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Geography. : ‘i 433 
a “have thus occupied more and more ground by migration. All plants 
are, however, not equally capable of migration ; if this were the 
case, the strongest would everywhere have replaced the rest, and have 
“S occupied the whole of the space devoted to vegetation. The unlimited 
distribution of plants is also prevented by seas, deserts, mountain- 
- chains, differences of climate, and even by the existence of other plants — 
and animals. 
A natural flora results from the interchange of a number of forms 
belonging to different centres of vegetation, and their union into a whole 
with definite characters. The limits of such a flora are determined 
by climate, or by the hindrance to the distribution of species afforded 
by a broad ocean or other obstacle ; and these floras are the more natural. 
the more sharply they are defined, and the less their aboriginal inhabi- 
tants have mingled with immigrant forms. The most effectual hindrance 
to this intermingling is presented by the ocean. Almost as completely 
the great Sahara separates the flora of tropical Africa from that: of 
the Mediterranean region ; and the forest-region of equatorial America 
is an insurmountable obstacle to the migration of the native plants of 
the grassy plains of Venezuela and Brazil. The chief factor, however, 
in the separation of floras is difference in climate. 
The comparison of different floras, and of the regions to which 
they belong, brings out the law that the species of a genus or the 
genera of an order are the more nearly related to one another—-in other 
words more closely resemble one another—the less the distance between 
their centres of distribution, or the more similar the climates of these 
centres. Itis scarcely possible, for example, to have greater differences 
of climate than those which subsist between the base and the stiummit of 
A 
, 
lofty mountains ; and yet the vegetable productions of the most ele- 
vated regions not unfrequently bear the closest systematic relationship 
to those of the warm valleys out of which the mountain rises ; a phe- 
nomenon which suggests the supposition that the plants of the heights 
have ascended from the valleys, and have adapted themselves to the 
new vital conditions as far as was necessary to their perpetuation. But 
the transitions from one species to another which might be expected 
in the gradually ascending mountain regions are most commonly want- 
ing ; the Alpine species make their appearance, and those of the plains 
disappear, suddenly, at particular elevations. Contiguity of locality 
is, however, except in the case of mountain-ranges, usually accompanied 
by resemblance in climate. But there are also resemblances of climate — 
without geographical contiguity ; and allied genera or species appear in 
' the most widely separated regions, if their climate is similar. Well- 
known examples of this fact are furnished by the beeches of Japan 
FF 
