probably only once produced, on one point or area once
in time; and that each diffuses itself, as far as barriers and its
conditions of life permit. If we look at any one main division of the
land, we find in the different parts, whether exposed to different
conditions or to the same conditions, many groups of species wholly or
nearly distinct as species, nevertheless intimately related. We find the
inhabitants of islands, though distinct as species, similarly related to
the inhabitants of the nearest continent; we find in some cases, that
even the different islands of one such group are inhabited by species
distinct, though intimately related to one another and to those of the
nearest continent:--thus typifying the distribution of organic beings
over the whole world. We find the floras of distant mountain-summits
either very similar (which seems to admit, as shown, of a simple
explanation) or very distinct but related to the floras of the
surrounding region; and hence, in this latter case, the floras of two
mountain-summits, although exposed to closely similar conditions, will
be very different. On the mountain-summits of islands, characterised by
peculiar faunas and floras, the plants are often eminently peculiar. The
dissimilarity of the organic beings inhabiting nearly similar countries
is best seen by comparing the main divisions of the world; in each of
which some districts may be found very similarly exposed, yet the
inhabitants are wholly unlike;--far more unlike than those in very
dissimilar districts in the same main division. We see this strikingly
in comparing two volcanic archipelagoes, with nearly the same climate,
but situated not very far from two different continents; in which case
their inhabitants are totally unlike. In the different main divisions of
the world, the amount of difference between the organisms, even in the
same class, is widely different, each main division having only the
species distinct in some families, in other families having the genera
distinct. The distribution of aquatic organisms is very different from
that of the terrestrial organisms; and necessarily so, from the barriers
to their progress being quite unlike. The nature of the conditions in an
isolated district will not explain the number of species inhabiting it;
nor the absence of one class or the presence of another class. We find
that terrestrial mammifers are not present on islands far removed from
other land. We see in