ancestors. Th2 conditions 
which make the Pacific coast the home of many peculiar genera and 
species are ancestral. The heat and moisture requirements of these 
plants are duplicated in other portions of the earth, where, however, 
totally different congeries of species occur. 
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