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On the Origin of Species

On The Geological Succession Of Organic Beings
On the slow and successive appearance of new species -- On their different rates of
change -- Species once lost do not reappear -- Groups of species follow the same general
rules in their appearance and disappearance as do single species -- On extinction -- On
simultaneous changes in the forms of life throughout the world -- On the affinities of
extinct species to each other and to living species -- On the state of development of
ancient forms -- On the succession of the same types within the same areas -- Summary
of preceding and present chapters.
Let us now see whether the several facts and laws relating to the geological succession of
organic beings accord best with the common view of the immutability of species, or with
that of their slow and gradual modification, through variation and natural selection.
New species have appeared very slowly, one after another, both on the land and in the
waters. Lyell has shown that it is hardly possible to resist the evidence on this head in the
case of the several tertiary stages; and every year tends to fill up the blanks between the
stages, and to make the proportion between the lost and existing forms more gradual. In
some of the most recent beds, though undoubtedly of high antiquity if measured by years,
only one or two species are extinct, and only one or two are new, having appeared there
for the first time, either locally, or, as far as we know, on the face of the earth. The
secondary formations are more broken; but, as Bronn has remarked, neither the
appearance nor disappearance of the many species embedded in each formation has been
simultaneous.
Species belonging to different genera and classes have not changed at the same rate, or in
the same degree. In the older tertiary beds a few living shells may still be found in the
midst of a multitude of extinct forms. Falconer has given a striking instance of a similar
fact, for an existing crocodile is associated with many lost mammals and reptiles in the
sub-Himalayan deposits. The Silurian Lingula differs but little from the living species of
this genus; whereas most of the other Silurian Molluscs and all the Crustaceans have
changed greatly. The productions of the land seem to have changed at a quicker rate than
those of the sea, of which a striking instance has been observed in Switzerland. There is
some reason to believe that organisms high in the scale, change more quickly than those
that are low: though there are exceptions to this rule. The amount of organic change, as
Pictet has remarked, is not the same in each successive so-called formation. Yet if we
compare any but the most closely related formations, all the species will be found to have
undergone some change. When a species has once disappeared from the face of the earth,
we have no reason to believe that the same identical form ever reappears. The strongest
apparent exception to this latter rule is that of the so-called "colonies" of M. Barrande,
which intrude for a period in the midst of an older formation, and then allow the pre-
existing fauna to reappear; but Lyell's explanation, namely, that it is a case of temporary
migration from a distinct geographical province, seems satisfactory.
 
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