other hand, ancient and important traits of structure may, in some
species, have dwindled into inconspicuous survivals or be still found
only in the embryo; so that only great knowledge and sagacity can
identify them; yet upon ancient traits, though hidden, classification
depends. The seal seems nearer allied to the porpoise than to the tiger,
the shrew nearer to the mouse than to the hedgehog; and the Tasmanian
wolf looks more like a true wolf, the Tasmanian devil more like a
badger, than like a kangaroo: yet the seal is nearer akin to the tiger,
the shrew to the hedgehog, and the Tasmanian flesh-eaters are marsupial,
like the kangaroo. To overcome this difficulty we must understand the
resemblance upon which classification is based to include resemblance of
Causation, that is, the fact itself of descent from common ancestors.
For organic beings, all other rules of classification are subordinate to
one: trace the genealogy of every form.

By this rule we get a definite meaning for the phrase 'important or
fundamental attribute' as determining organic classes; namely, most
ancient, or 'best serving to indicate community of origin.' Grades of
classification will be determined by such fundamental characters, and
may correspond approximately to the more general types (now extinct)
from which existing animals have descended.

(2) By the hypothesis of development the fixity of species is
discredited. The lowest grade of a classification is made up not of
well-defined types unchanging from age to age, but of temporary species,
often connected by uncertain and indistinct varieties: some of which
may, in turn, if the conditions of their existence alter, undergo such
changes as to produce new species. Hence the notion that Kinds exist in
organic nature must be greatly modified. During a given period of a few
thousand years, Kinds may be recognised, because, under such conditions
as now prevail in the world, that period of time is insufficient to
bring about great changes. But, if it be true that lions, tigers, and
leopards have had a common ancestor, from whose type they have gradually
diverged, it is plain that their present distinctness results only from
the death of intermediate specimens and the destruction of intermediate
varieties. Were it possible to restore, by the evidence of fossils, all
the ranks of the great processions that have descended from the common
ancestor, there would nowhere occur a greater difference than