dants one from the other, and all from a common progenitor. In
the genus Lingula, for instance, the species which have successively
appeared at all ages must have been connected by an unbroken series of
generations, from the lowest Silurian stratum to the present day.

We have seen in the last chapter that whole groups of species sometimes
falsely appear to have been abruptly developed; and I have attempted
to give an explanation of this fact, which if true would be fatal to my
views. But such cases are certainly exceptional; the general rule being
a gradual increase in number, until the group reaches its maximum, and
then, sooner or later, a gradual decrease. If the number of the species
included within a genus, or the number of the genera within a family, be
represented by a vertical line of varying thickness, ascending through
the successive geological formations, in which the species are found,
the line will sometimes falsely appear to begin at its lower end, not in
a sharp point, but abruptly; it then gradually thickens upwards, often
keeping of equal thickness for a space, and ultimately thins out in the
upper beds, marking the decrease and final extinction of the species.
This gradual increase in number of the species of a group is strictly
conformable with the theory; for the species of the same genus, and the
genera of the same family, can increase only slowly and progressively;
the process of modification and the production of a number of allied
forms necessarily being a slow and gradual process, one species first
giving rise to two or three varieties, these being slowly converted
into species, which in their turn produce by equally slow steps other
varieties and species, and so on, like the branching of a great tree
from a single stem, till the group becomes large.

ON EXTINCTION.

We have as yet only spoken incidentally of the disappearance of species
and of groups of species. On the theory of natural selection, the
extinction of old forms and the production of new and improved forms are
intimately connected together. The old notion of all the inhabitants of
the earth having been swept away by catastrophes at successive periods
is very generally given up, even by those geologists, as Elie de
Beaumont, Murchison, Barrande, etc., whose general views would naturally
lead them to this conclusion. On the contrary, we have every reason to
believe, from the study of the tertiary formations, that species and
group