and at many instincts causing other
animals to suffer.

If species be only well-marked and permanent varieties, we can see at
once why their crossed offspring should follow the same complex laws in
their degrees and kinds of resemblance to their parents--in being
absorbed into each other by successive crosses, and in other such
points--as do the crossed offspring of acknowledged varieties. This
similarity would be a strange fact, if species had been independently
created and varieties had been produced through secondary laws.

If we admit that the geological record is imperfect to an extreme
degree, then the facts, which the record does give, strongly support the
theory of descent with modification. New species have come on the stage
slowly and at successive intervals; and the amount of change after equal
intervals of time, is widely different in different groups. The
extinction of species and of whole groups of species, which has played
so conspicuous a part in the history of the organic world, almost
inevitably follows from the principle of natural selection; for old
forms are supplanted by new and improved forms. Neither single species
nor groups of species reappear when the chain of ordinary generation is
once 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 beings can be classed with all recent beings, naturally follows
from the living and the extinct being the offspring of common parents.
As species have generally diverged in character during their long course
of descent and modification, we can understand why it is that the more
ancient forms, or early progenitors of each group, so often occupy a
position in some degree intermediate between existing groups. Recent
forms are generally looked upon as being, on the whole, higher in the
scale of organization than ancient forms; and they must be higher, in so
far as the later and more improved forms have conquered the older and
less improved forms in the struggle for life; they have also generally
had their organs more specialized for differ