On the Origin of Species by Means of Natural Selection
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Chapter X
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 rules relating to the
geological succession of organic beings, better accord with the common view
of the immutability of species, or with that of their slow and gradual
modification, through descent 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 them, and to make the
percentage system of lost and new 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 lost forms, and only one or two are new forms,
having here appeared for the first time, either locally, or, as far as we
know, on the face of the earth. If we may trust the observations of
Philippi in Sicily, the successive changes in the marine inhabitants of
that island have been many and most gradual. The secondary formations are
more broken; but, as Bronn has remarked, neither the appearance nor
disappearance of their many now extinct species has been simultaneous in
each separate formation.
Species of different genera and classes have not changed at the same rate,
or in the same degree. In the oldest 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, in an existing crocodile
associated with many strange and 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 change at a quicker rate than those of the sea, of which a striking
instance has lately been observed in Switzerland. There is some reason to
believe that organisms, considered high in the scale of nature, change more
quickly than those that are low: though there are exceptions to this rule.
The amount of organic change, as Pictet has remarked, does not strictly
correspond with the succession of our geological formations; so that
between each two consecutive formations, the forms of life have seldom
changed in exactly the same degree. 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 reason to believe that the same identical form never
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 to me satisfactory.
These several facts accord well with my theory. I believe in no fixed law
of development, causing all the inhabitants of a country to change
abruptly, or simultaneously, or to an equal degree. The process of
modification must be extremely slow. The variability of each species is
quite independent of that of all others. Whether such variability be taken
advantage of by natural selection, and whether the variations be
accumulated to a greater or lesser amount, thus causing a greater or lesser
amount of modification in the varying species, depends on many complex
contingencies,--on the variability being of a beneficial nature, on the
power of intercrossing, on the rate of breeding, on the slowly changing
physical conditions of the country, and more especially on the nature of
the other inhabitants with which the varying species comes 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,
that it should change less. We see the same fact in geographical
distribution; for instance, in the land-shells and coleopterous insects of
Madeira having 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 a country have become
modified and improved, we can understand, on the principle of competition,
and on that of the many all-important relations of organism to organism,
that any form which does not become in some degree modified and improved,
will be liable to be exterminated. Hence we can see why all the species in
the same region do at last, if we look to wide enough intervals of time,
become modified; for those which do not change will 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 long-enduring fossiliferous formations depends on great
masses of sediment having been deposited on areas whilst subsiding, our
formations have been almost necessarily accumulated at wide and irregularly
intermittent intervals; 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 a 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 exact
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. For instance, it is just possible, if our
fantail-pigeons were all destroyed, that fanciers, by striving during long
ages for the same object, might make a new breed hardly distinguishable
from our present fantail; but if the parent rock-pigeon were also
destroyed, and in nature we have every reason to believe that the
parent-form will generally be supplanted and exterminated by its improved
offspring, it is quite incredible that a fantail, identical with the
existing breed, could be raised from any other species of pigeon, or even
from the other well-established races of the domestic pigeon, for the
newly-formed fantail would be almost sure to inherit from its new
progenitor some slight 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 does not
reappear after it has once disappeared; or 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 my theory.
For as all the species of the same group have descended from some one
species, it is clear that as long as any species of the group have appeared
in the long succession of ages, so long must its members have continuously
existed, in order to have generated either new and modified or the same old
and unmodified forms. Species of the genus Lingula, for instance, must
have continuously existed by an unbroken succession of generations, from
the lowest Silurian stratum to the present day.
We have seen in the last chapter that the species of a group sometimes
falsely appear to have come in abruptly; and I have attempted to give an
explanation of this fact, which if true would have been fatal to my views.
But such cases are certainly exceptional; the general rule being a gradual
increase in number, till the group reaches its maximum, and then, sooner or
later, it gradually decreases. If the number of the species of a genus, or
the number of the genera of a family, be represented by a vertical line of
varying thickness, crossing 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, sometimes keeping for a space of equal thickness, 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 my theory; as the species of the
same genus, and the genera of the same family, can increase only slowly and
progressively; for the process of modification and the production of a
number of allied forms must be slow and gradual,--one species giving rise
first to two or three varieties, these being slowly converted into species,
which in their turn produce by equally slow steps other species, and so on,
like the branching of a great tree from a single stem, till the group
becomes large.
On Extinction. -- We have as yet spoken only incidentally of the
disappearance of species and of groups of species. On the theory of
natural selection the extinction of old forms and the production of new and
improved forms are intimately connected together. The old notion of all
the inhabitants of the earth having been swept away at successive periods
by catastrophes, is very generally given up, even by those geologists, as
Elie de Beaumont, Murchison, Barrande, &c., whose general views would
naturally lead them to this conclusion. On the contrary, we have every
reason to believe, from the study of the 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. Both single
species and whole groups of species last for very unequal periods; some
groups, as we have seen, having endured from the earliest known dawn of
life to the present day; some having disappeared before the close of the
palaeozoic 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 extinction of the species of a group is
generally a slower process than their production: if 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 numbers
of the species. In some cases, however, the extermination of whole groups
of beings, as of 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 as the
individual has a definite length of life, so have species a definite
duration. No one 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 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 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 certain from the
analogy of all other mammals, even of the slow-breeding elephant, and from
the history of the naturalisation of the domestic horse in South America,
that under more favourable conditions it would in a very few years have
stocked the whole continent. But we could not have told what the
unfavourable conditions were which checked its increase, whether some one
or several contingencies, and at what period of the horse's life, and in
what degree, they severally acted. If the conditions had gone on, however
slowly, becoming less and less favourable, we assuredly should not have
perceived the fact, yet the fossil horse would certainly have become rarer
and rarer, and finally extinct;--its place being seized on by some more
successful competitor.
It is most difficult always to remember that the increase of every living
being is constantly being checked by unperceived injurious agencies; and
that these same unperceived agencies are amply sufficient to cause rarity,
and finally extinction. We see in many cases in the more recent tertiary
formations, that rarity precedes extinction; and we know that this has been
the progress of events with those animals which have been exterminated,
either locally or wholly, through man's agency. I may repeat what I
published in 1845, namely, that to admit that species generally become rare
before they become extinct--to feel no surprise at the rarity of a species,
and yet to marvel greatly when it ceases to exist, is much the same as to
admit that sickness in the individual is the forerunner of death--to feel
no surprise at sickness, but when the sick man dies, to wonder and to
suspect that he died by some unknown deed of violence.
The theory of natural selection is grounded on the belief that each new
variety, and ultimately each new species, is produced and maintained by
having some advantage over those with which it comes into competition; and
the consequent extinction of less-favoured forms almost inevitably follows.
It is the same with our domestic productions: when a new and slightly
improved variety has been raised, it at first supplants the less improved
varieties in the same neighbourhood; when much improved it is transported
far and near, like our short-horn cattle, and takes the place of other
breeds in other countries. Thus the appearance of new forms and the
disappearance of old forms, both natural and artificial, are bound
together. In certain flourishing groups, the number of new specific forms
which have been produced within a given time is probably greater than that
of the old forms which have been exterminated; but we know that the number
of species has not gone on indefinitely increasing, at least during the
later geological periods, so that looking to later times we may believe
that the production of new forms has caused the extinction of about the
same number of old forms.
The competition will generally be most severe, as formerly explained and
illustrated by examples, between the forms which are most like each other
in all respects. Hence the improved and modified descendants of a species
will generally cause the extermination of the parent-species; and if many
new forms have been developed from any one species, the nearest allies of
that species, i.e. the species of the same genus, will be the most liable
to extermination. Thus, as I believe, a number of new species descended
from one species, that is a new genus, comes to supplant an old genus,
belonging to the same family. But it must often have happened that a new
species belonging to some one group will have seized on the place occupied
by a species belonging to a distinct group, and thus caused its
extermination; and if many allied forms be developed from the successful
intruder, many will have to yield their places; and it will generally be
allied forms, which will suffer from some inherited inferiority in common.
But whether it be species belonging to the same or to a distinct class,
which yield their places to other species which have been modified and
improved, a few of the sufferers may often long be preserved, from being
fitted to some peculiar line of life, or from inhabiting some distant and
isolated station, where they have escaped severe competition. For
instance, a single species of Trigonia, a great genus of shells in the
secondary formations, survives in the Australian seas; and a few members of
the great and almost extinct group of Ganoid fishes still inhabit our fresh
waters. Therefore the utter extinction of a group is generally, as we have
seen, a slower process than its production.
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