governing the succession of forms in past times being nearly the same
with those governing at the present time the differences in different
areas. We see this in many facts. The endurance of each species and
group of species is continuous in time; for the exceptions to the rule
are so few, that they may
fairly be attributed to our not having as yet discovered in an
intermediate deposit the forms which are therein absent, but which
occur above and below: so in space, it certainly is the general rule
that the area inhabited by a single species, or by a group of species,
is continuous; and the exceptions, which are not rare, may, as I have
attempted to show, be accounted for by migration at some former period
under different conditions or by occasional means of transport, and by
the species having become extinct in the intermediate tracts. Both in
time and space, species and groups of species have their points of
maximum development. Groups of species, belonging either to a certain
period of time, or to a certain area, are often characterised by
trifling characters in common, as of sculpture or colour. In looking to
the long succession of ages, as in now looking to distant provinces
throughout the world, we find that some organisms differ little, whilst
others belonging to a different class, or to a different order, or even
only to a different family of the same order, differ greatly. In both
time and space the lower members of each class generally change less
than the higher; but there are in both cases marked exceptions to the
rule. On my theory these several relations throughout time and space
are intelligible; for whether we look to the forms of life which have
changed during successive ages within the same quarter of the world, or
to those which have changed after having migrated into distant
quarters, in both cases the forms within each class have been connected
by the same bond of ordinary generation; and the more nearly any two
forms are related in blood, the nearer they will generally stand to
each other in time and space; in both cases the laws of variation have
been the same, and modifications have been accumulated by the same
power of natural selection.
CHAPTER XIII.
MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY: EMBRYOLOGY:
RUDIMENTARY ORGANS.
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