sented by a vertical line of varying thickness, crossing 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, sometimes
keeping for a space of equal thickness, 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 my theory; as the species of the same genus, and the
genera of the same family, can increase only slowly and progressively;
for the process of modification and the production of a number of allied
forms must be slow and gradual,--one species giving rise first to two
or three varieties, these being slowly converted into species, which in
their turn produce by equally slow steps other 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 spoken only 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 at successive periods by catastrophes,
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
groups of species gradually disappear, one after another, first from one
spot, then from another, and finally from the world. Both single species
and whole groups of species last for very unequal periods; some groups,
as we have seen, having endured from the earliest known dawn of life
to the present day; some having disappeared before the close of the
palaeozoic period. No fixed law seems to determine the length of time
during which any single species or any single genus endures. There is
reason to believe that the complete extinction of the species of a group
is generally a slower process than their production: if the appearance
and disappearance of a group of species be represented, as before, by
a vertical line of varying thickness, the line is found to taper more
gradually at its upper end, which marks