same group have descended from some one species, it is clear that as
long as any species of the group have appeared in the long succession
of ages, so long must its members have continuously existed, in order
to have generated either new and modified or the same old and
unmodified forms. Species of the genus Lingula, for instance, must have
continuously existed by an unbroken succession of generations, from the
lowest Silurian stratum to the present day.
We have seen in the last chapter that the species of a group sometimes
falsely appear to have come in abruptly; and I have attempted to give
an explanation of this fact, which if true would have been fatal to my
views. But such 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 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, 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
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