s 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.

CLASSIFICATION.

From the most remote period in the history of the world organic beings
have been found to resemble each other in descending degrees, so that
they can be classed in groups under groups. This classification is
not arbitrary like the grouping of the stars in constellations. The
existence of groups would have been of simple significance, if one group
had been exclusively fitted to inhabit the land, and another the water;
one to feed on flesh, another on vegetable matter, and so on; but the
case is widely different, for it is notorious how commonly members of
even the