VOL. II. | Naturalized Plants. 33 
While, therefore, the floral aspect of this region is strongly tinged 
by foreign admixture, it is- predominately indigenous except in 
the immediate precincts of habitation. How long it may so con- 
tinue cannot be foretold. Some evil emigrants are rapidly spread- 
ing, and new ones are constantly arriving, . 
The prevalence of intruding plants upon the other side of the 
_ continent has been explained as resulting from the originally sylvan 
character of that region. The native herbs were adapted to the 
shade of woodlands and could not survive the clearing of the forests. 
Thus there being no native campestrine vegetation to occupy the 
clearings, a free field was left for the incoming weeds of Europe* 
If it be, indeed, not by reason of any superior inherent vigor or 
capability of adaptation, but from the lack of suitable competition 
that intruding weeds obtain possession of the soil, our native flora 
should be able to maintain its predominance. This has never been 
a forest region, and possesses a large number of campestrine plants. 
These native herbs are likewise adapted to the condition of scanty 
and uncertain rainfall which here prevails. With the first showers 
they spring up and are in flower almost as soon as they are out of 
the ground, so that the production of seed is assured in the dryest 
season. With a continuance of moisture, growth continues and 
constant blossoming, until the full grown plant can hardly be re- 
cognized as identical with the pigmies which first appear. This 
character exercises its protective influence especially in the deserts, 
and among the plants of the fertile region is most developed in those 
which grow on dry and barren soils. In such tracts the proportion 
of introduced plants to indigenous ones is much smaller, not only 
numerically but particularly in the number of species, than it is in 
rich and moist places. And those exotics which are widest spread 
and most abundant are precisely the ones which possess this char- 
acteristic, as is noticeable in the Erodiums, the most abundant of 
all. Many of the most objectionable weeds require a long season of 
moisture to perfect their fruit, and climatic conditions may be ex- 
pected to confine these to meadows or irrigated fields. Elsewhere 
_ the native herbs should have no difficulty in maintaining their pos- 
session of the soil. 
_ CorrecTions.—The Cenchrus mentioned in vol. i, page 187, as C. 
*Gray, Characteristics of the N, Am, Flora; Am. Jour. Sci. xxviii, 324. _ 
