Charles Darwin.

The origin of species online

. (page 31 of 50)
Online LibraryCharles DarwinThe origin of species → online text (page 31 of 50)
Font size
QR-code for this ebook

hall of the above dimensions. Several eminent breeders, during a single life-
time, have so largely modified some of the higher animals, which propagate
their kind much more slowly than most of the lower animals, that they have
formed what well deserves to be called a new sub-breed. Few men have
attended with due care to any one strain for more than half a century, so
that a hundred years represents the work of two breeders in succession. It is
not to be supposed that species in a state of nature ever change so quickly
as domestic animals under the guidance of methodical selection. The com-
parison would be in every way fairer with the effects which follow from
unconscious selection, that is, the preservation of the most useful or beautiful
animals, with no intention of modifying the breed; but by this process of
unconscious selection, various breeds have been sensibly changed in the
course of two or three centuries.

Species, however, probably change much more slowly, and within the
same country only a few change at the same time. This slowness follows from

J all the inhabitants of the same country being already so well adapted to each
other, that new places in the polity of nature do not occur until after long
intervals, due to the occurrence of physical changes of some kind, or through


the immigration of new forms. Moreover, variations or individual differences
of the right nature, by which some of the inhabitants might be better fitted to
their new places under the altered circumstances, would not always occur at
once. Unfortunately we have no means of determining, according to the
standard of years, how long a period it takes to modify a species; but to the
subject of time we must return.


Now let us turn to our richest geological museums, and what a paltry
display we behold! That our collections are imperfect, is admitted by every
one. The remark of that admirable palaeontologist, Edward Forbes, should
never be forgotten, namely, that very many fossil species are known and
named from single and often broken specimens, or from a few specimens
collected on some one spot. Only a small portion of the surface of the earth
has been geologically explored, and no part with sufficient care, as the im-
portant discoveries made every year in Europe prove. No organism wholly
soft can be preserved. Shells and bones decay and disappear when left on the
bottom of the sea, where sediment is not accumulating. We probably take a
quite erroneous view, when we assume that sediment is being deposited over
nearly the whole bed of the sea, at a rate sufficiently quick to embed and
preserve fossil remains. Throughout an enormously large proportion of the
ocean, the bright blue tint of the water bespeaks its purity. The many cases
on record of a formation conformably covered, after an immense interval
of time, by another and later formation, without the underlying bed having
suffered in the interval any wear and tear, seem explicable only on the view
of the bottom of the sea not rarely lying for ages in an unaltered condition.
The remains which do become embedded, if in sand or gravel, will, when
the beds are upraised, generally be dissolved by the percolation of rain water
charged with carbolic acid. Some of the many kinds of animals which live on
the beach between high and low water mark seem to be rarely preserved.
For instance, the several species of the Chthamalinse (a sub-family of sessile
cirripedes) coat the rocks all over the world in infinite numbers: they are
all strictly littoral, with the exception of a single Mediterranean species,
which inhabits deep water, and this has been found fossil in Sicily, whereas
not one other species has hitherto been found in any tertiary formation : yet
it is known that the genus Chthamalus existed during the Chalk period.
Lastly, many great deposits, requiring a vast length of time for their ac-
cumulation, are entirely destitute of organic remains, without our being able
to assign any reason : one of the most striking instances is that of the Flysch
formation, which consists of shale and sandstone, several thousand, oc-
casionally even six thousand, feet in thickness, and extending for at least 300
miles from Vienna to Switzerland; and although this great mass has been
most carefully searched, no fossils, except a few vegetable remains, have
been found.

With respect to the terrestrial productions which lived during the Sec-


ondary and Palaeozoic periods, it is superfluous to state that our evidence
is fragmentary in an extreme degree. For instance, until recently not a
land-shell was known belonging to either of these vast periods, with the
exception of one species discovered by Sir C. Lyell and Dr. Dawson in
the carboniferous strata of North America; but now land-shells have been
found in the lias. In regard to mammiferous remains, a glance at the
historical table published in Lyell's Manual will bring home the truth, how
accidental and rare is their preservation, far better than pages of detail. Nor
is their rarity surprising, when we remember how large a proportion of the
bones of tertiary mammals have been discovered either in caves or in
lacustrine deposits; and that not a cave or true lacustrine bed is known be-
longing to the age of our secondary or palaeozoic formations.

But the imperfection in the geological record largely results from another
and more important cause than any of the foregoing; namely, from the
several formations being separated from each other by wide intervals of time.
This doctrine has been emphatically admitted by many geologists and
palaeontologists, who, like E. Forbes, entirely disbelieve in the change of
species. When we see the formations tabulated in written works, or when
we follow them in nature, it is difficult to avoid believing that they are closely
consecutive. But we know, for instance, from Sir R. Murchison's great
work on Russia, what wide gaps there are in that country between the
superimposed formations; so it is in North America, and in many other
parts of the world. The most skilful geologist, if his attention had been
confined exclusively to these large territories, would never have suspected
that during the periods which were blank and barren in his own country,
great piles of sediment, charged with new and peculiar forms of life, had
elsewhere been accumulated. And if, in every separate territory, hardly any
idea can be formed of the length of time which has elapsed between the
consecutive formations, we may infer that this could nowhere be ascertained.
The frequent and great changes in the mineralogical composition of consecu-
tive formations, generally implying great changes in the geography of the sur-
rounding lands, whence the sediment was derived, accord with the belief
of vast intervals of time having elapsed between each formation.

We can, I think, see why the geological formations of each region are
almost invariably intermittent; that is, have not followed each other in close
sequence. Scarcely any fact struck me more when examining many hundred
miles of the South American coasts, which have been upraised several
hundred feet within the recent period, than the absence of any recent deposits
sufficiently extensive to last for even a short geological period. Along the
whole west coast, which is inhabited by a peculiar marine fauna, tertiary
beds are so poorly developed that no record of several successive and peculiar
marine faunas will probably be preserved to a distant age. A little reflection
will explain why, along the rising coast of the western side of South America,
no extensive formations with recent or tertiary remains can anywhere be
found, though the supply of sediment must for ages have been great, from
the enormous degradation of the coast rocks and from the muddy streams


entering the sea. The explanation, no doubt, is that the littoral and sub-
littoral deposits are continually worn away, as soon as they are brought up
by the slow and gradual rising of the land within the grinding action of the
coast waves.

We may, I think, conclude that sediment must be accumulated in ex-
tremely thick, solid, or extensive masses, in order to withstand the incessant
action of the waves, when first upraised and during successive oscillations of
level, as well as the subsequent subaerial degradation. Such thick and ex-
tensive accumulations of sediment may be formed in two ways; either in
profound depths of the sea, in which case the bottom will not be inhabited
by so many and such varied forms of life as the more shallow seas; and the
mass when upraised will give an imperfect record of the organisms which
existed in the neighborhood during the period of its accumulation. Or
sediment may be deposited to any thickness and extent over a shallow bottom,
if it continue slowly to subside. In this latter case, as long as the rate of sub-
sidence and the supply of sediment nearly balance each other, the sea will
remain shallow and favorable for many and varied forms, and thus a rich
fossiliferous formation, thick enough, when upraised, to resist a large amount
of denudation, may be formed.

I am convinced that nearly all our ancient formations, which are through-
out the greater part of their thickness rich in fossils, have thus been formed
during subsidence. Since publishing my views on this subject in 1845, I have
watched the progress of geology, and have been surprised to note how author
after author, in treating of this or that great formation, has come to the
conclusion that it was accumulated during subsidence. I may add, that the
only ancient tertiary formation on the west coast of South America, which
has been bulky enough to resist such degradation as it has as yet suffered,
but which will hardly last to a distant geological age, was deposited during
a downward oscillation of level, and thus gained considerable thickness.

All geological facts tell us plainly that each area has undergone numerous
slow oscillations of level, and apparently these oscillations have affected wide
spaces. Consequently formations rich in fossils and sufficiently thick and
extensive to resist subsequent degradation will have been formed over wide
spaces during periods of subsidence, but only where the supply of sediment
was sufficient to keep the sea shallow and to embed and preserve the remains
before they had time to decay. On the other hand, as long as the bed of the
sea remains stationary, thick deposits cannot have been accumulated in the
shallow parts, which are the most favorable to life. Still less can this have
happened during the alternate periods of elevation; or, to speak more accu-
rately, the beds which were then accumulated will generally have been
destroyed by being upraised and brought within the limits of the coast-action.

These remarks apply chiefly to littoral and sub-littoral deposits. In the
case of an extensive and shallow sea, such as that within a large part of the
Malay Archipelago, where the depth varies from thirty or forty to sixty
fathoms, a widely extended formation might be formed during a period of
elevation, and yet not suffer excessively from denudation during its slow


upheaval; but the thickness of the formation could not be great, for owing
to the elevatory movement it would be less than the depth in which it was
formed; nor would the deposit be much consolidated, nor be capped by
overlying formations, so that it would run a good chance of being worn away
by atmospheric degradation and by the action of the sea during subsequent
oscillations of level. It has, however, been suggested by Mr. Hopkins, that if
one part of the area, after rising and before being denuded, subsided, the
deposit formed during the rising movement, though not thick, might after-
ward become protected by fresh accumulations, and thus be preserved for a
long period.

Mr. Hopkins also expresses his belief that sedimentary beds of considerable
horizontal extent have rarely been completely destroyed. But all geologists,
excepting the few who believe that our present metamorphic schists and
plutonic rocks once formed the primordial nucleus of the globe, will admit
that these latter rocks have been stripped of their covering to an enormous
extent. For it is scarcely possible that such rocks could have been solidified
and crystallized while uncovered ; but if the metamorphic action occurred at
profound depths of the ocean, the former protecting mantle of rock may not
have been very thick. Admitting then that gneiss, mica-schist, granite, diorite,
etc., were once necessarily covered up, how can we account for the naked and
extensive areas of such rocks in any parts of the world, except on the belief
that they have subsequently been completely denuded of all overlying strata?
That such extensive areas do exist cannot be doubted: the granitic region
of Parime is described by Humboldt as being at least nineteen times as large
as Switzerland. South of the Amazon, Boue colors an area composed of rocks
of this nature as equal to that of Spain, France, Italy, part of Germany, and
the British Islands, all conjoined. This region has not been carefully explored,
but from the concurrent testimony of travellers, the granitic area is very
large : thus Von Eschwege gives a detailed section of these rocks, stretching
from Rio de Janeiro for 260 geographical miles inland in a straight line;
and I travelled for 150 miles in another direction, and saw nothing but
granitic rocks. Numerous specimens, collected along the whole coast, from
near Rio de Janeiro to the mouth of the Plata, a distance of 1,100 geographi-
cal miles, were examined by me, and they all belonged to this class. Inland,
along the whole northern bank of the Plata, I saw, besides modern tertiary
beds, only one small patch of slightly metamorphosed rock, which alone
could have formed a part of the original capping of the granitic series. Turn-
ing to a well-known region, namely, to the United States and Canada, as
shown in Professor H. D. Rogers's beautiful map, I have estimated the areas
by cutting out and weighing the paper, and I find that the metamorphic
(excluding the "semi-metamorphic" ) and granite rocks exceed, in the pro-
portion of 19 to 12.5, the whole of the newer Palaeozoic formations. In many
regions the metamorphic and granite rocks would be found much more
widely extended than they appear to be, if all the sedimentary beds were
removed which rest unconformably on them, and which could not have
formed part of the original mantle under which they were crystallized.


Hence, it is probable that in some parts of the world whole formations have
been completely denuded, with not a wreck left behind.

One remark is here worth a passing notice. During periods of elevation,
the area of the land and of the adjoining shoal parts of the sea will be in-
creased, and new stations will often be formed — all circumstances favorable,
as previously explained, for the formation of new varieties and species; but
during such periods there will generally be a blank in the geological record.
On the other hand, during subsidence, the inhabited area and number of
inhabitants will decrease (excepting on the shores of a continent when first
broken up into an archipelago) , and consequently, during subsidence, though
there will be much extinction, few new varieties or species will be formed;
and it is during these very periods of subsidence that the deposits which are
richest in fossils have been accumulated.



From these several considerations it cannot be doubted that the geological
record, viewed as a whole, is extremely imperfect; but if we confine our
attention to any one formation, it becomes much more difficult to understand
why we do not therein find closely graduated varieties between the allied
species which lived at its commencement and at its close. Several cases are
on record of the same species presenting varieties in the upper and lower
parts of the same formation. Thus Trautschold gives a number of instances
with Ammonites, and Hilgendorf has described a most curious case of ten
graduated forms of Planorbis multiformis in the successive beds of a fresh-
water formation in Switzerland. Although each formation has indisputably
required a vast number of years for its deposition, several reasons can be
given why each should not commonly include a graduated series of links
between the species which lived at its commencement and close, but I can-
not assign due proportional weight to the following considerations.

Although each formation may mark a very long lapse of years, each
probably is short compared with the period requisite to change one species
into another. I am aware that two palaeontologists, whose opinions are worthy
of much deference, namely Bronn and Woodward, have concluded that tlic
average duration of each formation is twice or thrice as long as the average
duration of specific forms. But insuperable difficulties, as it seems to me,
prevent us from coming to any just conclusion on this head. When we see
a species first appearing in the middle of any formation, it would be rash
in the extreme to infer that it had not elsewhere previously existed. So again,
when we find a species disappearing before the last layers have been de-
posited, it would be equally rash to suppose that it then became extinct. We
forget how small the area of Europe is, compared with the rest of the world ;
nor have the several stages of the same formation throughout Europe been
correlated with perfect accuracy.

We may safely infer that with marine animals of all kinds there has been


a large amount of migration due to climatal and other changes; and when
we see a species first appearing in any formation, the probability is that it only
then first immigrated into that area. It is well known, for instance, that
several species appear somewhat earlier in the palaeozoic beds of North
America than in those of Europe; time having apparently been required
for their migration from the American to the European seas. In examining
the latest deposits, in various quarters of the world, it has everywhere been
noted, that some few still existing species are common in the deposit, but
have become extinct in the immediately surrounding sea; or, conversely, that
gome are now abundant in the neighboring sea, but are rare or absent in this
particular deposit. It is an excellent lesson to reflect on the ascertained
amount of migration of the inhabitants of Europe during the glacial epoch,
which forms only a part of one whole geological period; and likewise to
reflect on the changes of level, on the extreme change of climate, and on
the great lapse of time, all included within this same glacial period. Yet it
may be doubted whether, in any quarter of the world, sedimentary deposits,
including fossil remains, have gone on accumulating within the same area
during the whole of this period. It is not, for instance, probable that sediment
was deposited during the whole of the glacial period near the mouth of the
Mississippi, within that limit of depth at which marine animals can best
flourish: for we know that great geographical changes occurred in other
parts of America during this space of time. When such beds as were de-
posited in shallow water near the mouth of the Mississippi during some part
of the glacial period shall have been upraised, organic remains will probably
first appear and disappear at different levels, owing to the migrations of
species and to geographical changes. And in the distant future, a geologist,
examining these beds, would be tempted to conclude that the average dura-
tion of life of the embedded fossils had been less than that of the glacial
period, instead of having been really far greater, that is, extending from
before the glacial epoch to the present day.

In order to get a perfect gradation between two forms in the upper anc
lower parts of the same formation, the deposit must have gone on con-'
tinuously accumulating during a long period, sufficient for the slow process
of modification ; hence, the deposit must be a very thick one ; and the species
undergoing change must have lived in the same district throughout the whole
time. But we have seen that a thick formation, fossiliferous throughout its
entire thickness, can accumulate only during a period of subsidence; and to
keep the depth approximately the same, which is necessary that the same
marine species may live on the same space, the supply of sediment must
nearly counterbalance the amount of subsidence. But this same movement
of subsidence will tend to submerge the area whence the sediment is derived,
and thus diminish the supply, while the downward movement continues. In
fact, this nearly exact balancing between the supply of sediment and the
amount of subsidence is probably a rare contingency; for it has been observed
by more than o^e palaeontologist that very thick deposits are usually barren
of organic remains, except near their upper or lower limits.


It would seem that each separate formation, Hke the whole pile of forma-
tion in any country, has generally been intermittent in its accumulation.
When we see, as is so often the case, a formation composed of beds of widely
different mineralogical composition, we may reasonably suspect that the
process of deposition has been more or less interrupted. Nor will the closest
inspection of a formation give us any idea of the length of time which its
deposition may have consumed. Many instances could be given of beds, only
a few feet in thickness, representing formations which are elsewhere thou-
sands of feet in thickness, and which must have required an enormous period
for their accumulation; yet no one ignorant of this fact would have even
suspected the vast lapse of time represented by the thinner formation. Many
cases could be given of the lower beds of a formation having been upraised,
denuded, submerged, and then re-covered by the upper beds of the same
formation — facts, showing what wide, yet easily overlooked, intervals have
occurred in its accumulation. In other cases we have the plainest evidence
in great fossilized trees, still standing upright as they grew, of many long in-
tervals of time and changes of level during the process of deposition, which
would not have been suspected, had not the trees been preserved: thus Sir
C. Lyell and Dr. Dawson found carboniferous beds 1,400 feet thick in Nova
Scotia, with ancient root-bearing strata, one above the other, at no less than
eighty-six different levels. Hence, when the same species occurs at the bottom,
middle, and top of a formation, the probability is that it has not lived on the
same spot during the whole period of deposition, but has disappeared and
reappeared, perhaps many times, during the same geological period. Con-
sequently if it were to undergo a considerable amount of modification during
the .deposition of any one geological formation, a section would not include
all the fine intermediate gradations which must, on our theory, have existed,
but abrupt, though perhaps slight, changes of form.

It is all-important to remember that naturalists have no golden rule by
which to distinguish species and varieties; they grant some little variability
to each species, but when they meet with a somewhat greater amount of
difference between any two forms, they rank both as species, unless they are
enabled to connect them together by the closest intermediate gradations;
and this, from the reasons just assigned, we can seldom hope to effect in any
one geological section. Supposing B and C to be two species, and a third. A,
to be found in an older and underlying bed; even if A were strictly inter-
mediate between B and C, it would simply be ranked as a third and distinct
species, unless at the same time it could be closely connected by intermediate
varieties with either one or both forms. Nor should it be forgotten, as before
explained, that A might be the actual progenitor of B and C, and yet would
not necessarily be strictly intermediate between them in all respects. So that
we might obtain the parent-species and its several modified descendants
from the lower and upper beds of the same formation, and unless we ob-

Online LibraryCharles DarwinThe origin of species → online text (page 31 of 50)