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and the eastern shores of
on almost exactly opposite meridians of longitude. |
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| A third great fact, partly included in the foregoing
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
in travelling, for instance, from north to
never fails to be struck by the manner in which successive groups of beings, specifically distinct,
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
the plains of La Plata by another species of the same genus; and not by a true ostrich or
like those
Africa and Australia under the same latitude. On these same plains of La
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
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
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
→though
they may be all peculiar
We may look back to past ages, as shown in the last chapter, and we find American types then
on the American continent and in the American seas. We see in these facts some deep organic bond,
throughout space and time, over the same areas of land and water,
of
physical conditions. The naturalist must
→feel little curiosity,
who is not led to inquire what this bond is. |
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|
→on my theory,
is simply inheritance, that cause which alone, as far as we positively know, produces organisms quite
→or,
as we see in the case of
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
influence of different physical conditions. The
of dissimilarity will depend on the migration of the more dominant forms of life from one region into another having been
more or less
at periods more or less remote;— on the nature and number of the former
and on
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
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
are confined to the same areas, as is so commonly and notoriously the case. |
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→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
individual in its complex struggle for life, so the
of modification in different species will be no uniform quantity.
→If, for instance,
a number of species,
→which stand in direct competition
with each
→migrate in
a body into a new and afterwards isolated country, they
be little liable to modification; for neither migration nor isolation in themselves
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.
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these views, it is
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
descended from the same progenitor. In the case of those species, which have undergone during whole geological periods
little modification, there is not much difficulty in believing that they
have migrated from the same region; for during the vast geographical and climatal changes which
have supervened since ancient times, almost any amount of migration is possible. But in many other cases, in which we have reason to
that the species of a genus have been produced within comparatively recent times, there is great difficulty on this head. It
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