, till finally the group becomes extinct, in the same
way as a species becomes extinct, by the individuals becoming rarer and
rarer. Moreover, groups, like the individuals of a species, appear to
become extinct at different times in different countries. The
Palaeotherium was extinct much sooner in Europe than in India: the
Trigonia{428} was extinct in early ages in Europe, but now lives in the
seas of Australia. As it happens that one species of a family will
endure for a much longer period than another species, so we find that
some whole groups, such as Mollusca, tend to retain their forms, or to
remain persistent, for longer periods than other groups, for instance
than the Mammalia. Groups therefore, in their appearance, extinction,
and rate of change or succession, seem to follow nearly the same laws
with the individuals of a species{429}.

{426} _Origin_, Ed. i. p. 411, vi. p. 566.

{427} _Origin_, Ed. i. p. 316, vi. p. 457.

{428} _Origin_, Ed. i. p. 321, vi. p. 463.

{429} In the _Origin_, Ed. i. this preliminary matter is replaced
(pp. 411, 412, vi. pp. 566, 567) by a discussion in which
extinction is also treated, but chiefly from the point of view of
the theory of divergence.

_What is the Natural System?_

The proper arrangement of species into groups, according to the natural
system, is the object of all naturalists; but scarcely two naturalists
will give the same answer to the question, What is the natural system
and how are we to recognise it? The most important characters{430} it
might be thought (as it was by the earliest classifiers) ought to be
drawn from those parts of the structure which determine its habits and
place in the economy of nature, which we may call the final end of its
existence. But nothing is further from the truth than this; how much
external resemblance there is between the little otter (Chironectes) of
Guiana and the common otter; or again between the common swallow and the
swift; and who can doubt that the means and ends of their existence are
closely similar, yet how grossly wrong would be the classification,
which put close to each other a Marsupial and Placental animal, and two
birds with widely different skeletons. Relations, such as in the two
latter cases, or as that between the whale and fishes, are denominated
"analogical{431}," or are sometimes described as "relations of
adaption." They are infinitely numerous and often very singul