the species would become rarer: we
feel not the least surprise at one species of a genus being rare and
another abundant; why then should we be surprised at its extinction,
when we have good reason to believe that this very rarity is its regular
precursor and cause.

{513} <The following words:> The fauna changes singly <were inserted
by the author, apparently to replace a doubtful erasure>.

In the sixth chapter the leading facts in the geographical distribution
of organic beings were considered--namely, the dissimilarity in areas
widely and effectually separated, of the organic beings being exposed to
very similar conditions (as for instance, within the tropical forests of
Africa and America, or on the volcanic islands adjoining them). Also the
striking similarity and general relations of the inhabitants of the same
great continents, conjoined with a lesser degree of dissimilarity in the
inhabitants living on opposite sides of the barriers intersecting
it--whether or not these opposite sides are exposed to similar
conditions. Also the dissimilarity, though in a still lesser degree, in
the inhabitants of different islands in the same archipelago, together
with their similarity taken as a whole with the inhabitants of the
nearest continent, whatever its character may be. Again, the peculiar
relations of Alpine floras; the absence of mammifers on the smaller
isolated islands; and the comparative fewness of the plants and other
organisms on islands with diversified stations; the connection between
the possibility of occasional transportal from one country to another,
with an affinity, though not identity, of the organic beings inhabiting
them. And lastly, the clear and striking relations between the living
and the extinct in the same great divisions of the world; which
relation, if we look very far backward, seems to die away. These facts,
if we bear in mind the geological changes in progress, all simply follow
from the proposition of allied organic beings having lineally descended
from common parent-stocks. On the theory of independent creations they
must remain, though evidently connected together, inexplicable and
disconnected.

In the seventh chapter, the relationship or grouping of extinct and
recent species; the appearance and disappearance of groups; the
ill-defined objects of the natural classification, not depending on the
similarity of organs physiologically important, not being influenced by
adapti