Charles Darwin.

The origin of species online

. (page 33 of 50)
Online LibraryCharles DarwinThe origin of species → online text (page 33 of 50)
Font size
QR-code for this ebook

great numbers. Thus the words, which I wrote in 1859, about the existence
of living beings long before the Cambrian period, and which are almost the
same with those since used by Sir W. Logan, have proved true. Nevertheless,
the difficulty of assigning any good reason for the absence of vast piles of
strata rich in fossils beneath the Cambrian system is very great. It does not
seem probable that the most ancient beds have been quite worn away by
denudation, or that their fossils have been wholly obHterated by metamorphic
action, for if this had been the case we should have found only small rem-
nants of the formations next succeeding them in age, and these would always
have existed in a partially metamorphosed condition. But the descriptions
which we possess of the Silurian deposits over immense territories in Russia
and in North America, do not support the view that the older a forma-
tion is, the more invariably it has suffered extreme denudation and meta-

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 in-
habited 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 neighborhood of the now existing continents of Europe and North
America. This same view has since been maintained by Agassiz and others.
But we do not know what was the state of things in the intervals between
the several 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 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 hardly one truly oceanic island
(with the exception of New Zealand, if this can be called a truly oceanic
island) is as yet known to afford even a remnant of any palaeozoic or second-


ary formation. Hence, we may perhaps infer, that during the palaeozoic and I
secondary periods, neither continents nor continental islands existed where
our oceans now extend ; for had they existed, palaeozoic and secondary forma-
tions would in all probability have been accumulated from sediment derived
from their wear and tear; and these would have been at least partially up-
heaved by the oscillations of level, which 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 Cambrian period.
The colored map appended to my volume on Coral Reefs led me to con-
clude 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 we have no reason to assume that things have thus remained
from the beginning of the world. Our continents seem to have been formed
by a preponderance, during many oscillations of level, of the force of eleva-
tion. But may not the areas of preponderant movement have changed in
the lapse of ages? At a period long antecedent to the Cambrian epoch, con-
tinents 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 sedimentary
formations, in recognizable condition, older than the Cambrian strata, sup-
posing 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 naked 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 Cambrian
epoch in a completely metamorphosed and denuded condition.

The several difficulties here discussed, namely, that, though we find in
our geological formations many links between the species which now exist
and which formerly existed, we do not find infinitely numerous fine transi-
tional forms closely joining them all together, the sudden manner in which
several groups of species first appear in our European formations, the almost
entire absence, as at present known, of formations rich in fossils beneath
the Cambrian strata, are all undoubtedly of the most serious nature. We see
this in the fact that the most eminent palaeontologists, namely, Cuvier,
Agassiz, Barrande, Pictet, Falconer, E. Forbes, etc., and all our greatest
geologists, as Lyell, Murchison, Sedgwick, etc., have unanimously, often
vehemently, maintained the immutability of species. But Sir Charles Lyell
now gives the support of his high authority to the opposite side, and most


geologists and palaeontologists are much shaken in their former belief. Those
who believe that the geological record is in any degree perfect, will un-
doubtedly at once reject the theory. For my part, following out Lyell's meta-
phor, I look at the 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,
more or less different in the successive chapters, may represent the forms
of life, which are entombed in our consecutive formations, and which falsely
appear to have been abruptly introduced. On this view the difficulties above
discussed are greatly diminished or even disappear.


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 Chap-

Let us now see whether the several facts and laws relating to the geological
succession of organic beings accord best with the common view of the im-
mutability of species, or with that of their slow and gradual modification
through variation and natural selection.

New species have appeared very slowly, one after another, both on the
land and in the waters. Lyell has shown that it is hardly possible to resist
the evidence on this head in the case of the several tertiary stages; and every
year tends to fill up the blanks between the stages, and to make the propor-
tion between the lost and existing forms more gradual. In some of the most
recent beds, though undoubtedly of high antiquity if measured by years,
only one or two species are extinct, and only one or two are new, having
appeared there for the first time, either locally, or, as far as we know,' on
the face of the earth. The secondary formations are more broken; but, as
Bronn has remarked, neither the appearance nor disappearance of the many
species embedded in each formation has been simultaneous.

Species belonging to different genera and classes have not changed at the
same rate, or in the same degree. In the older tertiary beds a few living
shells may still be found in the midst of a multitude of extinct forms.
Falconer has given a striking instance of a similar fact, for an existing
crocodile is associated with many lost mammals and reptiles in the sub-
Himalayan deposits. The Silurian Lingula differs but little from the living
species of this genus; whereas most of the other Silurian Mollusks and all
the Crustaceans have changed greatly. The productions of the land seem
to have changed at a quicker rate than those of the sea, of which a striking
instance has been observed in Switzerland. There is some reason to believe
that organisms high in the scale, change more quickly than those that are
low : though there are exceptions to this rule. The amount of organic change,
as Pictet has remarked, is not the same in each successive so-called formation.
Yet if we compare any but the most closely related formations, all the species
will be found to have undergone some change. When a species has once dis-
appeared from the face of the earth, we have no reason to believe that the
same identical form ever reappears. The strongest apparent exception to
this latter rule is that of the so-called "colonies" of M. Barrande, which
intrude for a period in the midst of an older formation, and then allow the
pre-existing fauna to reappear; but Lyell's explanation, namely, that it is a



case of temporary migration from a distinct geographical province, seems

These several facts accord well with our theory, which includes no fixed
law of development, causing all the inhabitants of an area to change ab-
ruptly, or simultaneously, or to an equal degree. The process of modifica-
tion must be slow, and will generally affect only a few species at the same
time; for the variability of each species is independent of that of all others.
Whether such variations or individual differences as may arise will be accu-
mulated through natural selection in a greater or lesser degree, thus causing
a greater or lesser amount of permanent modification, will depend on many
complex contingencies — on the variations being of a beneficial nature, on
the freedom of intercrossing, on the slowly changing physical conditions of
the country, on the immigration of new colonists, and on the nature of
the other inhabitants with which the varying species come into competition.
Hence it is by no means surprising that one species should retain the same
identical form much longer than others; or, if changing, should change in
a less degree. We find similar relations between the existing inhabitants of
distinct countries; for instance, the land-shells and coleopterous insects of
Madeira have come to differ considerably from their nearest allies on the
continent of Europe, whereas the marine shells and birds have remained
unaltered. We can perhaps understand the apparently quicker rate of change
in terrestrial and in more highly organized productions compared with
marine and lower productions, by the more complex relations of the
higher beings to their organic and inorganic conditions of life, as explained
in a former chapter. When many of the inhabitants of any area have become
modified and improved, we can understand, on the principle of competition,
and from the all-important relations of organism to organism in the struggle
for life, that any form which did not become in some degree modified and
improved, would be liable to extermination. Hence, we see why all the species
in the same region do at last, if we look to long enough intervals of time^
become modified, for otherwise they would become extinct.

In members of the same class the average amount of change, during long
and equal periods of time, may, perhaps, be nearly the same; but as the ac-
cumulation of enduring formations, rich in fossils, depends on great masses
of sediment being deposited on subsiding areas, our formations have been
almost necessarily accumulated at wide and irregularly intermittent intervals
of time; consequently the amount of organic change exhibited by the fossils
embedded in consecutive formations is not equal. Each formation, on this
view, does not mark a new and complete act of creation, but only an occa-
sional scene, taken almost at hazard, in an ever slowly changing drama.

We can clearly understand why a species when once lost should never
reappear, even if the very same conditions of life, organic and inorganic,
should recur. For though the ofTspring of one species might be adapted (and
no doubt this has occurred in innumerable instances) to fill the place of
another species in the economy of nature, and thus supplant it; yet the two
forms — the old and the new — would not be identically the same; for both


would almost certainly inherit different characters from their distinct pro-
genitors; and organisms already differing would vary in a different manner.
For instance, it is possible, if all our fantail pigeons were destroyed, that
fanciers might make a new breed hardly distinguishable from the present
breed; but if the parent rock-pigeon were likewise destroyed, and under
nature we have every reason to believe that parent-forms are generally sup-
planted and exterminated by their improved offspring, it is incredible that t
a fantail, identical with the existing breed, could be raised from any other
species of pigeon, or even from any other well-established race of the domestic ^
pigeon, for the successive variations would almost certainly be in some de- I
gree different, and the newly-formed variety would probably inherit from |
its progenitor some characteristic differences.

Groups of species, that is, genera and families, follow the same general ,
rules in their appearance and disappearance as do single species, changing.;
more or less quickly, and in a greater or lesser degree. A group, when it has
once disappeared, never reappears; that is, its existence, as long as it lasts,
is continuous. I am aware that there are some apparent exceptions to this
rule, but the exceptions are surprisingly few, so few that E. Forbes, Pictet,
and Woodward (though all strongly opposed to such views as I maintain)
admit its truth; and the rule strictly accords with the theory. For all the spe-
cies of the same group, however long it may have lasted, are the modified de-
scendants one from the other, and all from a common progenitor. In the
genus Lingula, for instance, the species which have successively appeared
at all ages must have been connected by an unbroken series of generations,
from the lowest Silurian stratum to the present day.

We have seen in the last chapter that whole groups of species sometimes
falsely appear to have been abruptly developed; and I have attempted to
give an explanation of this fact, which if true would be fatal to my views.
But such cases are certainly exceptional; the general rule being a gradual
increase in number, until the group reaches its maximum, and then, sooner
or later, a gradual decrease. If the number of the species included within
a genus, or the number of the genera within a family, be represented by a
vertical line of varying thickness, ascending through the successive geological
formations in which the species are found, the line will sometimes falsely
appear to begin at its lower end, not in a sharp point, but abruptly; it then
gradually thickens upward, often keeping of equal thickness for a space,
and ultimately thins out in the upper beds, marking the decrease and final
extinction of the species. This graciual increase in number of the species of
a group is strictly tconformable with the theory, for the species of the same
genus, and the genera of the same family, can increase only slowly and
progressively; the process of modification and the production of a number
of allied forms necessarily being a slow and gradual process, one species
first giving rise to two or three varieties, these being slowly converted into
species, which in their turn produce by equally slow steps other varieties
and species, and so on, like the branching of a great tree from a single stem,
till the group becomes large.



We have as yet only spoken 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 hav-
ing been swept away by catastrophes at successive periods is very gener-
ally given up, even by those geologists, as Elie de Beaumont, Murchison,
Barrande, etc., whose general views would naturally lead them to this con-
clusion. On the contrary, we have every reason to believe, from the study
of the tertiary formations, that species and groups of species gradually dis-
appear, one after another, first from one spot, then from another, and finally
from the world. In some few cases, however, as by the breaking of an isthmus
and the consequent irruption of a multitude of new inhabitants into an ad-
joining sea, or by the final subsidence of an island, the process of extinction
may have been rapid. Both single species and whole groups of species last
for very unequal periods; some groups, as we have seen, have endured from
the earliest known dawn of life to the present day; some have 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 en-
dures. There is reason to believe that the extinction of a whole group of
species is generally a slower process than their production: if their ap-
pearance and disappearance 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 the early increase in number of the species.
In some cases, however, the extermination of whole groups, as of Ammonites,
toward the close of the secondary period, has been wonderfully sudden.

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 can have
marvelled more than I have done at the extinction of species. 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 of 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 ap-
parently so favorable. But my astonishment was groundless. Professor Owen
soon perceived that the tooth, though so like that of the existing horse, be-
longed to an extinct species. Had this horse been still living, but in some
degree rare, no naturalist would have felt the least surprised 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 unfavorable in its conditions of life; but what that some-
thing 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 naturalization of the domestic horse in South America, that under more
favorable conditions it would in a very few years have stocked the whole
continent. But we could not have told what the unfavorable conditions
were which checked its increase, whether some one or several contingencies,
and at ^vhat 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 favorable, 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 ahyays to remember that the increase of every creature-
is constantly being checked by unperceived hostile agencies; and that these-
same unperceived agencies are amply sufficient to cause rarity, and finally
extinction. So little is this subject understood, that I have heard surprise
repeatedly expressed at such great monsters as the Mastodon and the more
ancient Dinosaurians having become extinct; as if mere bodily strength,
gave \dctory in the battle of life. Mere size, on the contrary, w^ould in some
cases determine, as has been remarked by Owen, quicker extermination,
from the greater amount of requisite food. Before man inhabited India or
Africa, some cause must have checked the continued increase of the exist-
ing elephant. A highly capable judge. Dr. Falconer, believes that it is chiefly
insects, which, from incessantly harassing and weaking the elephant in India,
check its increase; and this was Bruce's conclusion with respect to the African
elephant in Abyssina. It is certain that insects and bloodsucking bats de-
termine the existence of the larger naturalized quadrupeds in several parts
of South America.

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
the species ceases to exist, is much the same as to admit that sickness in the
indi\ddual 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
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 the less-favored 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 neighborhood; 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 those naturally and those artificially produced, are bound
together. In flourishing groups, the number of new specific forms which
have been produced within a given time has at some periods probably been
greater than the number of the old specific forms which have been ex-
terminated; but we know that species have not gone on indefinitely increas-
ing, at least during the later geological epochs, 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 beheve, a number of new species descended from

Online LibraryCharles DarwinThe origin of species → online text (page 33 of 50)