CHAPTER XI - ON THE GEOLOGICAL SUCCESSION
OF ORGANIC BEINGS
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
lace 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.
These several facts accord well with our theory, which includes no
fixed law of development, causing all the inhabitants of an area to
change abruptly, or simultaneously, or to an equal degree. The process
of modification must be slow, and will generally affect only a few
species at the same time; for the variability of each species is
independent of that of all others. Whether such variations or
individual differences as may arise will be accumulated through
natural selection in a greater or less degree, thus causing a
greater or less amount of permanent modification, will depend on
many complex contingencies- on the variations being of a beneficial
nature, on the freedom of intercrossing, on the slowly changing
physical conditions of the country, on the immigration of new
colonists, and on the nature of the other inhabitants with which the
varying species come into competition. Hence it is by no means
surprising that one species should retain the same identical form much
longer than others; or, if changing, should change in a less degree.
We find similar relations between the existing inhabitants of distinct
countries; for instance, the land-shells and coleopterous insects of
Madeira have come to differ considerably from their nearest allies
on the continent of Europe, whereas the marine shells and birds have
remained unaltered. We can perhaps understand the apparently quicker
rate of change in terrestrial and in more highly organised productions
compared with marine and lower productions, by the more complex
relations of the higher beings to their organic and inorganic
conditions of life, as explained in a former chapter. When many of the
inhabitants of any area have become modified and improved, we can
understand, on the principle of competition, and from the
all-important relations of organism to organism in the struggle for
life, that any form which did not become in some degree modified and
improved, would be liable to extermination. Hence we see why all the
species in the same region do at last, if we look to long enough
intervals of time, become modified, for otherwise they would become
extinct.
In members of the same class the average amount of change, during
long and equal periods of time, may, perhaps, be nearly the same;
but as the accumulation of enduring formations, rich in fossils,
depends on great masses of sediment being deposited on subsiding
areas, our formations have been almost necessarily accumulated at wide
and irregularly intermittent intervals of time; consequently the
amount of organic change exhibited by the fossils embedded in
consecutive formations is not equal. Each formation, on this view,
does not mark a new and complete act of creation, but only an
occasional scene, taken almost at hazard, in an ever slowly changing
drama.
We can clearly understand why a species when once lost should
never reappear, even if the very same conditions of life, organic
and inorganic, should recur. For though the offspring of one species
might be adapted (and no doubt this has occurred in innumerable
instances) to fill the place of another species in the economy of
nature, and thus supplant it; yet the two forms- the old and the
new- would not be identically the same; for both would almost
certainly inherit different characters from their distinct
progenitors; and organisms already differing would vary in a different
manner. For instance, it is possible, if all our fantail pigeons
were destroyed, that fanciers might make a new breed hardly
distinguishable from the present breed; but if the parent
rock-pigeon were likewise destroyed, and under nature we have every
reason to believe that parent-forms are generally supplanted and
exterminated by their improved off spring, it is incredible that a
fantail, identical with the existing breed, could be raised from any
other species of pigeon, or even from any other well-established
race of the domestic pigeon, for the successive variations would
almost certainly be in some degree different, and the newly-formed
variety would probably inherit from its progenitor some characteristic
differences.
Groups of species, that is, genera and families, follow the same
general rules in their appearance and disappearance as do single
species, changing more or less quickly, and in a greater or lesser
degree. A group, when it has once disappeared, never reappears; that
is, its existence, as long as it lasts, is continuous. I am aware that
there are some apparent exceptions to this rule, but the exceptions
are surprisingly few, so few that E. Forbes, Pictet, and Woodward
(though all strongly opposed to such views as I maintain) admit its
truth; and the rule strictly accords with the theory. For all the
species of the same group, however long it may have lasted, are the
modified descendants one from the other, and all from a common
progenitor. In the genus Lingula, for instance, the species which have
successively appeared at all ages must have been connected by an
unbroken series of generations, from the lowest Silurian stratum to
the present day.
We have seen in the last chapter that whole groups of species
sometimes falsely appear to have been abruptly developed; and I have
attempted to give an explanation of this fact, which if true would
be fatal to my views. But such cases are certainly exceptional; the
general rule being a gradual increase in number, until the group
reaches its maximum, and then, sooner or later, a gradual decrease. If
the number of the species included within a genus, or the number of
the genera within a family, be represented by a vertical line of
varying thickness, ascending through the successive geological
formations, in which the species are found, the line will sometimes
falsely appear to begin at its lower end, not in a sharp point, but
abruptly; it then gradually thickens upwards, often keeping of equal
thickness for a space, and ultimately thins out in the upper beds,
marking the decrease and final extinction of the species. This gradual
increase in number of the species of a group is strictly conformable
with the theory, for the species of the same genus, and the genera
of the same family, can increase only slowly and progressively; the
process of modification and the production of a number of allied forms
necessarily being a slow and gradual process,- one species first
giving rise to two or three varieties, these being slowly converted
into species, which in their turn produce by equally slow steps
other varieties and species, and so on, like the branching of a
great tree from a single stem, till the group becomes large.
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