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tained numerous transitional gradations, we should not recognize their blood-
relationship, and should consequently rank them as distinct species.

It is notorious on what excessively slight differences many palaeontologists


have founded their species; and they do this the more readily if the specimens
come from different sub-stages of the same formation. Some experienced
conchologists are now sinking many of the very fine species of D'Orbigny
and others into the rank of varieties; and on this view we do find the kind
of evidence of change which on the theory we ought to find. Look again at
the later tertiary deposits, which include many shells believed by the majority
of naturalists to be identical with existing species; but some excellent
naturalists, as Agassiz and Pictet, maintain that all these tertiary species are
specifically distinct, though the distinction is admitted to be very slight; so
that here, unless we believe that these eminent naturalists have been misled
by their imaginations, and that these late tertiary species really present no
difference whatever from their living representatives, or unless we admit, in
opposition to the judgment of most naturalists, that these tertiary species are
all truly distinct from the recent, we have evidence of the frequent occur-
rence of slight modifications of the kind required. If we look to rather wider
intervals of time, namely, to distinct but consecutive stages of the same great
formation, we find that the embedded fossils, though universally ranked as
specifically different, yet are far more closely related to each other than are
the species found in more widely separated formations; so that here again we
have undoubted evidence of change in the direction required by the theory;
but to this latter subject I shall return in the following chapter.

With animals and plants that propagate rapidly and do not wander much,
there is reason to suspect, as we have formerly seen, that their varieties are
generally at first local; and that such local varieties do not spread widely
and supplant their parent-form until they have been modified and perfected
in some considerable degree. According to this view, the chance of discover-
ing in a formation in any one country all the early stages of transition be-
tween any two forms, is small, for the successive changes are supposed to have
been local or confined to some one spot. Most marine animals have a wide
range; and we have seen that with plants it is those which have the widest
range, that oftenest present varieties; so that, with shells and other marine
animals, it is probable that those which had the widest range, far exceeding
the limits of the known geological formations in Europe, have oftenest given
rise, first to local varieties and ultimately to new species; and this again
would greatly lessen the chance of our being able to trace the stages of transi-
tion in any one geological formation.

It is a more important consideration, leading to the same result, as lately
insisted on by Dr. Falconer, namely, that the period during which each
species underwent modification, though long as measured by years, was
probably short in comparison with that during which it remained without
undergoing any change.

It should not be forgotten, that at the present day, with perfect specimens
for examination, two forms can seldom be connected by intermediate
varieties, and thus proved to be the same species, until many specimens are
collected from many places; and with fossil species this can rarely be done.
We shall, perhaps, best perceive the improbability of our being enabled to


connect species by numerous, fine, intermediate, fossil links, by asking our-
selves whether, for instance, geologists at some future period will be able to
prove that our different breeds of cattle, sheep, horses, and dogs are de-
scended from a single stock or from several aboriginal stocks; or again,
whether certain sea-shells inhabiting the shores of North America, which are
ranked by some conchologists as distinct species from their European rep-
resentatives, and by other conchologists as only varieties, are really varieties,
or are, as it is called, specifically distinct. This could be effected by the future
geologist only by his discovering in a fossil state numerous intermediate
gradations; and such success is improbable in the highest degree.

It has been asserted over and over again, by writers who believe in the
immutability of species, that geology yields no linking forms. This assertion,
as we shall see in the next chapter, is certainly erroneous. As Sir J. Lubbock
has remarked, "Every species is a link between other allied forms." If we take
a genus having a score of species, recent and extinct, and destroy four-fifths
of them, no one doubts that the remainder will stand much more distinct
from each other. If the extreme forms in the genus happen to have been thus
destroyed, the genus itself will stand more distinct from other allied genera.
What geological research has not revealed, is the former existence of infinitely
numerous gradations, as fine as existing varieties, connecting together nearly
all existing and extinct species. But this ought not to be expected; yet this
has been repeatedly advanced as a most serious objection against my views.

It may be worth while to sum up the foregoing remarks on the causes of
the imperfection of the geological record under an imaginary illustration.
The Malay Archipelago is about the size of Europe from the North Cape to
the Mediterranean, and from Britain to Russia, and therefore equals all the
geological formations which have been examined with any accuracy, except-
ing those of the United States of America. I fully agree with Mr. Godwin-
Austen, that the present condition of the Malay Archipelago, with its
numerous large islands separated by wide and shallow seas, probably rep-
resents the former state of Europe, while most of our formations were ac-
cumulating. The Malay Archipelago is one of the richest regions in organic
beings ; yet if all the species were to be collected which have ever lived there,
how imperfectly would they represent the natural history of the world!

But we have every reason to believe that the terrestrial productions of the
archipelago would be preserved in an extremely imperfect manner in the
formations which we suppose to be there accumulating. Not many of the
strictly littoral animals, or of those which lived on naked submarine rocks,
would be embedded ; and those embedded in gravel or sand would not endure
to a distant epoch. Wherever sediment did not accumulate on the bed of the
sea, or where it did not accumulate at a sufficient rate to protect organic
bodies from decay, no remains could be preserved.

Formations rich in fossils of many kinds, and of thickness sufficient to last
to an age as distant in futurity as the secondary formations lie in the past,
would generally be formed in the archipelago only during periods of
subsidence. These periods of subsidence would be separated from each other


by immense intervals of time, during which the area would be either
stationary or rising; while rising, the fossiliferous formations on the steeper
shores would be destroyed, almost as soon as accumulated, by the incessant
coast-action, as we now see on the shores of South America. Even throughout
the extensive and shallow seas within the archipelago, sedimentary beds could
hardly be accumulated of great thickness during the periods of elevation, or
become capped and protected by subsequent deposits, so as to have a good
chance of enduring to a very distant future. During the periods of subsidence,
there would probably be much extinction of life; during the periods of eleva-
tion, there would be much variation, but the geological record would then be
less perfect.

It may be doubted whether the duration of any one great period of sub-
sidence over the whole or part of the archipelago, together with a con-
temporaneous accumulation of sediment, would exceed the average duration
of the same specific forms; and these contingencies are indispensable for the
preservation of all the transitional gradations between any two or more
species. If such gradations were not all fully preserved, transitional varieties
would merely appear as so many new, though closely allied species. It is also
probable that each great period of subsidence would be interrupted by
oscillations of level, and that slight climatical changes would intervene
during such lengthy periods; and in these cases the inhabitants of the archi-
pelago would migrate, and no closely consecutive record of their modifica-
tions could be preserved in any one formation.

Very many of the marine inhabitants of the archipelago now range thou-
sands of miles beyond its confines; and analogy plainly leads to the belief
that it would be chiefly these far-ranging species, though only some of them,
which would of tenest produce new varieties ; and the varieties would at first
be local or confined to one place, but if possessed of any decided advantage,
or when further modified and improved, they would slowly spread and sup-
plant their parent forms. When such varieties returned to their ancient
homes, as they would differ from their former state in a nearly uniform,
though perhaps extremely shght degree, and as they would be found em-
bedded in slightly different sub-stages of the same formation, they would,
according to the principles followed by many palaeontologists, be ranked as.
new and distinct species.

If then there be some degree of truth in these remarks, we have no right
to expect to find, in our geological formations, an infinite number of those
fine transitional forms which, on our theory, have connected all the past and
present species of the same group into one long and branching chain of life.
We ought only to look for a few links, and such assuredly we do find — some
more distantly, some more closely, related to each other; and these links, let
them be ever so close, if found in different stages of the same formation,
would, by many palaeontologists, be ranked as distinct species. But I do not
pretend that I should ever have suspected how poor was the record in the
best preserved geological sections, had not the absence of innumerable


transitional links between the species which lived at the commencement and
close of each formation, pressed so hardly on my theory.


The abrupt manner in which whole groups of species suddenly appear in
certain formations, has been urged by several palaeontologists — for instance,
by Agassiz, Pictet, and Sedgwick — as a fatal objection to the belief in the
transmutation of species. If numerous species, belonging to the same genera
or families, have really started into life at once, the fact would be fatal to the
theory of evolution through natural selection. For the development by this
means of a group of forms, all of which are descended from some one
progenitor, must have been an extremely slow process; and the progenitors
must have lived long before their modified descendants. But we continually
overrate the perfection of the geological record, and falsely infer, because
certain genera or families have not been found beneath a certain stage, that
they did not exist before that stage. In all cases positive palaeontological
evidence may be implicitly trusted; negative evidence is worthless, as ex-
perience has so often shown. We continually forget how large the world is,
compared with the area over which our geological formations have been
carefully examined; we forget that groups of species may elsewhere have
long existed, and have slowly multiplied, before they invaded the ancient
archipelagoes of Europe and the United States. We do not make due
allowance for the intervals of time which have elapsed between our consecu-
tive formations, longer perhaps in many cases than the time required for the
accumulation of each formation. These intervals will have given time for the
multiplication of species from some one parent-form: and in the succeeding
formation, such groups or species will appear as if suddenly created.

I may here recall a remark formerly made, namely, that it might require
a long succession of ages to adapt an organism to some new and peculiar line
of life, for instance, to fly through the air; and consequently that the transi-
tional forms would often long remain confined to some one region; but that,
when this adaptation had once been effected, and a few species had thus
acquired a great advantage over other organisms, a comparatively short time
would be necessary to produce many divergent forms, which would spread
rapidly and widely throughout the world. Professor Pictet, in his excellent
review of this work, in commenting on early transitional forms, and taking
birds as an illustration, cannot see how the successive modifications of the
anterior limbs of a supposed prototype could possibly have been of any
advantage. But look at the penguins of the Southern Ocean; have not these
birds their front limbs in this precise intermediate state of "neither true arms
nor true wings"? Yet these birds hold their place victoriously in the battle
for life ; for they exist in infinite numbers and of many kinds. I do not suppose
that we here see the real transitional grades through which the wings of birds
have passed; but what special difficulty is there in believing that it might
profit the modified descendants of the penguin, first to become enabled to


flap along the surface of the sea like the logger-headed duck, and ultimately
to rise from its surface and glide through the air?

I will now give a few examples to illustrate the foregoing remarks, and
to show how liable we are to error in supposing that whole groups of species
have suddenly been produced. Even in so short an interval as that between
the first and second editions of Pictet's great work on Palaeontology, pub-
lished in 1844-46 and in 1853-57, the conclusions on the first appearance
and disappearance of several groups of animals have been considerably
modified; and a third edition would require still further changes. I may
recall the well-known fact that in geological treatises, published not many
years ago, mammals were always spoken of as having abruptly come in at the
commencement of the tertiary series. And now one of the richest known
accumulations of fossil mammals belongs to the middle of the secondary
series; and true mammals have been discovered in the new red sandstone at
nearly the commencement of this great series. Guvier used to ursje that no
monkey occurred in any tertiary stratum; but now extinct species have been
discovered in India, South America, and ia Europe, as far back as the
miocene stage. Had it not been for the rare accident of the preservation
of footsteps in the new red sandstone of the United States, who would have
ventured to suppose that no less than at least thirty different bird-like ani-
mals, some of gigantic size, existed during that period? Not a fragment of
bone has been discovered in these beds. Not long ago, palaeontologists main-
tained that the whole class of birds came suddenly into existence during the
eocene period ; but now we know, on the authority of Prof essol* Owen, that a
bird certainly lived during the deposition of the upper green sand; and still
more recently, that strange bird, the Archeopteryx, with a long lizard-like
tail, bearing a pair of feathers on each joint, and with its wings furnished
with two free claws, has been discovered in the oolitic slates of Solenhofen.
Hardly any recent discovery shows more forcibly than this how little we as
yet know of the former inhabitants of the world.

I may give another instance, which, from having passed under my own
eyes, has much struck me. In a memoir on Fossil Sessile Girripedes, I stated
that, from the large number of existing and extinct tertiary species; from
the extraordinary abundance of the individuals of many species all over the
world, from the arctic regions to the equator, inhabiting various zones of
depths, from the upper tidal limits to fifty fathoms; from the perfect manner
in which specimens are preserved in the oldest tertiary beds; from the ease
with which even a fragment of a valve can be recognized ; from all these cir-
cumstances, I inferred that, had sessile cirripedes existed during the sec-
ondary periods, they would certainly have been preserved and discovered;
and as not one species had then been discovered in beds of this age, I con-
cluded that this great group had been suddenly developed at the commence-
ment of the tertiary series. This was a sore trouble to me, adding, as I then
thought, one more instance of the abrupt appearance of a great group of
species. But my work had hardly been published, when a skilful palaeon-
tologist, M. Bosquet, sent me a drawing of a perfect specimen of an unmis-


takable sessile cirripede, which he had himself extracted from the chalk of
Belgium. And, as if to make the case as striking as possible, this cirripede
was a Chthamalus, a very common, large, and ubiquitous genus, of which
not one species has as yet been found even in any tertiary stratum. Still more
recently, a Pyrgoma, a member of a distinct sub-family of sessile cirripedes,
has been discovered by Mr. Woodward in the upper chalk; so that we now
have abundant evidence of the existence of this group of animals during
the secondary period.

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, according to Agassiz, in the Chalk period. This group
includes the large majority of existing species. But certain Jurassic and
Triassic forms are now commonly admitted to be teleostean; and even some
palaeozoic forms have thus been classed by one high authority. If the
teleosteans had really appeared suddenly in the northern hemisphere at the
commencement of the chalk formation, the fact would have been highly
remarkable; but it would not have formed an insuperable difficulty, unless
it could likewise have been shown that at the same period the species were
suddenly and simultaneously developed in other quarters of the world. 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 fishes
might formerly have had a similarly confined range, and after having been
largely developed in some one sea, 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 considerations, 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 knowledge effected by the discoveries
of the last dozen years, it seems to me to be about as rash to dogmatize on the
succession of organic forms throughout the world, as it would be for a
naturalist to land for five minutes on a barren point in Australia, and then
to discuss the number and range of its productions.


There is another and allied difficulty, which is much more serious. I allude
to the manner in which species belonging to several of the main divisions of


the animal kingdom 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 are descended from a single progenitor, apply with equal
force to the earliest known species. For instance, it cannot be doubted that
all the Cambrian and Silurian trilobites are descended from some one
crustacean, which must have lived long before the Cambrian age, and which
probably differed greatly from any known animal. Some of the most ancient
animals, as the Nautilus, Lingula, etc., do not differ much from living
species ; and it cannot on our theory be supposed, that these old species were
the progenitors of all the species belonging to the same groups which have
subsequently appeared, for they are not in any degree intermediate in

Consequently, if the theory be true, it is indisputable that before the
lowest. Cambrian stratum was deposited long periods elapsed, as long as, or
probably far longer than, the whole interval from the Cambrian age to the
present day; and that during these vast periods the world swarmed with
living creatures. Here we encounter a formidable objection; for it seems
doubtful whether the earth, in a fit state for the habitation of living creatures,
has lasted long enough. Sir W. Thompson concludes that the consolidation
of the crust can hardly have occurred less than twenty or more than four
hundred million years ago, but probably not less than ninety-eight or more,
than two hundred million years. These very wide hmits show how doubtful
the data are; and other elements may have hereafter to be introduced inta
the problem. Mr. Croll estimates that about sixty million years have elapsed
since the Cambrian period, but this, judging from the small amount of
organic change since the commencement of the Glacial epoch, appears a
very short time for the many great mutations of life, which have certainly
occurred since the Cambrian formation; and the previous one hundred
and forty million years can hardly be considered as sufficient for the de-
velopment of the varied forms of life which already existed during the
Cambrian period. It is however probable, as Sir William Thompson insists,
that the world at a very early period was subjected to more rapid and vio-
lent changes in its physical conditions than those now occurring; and such
changes would have tended to induce changes at a corresponding rate in
the organisms which then existed.

To the question why we do not find rich fossiliferous deposits belonging
to these assumed earliest periods prior to the Cambrian system, I can give
no satisfactory answer. Several eminent geologists, with Sir R. Murchison
at their head, were until recently convinced that we beheld in the organic
remains of the lowest Silurian stratum the first dawn of Hfe. Other highly
competent judges, as Lyell and E. Forbes, have disputed this conclusion.
We should not forget that only a small portion of the world is known with
accuracy. Not very long ago M. Barrande added another and lower stage,
abounding with new and peculiar species, beneath the then known Silurian
system; and now, still lower down in the Lower Cambrian formation, Mr.
Hicks has found South Wales beds rich in trilobites, and containing various


mollusks and annelids. The presence of phosphatic nodules and bituminous
matter, even in some of the lowest azotic rocks, probably indicates life at
these periods; and the existence of the Eozoon in the Laurentian formation
of Canada is generally admitted. There are three great series of strata be-
neath the Silurian system in Canada, in the lowest of which the Eozoon is
found. Sir W. Logan states that their "united thickness may possibly far
surpass that of all the succeeding rocks, from the base of the palaeozoic series
to the present time. We are thus carried to a period so remote, that the ap-
pearance of the so-called primordial fauna (of Barrande) may by some be
considered as a comparatively modern event." The Eozoon belongs to the
most lowly organized of all classes of animals, but is highly organized for
its class; it existed in countless numbers, and, as Dr. Dawson has remarked,
certainly preyed on other minute organic beings, which must have lived in

Online LibraryCharles DarwinThe origin of species → online text (page 32 of 50)