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for a space of equal thickness, 1859 1860 1861 1866 1869
of equal thickness for a space, 1872

must be 1859 1860 1861
necessarily being a 1866 1869 1872

giving rise first 1859 1860 1861
first giving rise 1866 1869 1872

so 1859 1860 1861
species, and so 1866 1869 1872

at successive periods by catastrophes, 1859 1860 1861
by catastrophes at successive periods 1866 1869 1872

number of the species
included within
of
a genus, or the number of the genera
within
of
a family, be represented by a vertical line of varying thickness,
ascending through
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,
often
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
the number
number
of the species of a group is strictly conformable with
the
my
theory,
theory;
for
as
the species of the same genus, and the genera of the same family, can increase only slowly and progressively;
for
for
the process of modification and the production of a number of allied forms must be slow and
gradual process,—
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
varieties
species,
and so on, like the branching of a great tree from a single stem, till the group becomes large.
On
On
Extinction.
Extinction.
Extinction .
Extinction .—
We have as yet spoken only incidentally of the disappearance of species and of groups of species. On the theory of natural
selection,
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