Origin of Species Variorum

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

of equal thickness for a space, 1872

→ necessarily being a 1866 1869 1872

must be 1859 1860 1861

→ first giving rise 1866 1869 1872

giving rise first 1859 1860 1861

→ species, and so 1866 1869 1872

so 1859 1860 1861

→ by catastrophes at successive periods 1866 1869 1872

at successive periods by catastrophes, 1859 1860 1861

→ world. 1859 1860 1869 1872

world; in some few cases, however, as by the breaking of an isthmus and the consequent irruption of a multitude of new inhabitants, or by the final subsidence of an island, the extinction may have been comparatively rapid. 1861 1866

thickens upwards,

sometimes

often

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

theory;

theory,

for

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 →necessarily being a slow and

gradual,—

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

species,

varieties

and →species, and so on, like the branching of a great tree from a single stem, till the group becomes large.

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 →by catastrophes at successive periods 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. In some few cases, however, as by the breaking of an isthmus and the consequent irruption of a multitude of new inhabitants into an adjoining sea, or by the