certain extent the territories of other species, should be those which
would have the best chance of spreading still further, and of giving
rise in new countries to new varieties and species. 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. But in the
course of time, the
forms dominant in the highest degree, wherever produced, would tend
everywhere to prevail. As they prevailed, they would cause the
extinction of other and inferior forms; and as these inferior forms
would be allied in groups by inheritance, whole groups would tend
slowly to disappear; though here and there a single member might long
be enabled to survive.
Thus, as it seems to me, the parallel, and, taken in a large sense,
simultaneous, succession of the same forms of life throughout the
world, accords well with the principle of new species having been
formed by dominant species spreading widely and varying; the new
246