Origin of Species Variorum

→ the species of a group 1859 1860 1861

a whole group of species 1866 1869 1872

→ of a group of species 1859 1860 1861

of a group 1866

OMIT 1869 1872

→ increase 1859 1860 1861 1866

the early increase 1869 1872

→ sudden relatively to that of most other groups. 1861 1866

sudden. 1859 1860 1869 1872

→ whole subject of the 1859 1860 1861 1866

OMIT 1869 1872

→ at the extinction of species, than I have done. 1859 1860 1861 1866

than I have done at the extinction of species. 1869 1872

Both single species and whole groups of species last for very unequal periods; some groups, as we have seen,

have

having

endured from the earliest known dawn of life to the present day; some

have

having

disappeared before the close of the palæozoic 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~~

complete

extinction of →the species of a group is generally a slower process than their production: if

their

the

appearance and disappearance →of a group of species be represented, as before, by a vertical line of varying thickness, the line is found to taper more gradually at its upper end, which marks the progress of extermination, than at its lower end, which marks the first appearance and →increase in

number

numbers

of the species. In some cases, however, the extermination of whole

groups,

groups

~~of beings,~~

of beings,

as of

ammonites,

ammonites

towards the close of the secondary period, has been wonderfully →sudden relatively to that of most other groups.

The →whole subject of the extinction of species has been involved in the most gratuitous mystery. Some authors have even supposed

that,

that

as the individual has

definite length of life, so have species a definite duration. No one

~~I think~~

I think

can have marvelled more →at the extinction of species, than I have done. When I found in La Plata the tooth of a horse embedded with the remains of Mastodon, Megatherium, Toxodon, and other extinct monsters, which all co-existed with still living shells at a very late geological period, I was filled with astonishment;

for,

for

seeing that the horse, since its introduction by the Spaniards into South America, has run