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Emile Souvestre - "An Attic Philosopher, v2"

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Books: On the Origin of Species

C >> Charles Darwin >> On the Origin of Species




The case most frequently insisted on by palaeontologists of the
apparently sudden appearance of a whole group of species, is that of
the teleostean fishes, low down in the Chalk period. This group
includes the large majority of existing species. Lately, Professor
Pictet has carried their existence one sub-stage further back; and
some palaeontologists believe that certain much older fishes, of which
the affinities are as yet imperfectly known, are really teleostean.
Assuming, however, that the whole of them did appear, as Agassiz
believes, at the commencement of the chalk formation, the fact would
certainly be highly remarkable; but I cannot see that it would be an
insuperable difficulty on my theory, unless it could likewise be shown
that the species of this group appeared suddenly and simultaneously
throughout the world at this same period. It is almost superfluous to
remark that hardly any fossil-fish are known from south of the
equator; and by running through Pictet's Palaeontology it will be seen
that very few species are known from several formations in Europe.
Some few families of fish now have a confined range; the teleostean
fish might formerly have had a similarly confined range, and after
having been largely developed in some one sea, might have spread
widely. Nor have we any right to suppose that the seas of the world
have always been so freely open from south to north as they are at
present. Even at this day, if the Malay Archipelago were converted
into land, the tropical parts of the Indian Ocean would form a large
and perfectly enclosed basin, in which any great group of marine
animals might be multiplied; and here they would remain confined,
until some of the species became adapted to a cooler climate, and were
enabled to double the southern capes of Africa or Australia, and thus
reach other and distant seas.

From these and similar considerations, but chiefly from our ignorance
of the geology of other countries beyond the confines of Europe and
the United States; and from the revolution in our palaeontological
ideas on many points, which the discoveries of even the last dozen
years have effected, it seems to me to be about as rash in us to
dogmatize on the succession of organic beings throughout the world, as
it would be for a naturalist to land for five minutes on some one
barren point in Australia, and then to discuss the number and range of
its productions.

ON THE SUDDEN APPEARANCE OF GROUPS OF ALLIED SPECIES IN THE LOWEST
KNOWN FOSSILIFEROUS STRATA.

There is another and allied difficulty, which is much graver. I allude
to the manner in which numbers of species of the same group, suddenly
appear in the lowest known fossiliferous rocks. Most of the arguments
which have convinced me that all the existing species of the same
group have descended from one progenitor, apply with nearly equal
force to the earliest known species. For instance, I cannot doubt that
all the Silurian trilobites have descended from some one crustacean,
which must have lived long before the Silurian age, and which probably
differed greatly from any known animal. Some of the most ancient
Silurian animals, as the Nautilus, Lingula, etc., do not differ much
from living species; and it cannot on my theory be supposed, that
these old species were the progenitors of all the species of the
orders to which they belong, for they do not present characters in any
degree intermediate between them. If, moreover, they had been the
progenitors of these orders, they would almost certainly have been
long ago supplanted and exterminated by their numerous and improved
descendants.

Consequently, if my theory be true, it is indisputable that before the
lowest Silurian stratum was deposited, long periods elapsed, as long
as, or probably far longer than, the whole interval from the Silurian
age to the present day; and that during these vast, yet quite unknown,
periods of time, the world swarmed with living creatures.

To the question why we do not find records of these vast primordial
periods, I can give no satisfactory answer. Several of the most
eminent geologists, with Sir R. Murchison at their head, are convinced
that we see in the organic remains of the lowest Silurian stratum the
dawn of life on this planet. Other highly competent judges, as Lyell
and the late E. Forbes, dispute this conclusion. We should not forget
that only a small portion of the world is known with accuracy. M.
Barrande has lately added another and lower stage to the Silurian
system, abounding with new and peculiar species. Traces of life have
been detected in the Longmynd beds beneath Barrande's so-called
primordial zone. The presence of phosphatic nodules and bituminous
matter in some of the lowest azoic rocks, probably indicates the
former existence of life at these periods. But the difficulty of
understanding the absence of vast piles of fossiliferous strata, which
on my theory no doubt were somewhere accumulated before the Silurian
epoch, is very great. If these most ancient beds had been wholly worn
away by denudation, or obliterated by metamorphic action, we ought to
find only small remnants of the formations next succeeding them in
age, and these ought to be very generally in a metamorphosed
condition. But the descriptions which we now possess of the Silurian
deposits over immense territories in Russia and in North America, do
not support the view, that the older a formation is, the more it has
suffered the extremity of denudation and metamorphism.

The case at present must remain inexplicable; and may be truly urged
as a valid argument against the views here entertained. To show that
it may hereafter receive some explanation, I will give the following
hypothesis. From the nature of the organic remains, which do not
appear to have inhabited profound depths, in the several formations of
Europe and of the United States; and from the amount of sediment,
miles in thickness, of which the formations are composed, we may infer
that from first to last large islands or tracts of land, whence the
sediment was derived, occurred in the neighbourhood of the existing
continents of Europe and North America. But we do not know what was
the state of things in the intervals between the successive
formations; whether Europe and the United States during these
intervals existed as dry land, or as a submarine surface near land, on
which sediment was not deposited, or again as the bed of an open and
unfathomable sea.

Looking to the existing oceans, which are thrice as extensive as the
land, we see them studded with many islands; but not one oceanic
island is as yet known to afford even a remnant of any palaeozoic or
secondary formation. Hence we may perhaps infer, that during the
palaeozoic and secondary periods, neither continents nor continental
islands existed where our oceans now extend; for had they existed
there, palaeozoic and secondary formations would in all probability
have been accumulated from sediment derived from their wear and tear;
and would have been at least partially upheaved by the oscillations of
level, which we may fairly conclude must have intervened during these
enormously long periods. If then we may infer anything from these
facts, we may infer that where our oceans now extend, oceans have
extended from the remotest period of which we have any record; and on
the other hand, that where continents now exist, large tracts of land
have existed, subjected no doubt to great oscillations of level, since
the earliest silurian period. The coloured map appended to my volume
on Coral Reefs, led me to conclude that the great oceans are still
mainly areas of subsidence, the great archipelagoes still areas of
oscillations of level, and the continents areas of elevation. But have
we any right to assume that things have thus remained from eternity?
Our continents seem to have been formed by a preponderance, during
many oscillations of level, of the force of elevation; but may not the
areas of preponderant movement have changed in the lapse of ages? At a
period immeasurably antecedent to the silurian epoch, continents may
have existed where oceans are now spread out; and clear and open
oceans may have existed where our continents now stand. Nor should we
be justified in assuming that if, for instance, the bed of the Pacific
Ocean were now converted into a continent, we should there find
formations older than the silurian strata, supposing such to have been
formerly deposited; for it might well happen that strata which had
subsided some miles nearer to the centre of the earth, and which had
been pressed on by an enormous weight of superincumbent water, might
have undergone far more metamorphic action than strata which have
always remained nearer to the surface. The immense areas in some parts
of the world, for instance in South America, of bare metamorphic
rocks, which must have been heated under great pressure, have always
seemed to me to require some special explanation; and we may perhaps
believe that we see in these large areas, the many formations long
anterior to the silurian epoch in a completely metamorphosed
condition.

The several difficulties here discussed, namely our not finding in the
successive formations infinitely numerous transitional links between
the many species which now exist or have existed; the sudden manner in
which whole groups of species appear in our European formations; the
almost entire absence, as at present known, of fossiliferous
formations beneath the Silurian strata, are all undoubtedly of the
gravest nature. We see this in the plainest manner by the fact that
all the most eminent palaeontologists, namely Cuvier, Owen, Agassiz,
Barrande, Falconer, E. Forbes, etc., and all our greatest geologists,
as Lyell, Murchison, Sedgwick, etc., have unanimously, often
vehemently, maintained the immutability of species. But I have reason
to believe that one great authority, Sir Charles Lyell, from further
reflexion entertains grave doubts on this subject. I feel how rash it
is to differ from these great authorities, to whom, with others, we
owe all our knowledge. Those who think the natural geological record
in any degree perfect, and who do not attach much weight to the facts
and arguments of other kinds given in this volume, will undoubtedly at
once reject my theory. For my part, following out Lyell's metaphor, I
look at the natural geological record, as a history of the world
imperfectly kept, and written in a changing dialect; of this history
we possess the last volume alone, relating only to two or three
countries. Of this volume, only here and there a short chapter has
been preserved; and of each page, only here and there a few lines.
Each word of the slowly-changing language, in which the history is
supposed to be written, being more or less different in the
interrupted succession of chapters, may represent the apparently
abruptly changed forms of life, entombed in our consecutive, but
widely separated formations. On this view, the difficulties above
discussed are greatly diminished, or even disappear.


CHAPTER 10. 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, etc., 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.