on; for old forms will be supplanted
by new and improved forms. Neither single species nor groups of species
reappear when the chain of ordinary generation has once been broken. The
gradual diffusion of dominant forms, with the slow modification of their
descendants, causes the forms of life, after long intervals of time, to
appear as if they had changed simultaneously throughout the world.
The fact of the fossil remains of each formation being in some degree
intermediate in character between the fossils in the formations above
and below, is simply explained by their intermediate position in the
chain of descent. The grand fact that all extinct organic beings belong
to the same system with recent beings, falling either into the same or
into intermediate groups, follows from the living and the extinct being
the offspring of common parents. As the groups which have descended
from an ancient progenitor have generally diverged in character, the
progenitor with its early descendants will often be intermediate in
character in comparison with its later descendants; and thus we can see
why the more ancient a fossil is, the oftener it stands in some degree
intermediate between existing and allied groups. Recent forms are
generally looked at as being, in some vague sense, higher than ancient
and extinct forms; and they are in so far higher as the later and more
improved forms have conquered the older and less improved organic beings
in the struggle for life. Lastly, the law of the long endurance of
allied forms on the same continent,--of marsupials in Australia, of
edentata in America, and other such cases,--is intelligible, for within
a confined country, the recent and the extinct will naturally be allied
by descent.

Looking to geographical distribution, if we admit that there has been
during the long course of ages much migration from one part of the world
to another, owing to former climatal and geographical changes and to the
many occasional and unknown means of dispersal, then we can understand,
on the theory of descent with modification, most of the great leading
facts in Distribution. We can see why there should be so striking a
parallelism in the distribution of organic beings throughout space, and
in their geological succession throughout time; for in both cases the
beings have been connected by the bond of ordinary generation, and the
means of modification have been the same. We see the full meaning of the
wonderful fa