Both in time and space
species and groups of species have their points of maximum development.
Groups of species, living during the same period of time, or living within
the same area, are often characterised by trifling features in common, as
of sculpture or colour. In looking to the long succession of past ages, as
in looking to distant provinces throughout the world, we find that species
in certain classes differ little from each other, whilst those in another
class, or only in a different section of the same order, differ greatly
from each other. In both time and space the lowly organised members of
each class generally change less than the highly organised; but there are
in both cases marked exceptions to the rule. According to our theory,
these several relations throughout time and space are intelligible; for
whether we look to the allied forms of life which have changed during
successive ages, or to those which have changed after having migrated into
distant quarters, in both cases they are connected by the same bond of
ordinary generation; in both cases the laws of variation have been the
same, and modifications have been accumulated by the same means of natural
selection.
CHAPTER XIV.
MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY -- EMBRYOLOGY --
RUDIMENTARY ORGANS.
Classification, groups subordinate to groups -- Natural system -- Rules and
difficulties in classification, explained on the theory of descent with
modification -- Classification of varieties -- Descent always used in
classification -- Analogical or adaptive characters -- Affinities, general,
complex and radiating -- Extinction separates and defines groups --
Morphology, between members of the same class, between parts of the same
individual -- Embryology, laws of, explained by variations not supervening
at an early age, and being inherited at a corresponding age -- Rudimentary
organs; their origin explained -- Summary.
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