Origin of Species Variorum

↑ 1 blocks not present in 1869 1872; present in 1859 1860 1861 1866

For as all the species of the 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.

→ one from the 1872

of each 1869

→ all from a 1872

of some 1869

→ the species which have successively appeared at all ages must have been connected 1869 1872

must have continuously existed 1859 1860 1861 1866

→ whole groups of species 1872

the species of a group 1859 1860 1861

many species of a group 1866 1869

→ been abruptly developed; 1872

come in abruptly; 1859 1860

come in abruptly 1861 1866 1869

→ and 1859 1860 1872

in a body; and 1861 1866 1869

→ of equal thickness for a space, 1872

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

→ 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

Woodward (though all strongly opposed to such views as I maintain) admit its truth; and the rule strictly accords with

the

theory. ↑ For all the species of the same group, however long it may have lasted, are the modified

descendants

descendants,

→one from the other, and →all from a common progenitor.

Species of

the genus Lingula, for instance, →the species which have successively appeared at all ages must have been connected

an unbroken

succession

series

of generations, from the lowest Silurian stratum to the present day.

We have seen in the last chapter that →whole groups of species sometimes falsely appear to have →been abruptly developed; →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

until

the group reaches its maximum, and then, sooner or later,

gradually

gradual

decreases.

decrease.

If the number of the species

included within

a genus, or the number of the genera

within

a family, be represented by a vertical line of varying thickness,

crossing

ascending through

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

often

keeping →of equal thickness for a space, 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