eline and canine genera are
widely distributed, and many of the individual species have enormous
ranges [the genus Mus I believe, however, is a strong exception to the
rule]. Mr Gould informs me that the rule holds with birds, as in the
owl genus, which is mundane, and many of the species range widely. The
rule holds also with land and fresh-water mollusca, with butterflies and
very generally with plants. As instances of the converse rule, I may
give that division of the monkeys which is confined to S. America, and
amongst plants, the Cacti, confined to the same continent, the species
of both of which have generally narrow ranges. On the ordinary theory of
the separate creation of each species, the cause of these relations is
not obvious; we can see no reason, because many allied species have been
created in the several main divisions of the world, that several of
these species should have wide ranges; and on the other hand, that
species of the same group should have narrow ranges if all have been
created in one main division of the world. As the result of such and
probably many other unknown relations, it is found that, even in the
same great classes of beings, the different divisions of the world are
characterised by either merely different species, or genera, or even
families: thus in cats, mice, foxes, S. America differs from Asia and
Africa only in species; in her pigs, camels and monkeys the difference
is generic or greater. Again, whilst southern Africa and Australia
differ more widely in their mammalia than do Africa and S. America, they
are more closely (though indeed very distantly) allied in their plants.

{347} <Note in original.> The same laws seem to govern distribution
of species and genera, and individuals in time and space. <See
_Origin_, Ed. i. p. 350, vi. p. 497, also a passage in the last
chapter, p. 146.>

{348} _Origin_, Ed. i. p. 404, vi. p. 559.

_Distribution of the inhabitants in the same continent._

If we now look at the distribution of the organisms in any one of the
above main divisions of the world, we shall find it split up into many
regions, with all or nearly all their species distinct, but yet
partaking of one common character. This similarity of type in the
subdivisions of a great region is equally well-known with the
dissimilarity of the inhabitants of the several great regions; but it
has been less often insisted on, though more worthy of remark. Thus f