cases are certainly exceptional; the general rule being a gradual
increase in number, till the group reaches its maximum, and then, sooner or
later, it gradually decreases. If the number of the species of a genus, or
the number of {317} the genera of a family, be represented 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, &c, 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 {318} 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