| → 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 |
|
| → a whole group of species 1866 1869 1872 |
| the species of a group 1859 1860 1861 |
|
| → OMIT 1869 1872 |
| of a group of species 1859 1860 1861 |
| of a group 1866 |
|
| → the early increase 1869 1872 |
| increase 1859 1860 1861 1866 |
|
| → sudden. 1859 1860 1869 1872 |
| sudden relatively to that of most other groups. 1861 1866 |
|
| → OMIT 1869 1872 |
| whole subject of the 1859 1860 1861 1866 |
|
| → than I have done at the extinction of species. 1869 1872 |
| at the extinction of species, than I have done. 1859 1860 1861 1866 |
|
| → OMIT 1869 1872 |
| how utterly groundless was 1859 1860 1861 1866 |
|
| → astonishment was groundless. 1869 1872 |
| astonishment! 1859 1860 1861 1866 |
|
|
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 final subsidence of an island, the process of extinction may have been rapid. Both single species and whole groups of species last for very unequal periods; some groups, as we have seen,
endured from the earliest known dawn of life to the present day; some
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
extinction of
→a whole group of species
is generally a slower process than their production: if
appearance and disappearance
→OMIT
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
→the early increase
in
of the species. In some cases, however, the extermination of whole
as of
towards the close of the secondary period, has been wonderfully
→sudden.
|
|
| The
→OMIT
extinction of species has been involved in the most gratuitous mystery. Some authors have even supposed
as the individual has
definite length of life, so have species a definite duration. No one
can have marvelled more
→than I have done at the extinction of species. 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;
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
→OMIT
my
→astonishment was groundless.
Owen soon perceived that the tooth, though so like that of the existing horse, belonged to an extinct species. Had this horse been still living, but in some degree rare, no naturalist would have felt the least surprise at its rarity; for rarity is the attribute of a vast number of species of all classes, in all countries. If we ask ourselves why this or that species is rare, we answer that something is unfavourable in its conditions of life; but what that something is, we can hardly ever tell. On the supposition of the fossil horse still existing as a rare species, we might have felt
|
