tween the provinces, whether of subaqueous or terrestrial plants,
relates strictly to _species_, and not to forms. In regard to the
numerical preponderance of certain forms, and many peculiarities of
internal structure, there is usually a marked agreement in the vegetable
productions of districts placed in corresponding latitudes, and under
similar physical circumstances, however remote their position. Thus
there are innumerable points of analogy between the vegetation of the
Brazils, equinoctial Africa, and India; and there are also points of
difference wherein the plants of these regions are distinguishable from
all extra-tropical groups. But there is a very small proportion of the
entire number of species common to the three continents. The same may be
said, if we compare the plants of the United States with that of the
middle of Europe; the species are distinct, but the forms are often so
analogous, as to have been styled "geographical representatives." There
are very few _species_ of phaenogamous plants, says Dr. J. Hooker, common
to Van Diemen's Land, New Zealand, and Fuegia, but a great many
_genera_, and some of them are confined to those three distant regions
of the southern hemisphere, being in many instances each severally
represented by a single species. The same naturalist also observes that
the southern temperate as well as the antarctic regions, possess each of
them representatives of some of the genera of the analogous climates of
the opposite hemisphere; but very few of the species are identical
unless they be such as are equally diffused over other countries, or
which inhabit the Andes, by the aid of which they have evidently
effected their passage southwards.

_Manner in which plants become diffused.--Winds._--Let us now consider
what means of diffusion, independently of the agency of man, are
possessed by plants, whereby, in the course of ages, they may be enabled
to stray from one of the botanical provinces above mentioned to another,
and to establish new colonies at a great distance from their birthplace.

The principal of the inanimate agents provided by nature for scattering
the seeds of plants over the globe, are the movements of the atmosphere
and of the ocean, and the constant flow of water from the mountains to
the sea. To begin with the winds: a great number of seeds, are furnished
with downy and feathery appendages, enabling them, when ripe, to float
in the air, and to be wafted easily to g