240 EVOLUTION OF PLANTS 
tainous districts of the north. Instead of dense forests, 
with an undergrowth of dogwoods, rhododendrons, 
brambles, roses, and with the ground carpeted with 
mosses and ferns, we find the floor of the valleys and 
the rolling foot-hills covered with annual grasses, with 
which, in the spring, are mingled numberless showy 
flowers, unfamiliar to the eastern botanist except in 
gardens. Fiery Eschscholtzias, blue Nemophilas, pink 
and yellow Mariposa lilies, and numberless other 
flowers, make masses of brilliant color of unrivalled 
beauty. Here and there are scattered spreading ever- 
green oaks, and on the hillsides are thickets of low- 
growing shrubs, “chaparral,” made up of Manzanita, 
Ceanothus, and other western types, while the streams 
are bordered with beautiful madrofios (Arbutus), bay- 
trees, and big-leaved maples, as well as the more fa- 
wiliar alders and cottonwoods. The central part of 
the state is the meeting-ground for the two diverse 
floras, the northern -types often following the cajions 
down to the valleys, where they mingle with the south- 
ern flora. 
While the natural conditions of topography and 
climate have, of course, been the most potent factors 
in the present distribution of plants, animals have also 
played an important part, and especially man. The 
advent of man into many regions has quite transformed 
them, so far as the flora is concerned. In the tropics 
many of the most characteristic plants, such as the 
banana, breadfruit, cocoa-palm, and mango, as well as 
many weeds, like the sensitive plant, have become nat- 
uralized everywhere. So in temperate regions many in- 
troduced weeds have taken possession of the soil to the 
