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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. 1859 1860 1869 1872
sudden relatively to that of most other groups. 1861 1866

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

how utterly groundless was 1859 1860 1861 1866
OMIT 1869 1872

astonishment! 1859 1860 1861 1866
astonishment was groundless. 1869 1872

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.
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
a
a
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 wild over the whole country and has increased in numbers at an unparalleled rate, I asked myself what could so recently have exterminated the former horse under conditions of life apparently so favourable. But how utterly groundless was my astonishment!
Professor
Pro- fessor