ession 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 species thus
produced being themselves dominant owing to inheritance, and to having
already had some advantage over their parents or over other species;
these again spreading, varying, and producing new species. The forms
which are beaten and which yield their places to the new and victorious
forms, will generally be allied in groups, from inheriting some
inferiority in common; and therefore as new and improved groups spread
throughout the world, old groups will disappear from the world; and the
succession of forms in both ways will everywhere tend to correspond.

There is one other remark connected with this subject worth making. I
have given my