ain
development--Local relations of fossil and living animals--Cause
of extinction of large animals--Indications of general progress
in plants and animals--The progressive development of
plants--Possible cause of sudden late appearance of
exogens--Geological distribution of insects--Geological
succession of vertebrata--Concluding remarks.

The theory of evolution in the organic world necessarily implies that
the forms of animals and plants have, broadly speaking, progressed from
a more generalised to a more specialised structure, and from simpler to
more complex forms. We know, however, that this progression has been by
no means regular, but has been accompanied by repeated degradation and
degeneration; while extinction on an enormous scale has again and again
stopped all progress in certain directions, and has often compelled a
fresh start in development from some comparatively low and imperfect
type.

The enormous extension of geological research in recent times has made
us acquainted with a vast number of extinct organisms, so vast that in
some important groups--such as the mollusca--the fossil are more
numerous than the living species; while in the mammalia they are not
much less numerous, the preponderance of living species being chiefly in
the smaller and in the arboreal forms which have not been so well
preserved as the members of the larger groups. With such a wealth of
material to illustrate the successive stages through which animals have
passed, it will naturally be expected that we should find important
evidence of evolution. We should hope to learn the steps by which some
isolated forms have been connected with their nearest allies, and in
many cases to have the gaps filled up which now separate genus from
genus, or species from species. In some cases these expectations are
fulfilled, but in many other cases we seek in vain for evidence of the
kind we desire; and this absence of evidence with such an apparent
wealth of material is held by many persons to throw doubt on the theory
of evolution itself. They urge, with much appearance of reason, that all
the arguments we have hitherto adduced fall short of demonstration, and
that the crucial test consists in being able to show, in a great number
of cases, those connecting links which we say must have existed. Many of
the gaps that still remain are so vast that it seems incredible to these
writers that they could ever have been fil