f varieties, of doubtful species, and of distinct
but representative species.

As the late Edward Forbes often insisted, there is a striking
parallelism in the laws of life throughout time and space: the laws
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.

13. MUTUAL AFFINITIES OF ORGANIC BEINGS: MORP