Comparison with 1866 |
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the same way as the productions
of real islands have everywhere
yielded to continental forms lately yielded to continental forms lately 1866 |
lately yielded to continental forms, 1859 1860 1861 |
yielded and are still yielding to continental forms 1869 1872 |
naturalised
there through there through 1866 | by 1859 1860 1861 | through 1869 1872 |
man's
agency. ↑2 blocks not present in 1859 1860 1861 1866; present in 1869 1872 | The same principles apply to the distribution of terrestrial animals and of marine productions, in the northern and southern temperate zones, and on the intertropical mountains.
When
during the height of the Glacial period
the ocean-currents were widely different to what they now are, some of the inhabitants of the temperate seas might have reached the equator; of these a few would perhaps at once be able to migrate southward, by keeping to the cooler currents, whilst others might remain and survive in the cooler
depths,
until the southern hemisphere was in its turn subjected to a glacial climate and permitted of
their further progress; in nearly the same manner as, according to Forbes, isolated spaces inhabited by Arctic productions exist to the present day in the deeper parts of the temperate
seas.
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I am
very far very far 1866 | far 1859 1860 1861 1869 1872 |
from supposing that all
difficulties difficulties 1866 1869 |
difficulties are removed on the view here given 1859 1860 1861 |
the difficulties 1872 |
in regard to the
distribution distribution 1866 1869 1872 | range 1859 1860 1861 |
and affinities of the allied
species, species, 1866 1869 1872 | species 1859 1860 1861 |
which live
in the northern
and
southern tem- perate zones and southern tem- perate zones and 1866 |
southern temperate zones and 1859 1860 1861 |
south, and sometimes 1869 1872 |
on the
mountains of the intertropical regions, are removed on the views above given. mountains of the intertropical regions, are removed on the views above given. 1866 |
mountains of the intertropical regions. 1859 1860 1861 |
intermediate mountain-ranges, are removed on the views above given. 1869 1872 |
It is extremely difficult to understand how a vast number of peculiar forms confined to the tropics could have been therein preserved during the coldest part of the Glacial period. The number of forms in Australia, which are related to European temperate forms, but which differ so greatly that it is impossible to believe that they could have been modified since the Glacial period, perhaps indicates some much more ancient cold period, even as far back as the miocene age, in accordance with the recent speculations of certain geologists. So again, as I am informed by Mr. Bates, the strongly marked character of several species of Carabus, inhabiting the southern parts of America, indicates that their common progenitor must have been introduced at some early period; and other analogous facts could be given. The exact lines and means
of migration during the recent Glacial period
cannot be indicated;
nor the reason why certain species and not others have migrated; why certain species have been modified and have given rise to new
groups of forms, and
others have remained unaltered. ↑1 blocks not present in 1866 1869 1872; present in 1859 1860 1861 | Very many difficulties remain to be solved.
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We cannot hope to explain such facts, until we can say why one species and not another becomes naturalised by man's
agency in a foreign land; why one
species ranges species ranges 1866 1869 1872 | ranges 1859 1860 1861 |
twice or thrice as far, and is twice or thrice as common, as another species within their own homes. ↑1 blocks not present in 1859 1860 1861 1866; present in 1869 1872 | Various special difficulties also remain to be solved:
for instance, the occurrence, as shown by Dr. Hooker, of the same plants at points so enormously remote as Kerguelen Land, New Zealand, and Fuegia; but icebergs, as suggested by Lyell, may have been concerned in their dispersal.
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I have said that many difficulties remain to be solved: some of the most remarkable are stated with admirable clearness by Dr. Hooker in his botanical works on the antarctic regions. These cannot be here discussed. I will only say
that, that, 1866 | that 1859 1860 1861 |
as far as regards the occurrence of identical species at points so enormously remote as Kerguelen Land, New Zealand, and Fuegia, I believe that towards the close of the Glacial period, icebergs, as suggested by Lyell, have been largely concerned in their dispersal. But the existence of several quite distinct species, belonging to genera exclusively confined to the south,
at these and other distant points of the southern hemisphere,
is, on the theory of descent with modification, a far is, on the theory of descent with modification, a far 1866 |
is, on my theory of descent with modification, a far 1859 1860 1861 |
of species, which, though distinct, belong to genera exclusively confined to the south, is a 1869 1872 |
more remarkable case
of difficulty.
For
some
of these species are so distinct, that we cannot
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