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1859
1860
1861
1866
1869
1872

Compare with:
1859
1860
1866
1869
1872

sea, 1859 1860 1861 1866
of the same sea, 1869 1872

though 1859 1860 1861 1866
are essentially American, though 1869 1872

feel little curiosity, 1859 1860 1861 1866
be dull, 1869 1872

on my theory, 1859 1860 1861 1866
OMIT 1869 1872

or, 1859 1860 1861 1866
each other, or, 1869 1872

like each other. 1859 1860 1861 1866
alike. 1869 1872

natural 1859 1860 1861 1866 1869
variation and natural 1872

in a quite 1859 1860 1861 1866
in a 1869
probably in a 1872

and reaction, in their mutual struggles for life;— 1859 1860 1861 1866
of the inhabitants on each other in leading to the preservation of different modifications; 1869 1872

being, 1859 1860 1861 1866
in the struggle for life being, 1869 1872

I believe, 1859 1860 1861 1866 1869
There is no evidence, 1872

in no 1859 1860 1861 1866 1869
of the existence of any 1872

If, for instance, 1859 1860 1861
If 1866 1869 1872

which stand in direct competition 1859 1860 1861
after having long competed 1866 1869 1872

migrate in 1859 1860 1861
in their old home, were to migrate in 1866 1869 1872

or at all modified. 1861 1866 1869 1872
modified. 1859 1860

and the eastern shores of
Africa
Africa,
on almost exactly opposite meridians of longitude.
A third great fact, partly included in the foregoing
statement,
statements,
is the affinity of the productions of the same continent or sea, though the species themselves are distinct at different points and stations. It is a law of the widest generality, and every continent offers innumerable instances. Nevertheless the
naturalist,
naturalist
in travelling, for instance, from north to
south,
south
never fails to be struck by the manner in which successive groups of beings, specifically distinct,
though
yet
yet
nearly
clearly
related, replace each other. He hears from closely allied, yet distinct kinds of birds, notes nearly similar, and sees their nests similarly constructed, but not quite alike, with eggs coloured in nearly the same manner. The plains near the Straits of Magellan are inhabited by one species of Rhea (American ostrich), and
northward
north-ward
the plains of La Plata by another species of the same genus; and not by a true ostrich or
emeu,
emu,
like those
inhabiting
found in
Africa and Australia under the same latitude. On these same plains of La
Plata
Plata,
we see the agouti and bizcacha, animals having nearly the same habits as our hares and rabbits and belonging to the same order of Rodents, but they plainly display an American type of structure. We ascend the lofty peaks of the
Cordillera,
Cordillera
and we find an alpine species of bizcacha; we look to the waters, and we do not find the beaver or musk-rat, but the coypu and capybara, rodents of the
S. American
American
type. Innumerable other instances could be given. If we look to the islands off the American shore, however much they may differ in geological structure, the
inhabitants
inhabitants,
though they may be all peculiar
species.
species,
are
are
essentially
essentially
American.
American.
We may look back to past ages, as shown in the last chapter, and we find American types then
prevailing
prevalent
on the American continent and in the American seas. We see in these facts some deep organic bond,
prevailing
prevailing
throughout space and time, over the same areas of land and water,
and
and
independently
independent
of
their
their
physical conditions. The naturalist must feel little curiosity, who is not led to inquire what this bond is.
The
This
bond
bond,
on my theory, is simply inheritance, that cause which alone, as far as we positively know, produces organisms quite
like
like,
or, as we see in the case of
varieties
varieties,
nearly like each other. The dissimilarity of the inhabitants of different regions may be attributed to modification through natural selection, and in a quite subordinate degree to the
definite
direct
influence of different physical conditions. The
degrees
degree
of dissimilarity will depend on the migration of the more dominant forms of life from one region into another having been
prevented
effected with
effected with
more or less
effectually,
effectually prevented,
ease,
at periods more or less remote;— on the nature and number of the former
immigrants; —
immigrants;—
and on
the
their
action and reaction, in their mutual struggles for life;— the relation of organism to organism being, as I have already often remarked, the most important of all relations. Thus the high importance of barriers comes into play by checking migration; as does time for the slow process of modification through natural selection. Widely-ranging species, abounding in individuals, which have already triumphed over many competitors in their own widely-extended
homes
homes,
will have the best chance of seizing on new places, when they spread into new countries. In their new homes they will be exposed to new conditions, and will frequently undergo further modification and improvement; and thus they will become still further victorious, and will produce groups of modified descendants. On this principle of inheritance with modification, we can understand how it is that sections of genera, whole genera, and even
families
families,
are confined to the same areas, as is so commonly and notoriously the case.
I believe, as was remarked in the last chapter, in no law of necessary development. As the variability of each species is an independent property, and will be taken advantage of by natural selection, only so far as it profits
each
the
individual in its complex struggle for life, so the
amount
degree
of modification in different species will be no uniform quantity. If, for instance, a number of species, which stand in direct competition with each
other
other,
migrate in a body into a new and afterwards isolated country, they
would
will
be little liable to modification; for neither migration nor isolation in themselves
can effect
effect
can do
anything. These principles come into play only by bringing organisms into new relations with each other, and in a lesser degree with the surrounding physical conditions. As we have seen in the last chapter that some forms have retained nearly the same character from an enormously remote geological period, so certain species have migrated over vast spaces, and have not become greatly or at all modified.
According to
On
these views, it is
obvious,
obvious
that the several species of the same genus, though inhabiting the most distant quarters of the world, must originally have proceeded from the same source, as they
are
have
descended from the same progenitor. In the case of those species, which have undergone during whole geological periods
but
but
little modification, there is not much difficulty in believing that they
may
may
have migrated from the same region; for during the vast geographical and climatal changes which
will
will
have supervened since ancient times, almost any amount of migration is possible. But in many other cases, in which we have reason to
believ
believe
that the species of a genus have been produced within comparatively recent times, there is great difficulty on this head. It