CHAPTER XI. ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS.
1. ON THE SLOW AND SUCCESSIVE APPEARANCE OF NEW SPECIES.
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.