The process of diffusion may often be very
slow, being dependent on climatal and geographical changes, or on
strange accidents, but in the long run the dominant forms will
generally succeed in spreading. The diffusion would, it is probable,
be slower with the terrestrial inhabitants of distinct continents than
with the marine inhabitants of the continuous sea. We might therefore
expect to find, as we apparently do find, a less strict degree of
parallel succession in the productions of the land than of the sea.
Dominant species spreading from any region might encounter still more
dominant species, and then their triumphant course, or even their
existence, would cease. We know not at all precisely what are all the
conditions most favourable for the multiplication of new and dominant
species; but we can, I think, clearly see that a number of
individuals, from giving a better chance of the appearance of
favourable variations, and that severe competition with many already
existing forms, would be highly favourable, as would be the power of
spreading into new territories. A certain amount of isolation,
recurring at long intervals of time, would probably be also
favourable, as before explained. One quarter of the world may have
been most favourable for the production of new and dominant species on
the land, and another for those in the waters of the sea. If two great
regions had been for a long period favourably circumstanced in an
equal degree, whenever their inhabitants met, the battle would be
prolonged and severe; and some from one birthplace and some from the
other might be victorious.
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