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one species, that is, a new genus, comes to supplant an old genus, belong-
ing to the same family. But it must often have happened that a new species
belonging to some one group has seized on the place occupied by a species
belonging to a distinct group, and thus have caused its extermination. If
many allied forms be developed from the successful intruder, many will
have to yield their places; and it will generally be the allied forms, which
will suffer from some inherited inferiority in common. But whether it be
species belonging to the same or to a distinct class, which have yielded their
places to other modified and improved species, a few of the suff"erers may
often be preser\'ed for a long time, from being fitted to some peculiar line
of life, or from inhabiting some distant and isolated station, where they
will have escaped severe competition. For instance, some species of Trigonia,
a great genus of shells in the secondar)^ formations, survive in the Australian
seas; and a few members of the great and almost extinct group of Ganoid
fishes still inhabit our fresh waters. Therefore, the utter extinction of a group
is generally, as we have seen, a slower process than its production.

With respect to the apparently sudden extermination of whole famihes
or orders, as of Trilobites at the close of the palaeozoic period, and of Am-
monites at the close of the secondan- period, we must remember what has
been already said on the probable wide inter\'als of time between our con-
secutive formations; and in these intervals there may have been much slow
extermination. Moreover, when, by sudden immigration or by unusually
rapid development, many species of a new group have taken possession
of an area, many of the older species will have been exterminated in a
correspondingly rapid manner; and the forms which thus yield their places
will commonly be alHed, for they will partake of the same inferiority in

Thus, as it seems to me, the manner in which single species and whole
groups of species become extinct accords well with the theor)' of natural


selection. We need not marvel at extinction; if we must marvel, let it be a
our own presumption in imagining for a moment that we understand th(
many complex contingencies on which the existence of each species depends
If we forget for an instant that each species tends to increase inordinately
and that some check is always in action, yet seldom perceived by us, the
whole economy of nature will be utterly obscured. Whenever we can pre
cisely say why this species is more abundant in individuals than that; wh^
this species and not another can be naturalized in a given country; then
and not until then, we may justly feel surprise why we cannot account foi
the extinction of any particular species or group of species.


Scarcely any palseontological discovery is more striking than the fact that^
the forms of life change almost simultaneously throughout the world. Thus'
our European Chalk formation can be recognized in many distant regions,
under the most different climates, where not a fragment of the mineral
chalk itself can be found; namely in North America, in equatorial South
America, in Tierra del Fuego, at the Cape of Good Hope, and in the penin-
sula of India. For at these distant points, the organic remains in certain
beds present an unmistakable resemblance to those of the Chalk. It is not
that the same species are met with; for in some cases not one species is
identically the same; but they belong to the same families, genera, and sec-
tions of genera, and sometimes are similarly characterized in such trifling
points as mere superficial sculpture. Moreover, other forms, which are not
found in the Chalk of Europe, but which occur in the formations either
above or below, occur in the same order at these distant points of the world.
In the several successive palaeozoic formations of Russia, Western Europe,
and North America, a similar parallelism in the forms of life has been ob-
served by several authors; so it is, according to Lyell, with the European
and North American tertiary deposits. Even if the few fossil species which
are common to the Old and New Worlds were kept wholly out of view, the
general parallelism in the successive forms of life, in the palaeozoic and
tertiary stages, would still be manifest, and the several formations could j
be easily correlated.

These observations, however, relate to the marine inhabitants of the
world : we have not sufficient data to judge whether the productions of the
land and of fresh water at distant points change in the same parallel manner.
We may doubt whether they have thus changed: if the Megatherium,
Mylodon, Macrauchenia, and Toxodon had been brought to Europe from i
La Plata, without any information in regard to their geological position,
no one would have suspected that they had co-existed with sea-shells all
still living; but as these anomalous monsters co-existed with the Masto-
don and Horse, it might at least have been inferred that they had lived dur-
ing one of the later tertiary stages.


When the marine forms of life are spoken of as having changed simul-
taneously throughout the world, it must not be supposed that this expression
relates to the same year, or to the same century, or even that it has a very
strict geological sense; for if all the marine animals now living in Europe,
and all those that lived in Europe during the pleistocene period (a very re-
mote period as measured by years, including the whole glacial epoch) were
compared with those now existing in South America or in Australia, the
most skilful naturalist would hardly be able to say whether the present or
the pleistocene inhabitants of Europe resembled most closely those of the
southern hemisphere. So, again, several highly competent observers main-
tain that the existing productions of the United States are more closely
related to those which lived in Europe during certain late tertiary stages,
than to the present inhabitants of Europe; and if this be so, it is evident
that fossiliferous beds now deposited on the shores of North America would
hereafter be liable to be classed with somewhat older European beds. Never-
theless, looking to a remotely future epoch, there can be little doubt that all
the more modern marine formations, namely, the upper pliocene, the
pleistocene, and strictly modern beds of Europe, North and South America,
and Australia, from containing fossil remains in some degree allied, and
from not including those forms which are found only in the older underlying
deposits, would be correctly ranked as simultaneous in a geological sense.

The fact of the forms of life changing simultaneously in the above large
sense, at distant parts of the world, has greatly struck those admirable ob-
servers, MM. de Verneuil and d'Archiac. After referring to the parallelism
of the palaeozoic forms of life in various parts of Europe, they add: "If,
struck by this strange sequence, we turn our attention to North America,
and there discover a series of analogous phenomena, it will appear certain
that all these modifications of species, their extinction, and the introduc-
tion of new ones, cannot be owing to mere changes in marine currents or
other causes more or less local and temporary, but depend on general laws
which govern the whole animal kingdom." M. Barrande has made forcible
remarks to precisely the same effect. It is, indeed, quite futile to look to
changes of currents, climate, or other physical conditions, as the cause of
these great mutations in the forms of life throughout the world, under the
most different climates. We must, as Barrande has remarked, look to some
special law. We shall see this more clearly when we treat of the present
distribution of organic beings, and find how slight is the relation between the
physical conditions of various countries and the nature of their inhabitants.

This great fact of the parallel succession of the forms of life throughout
the world, is explicable on the theory of natural selection. New species are
formed by having some advantage over older forms; and the forms which
are already dominant, or have some advantage over the other forms in their
own country, give birth to the greatest number of new varieties or incipient
species. We have distinct evidence on this head, in the plants which are
dominant, that is, which are commonest and most widely diffused, produc-
ing the greatest number of new varieties. It is also natural that the dominant.


varying and far-reaching species, which have already invaded, to a certain
extent, the territories of other species, should be those which would have
the best chance of spreading still farther, and of giving rise in new countries
to other new varieties and species. The process of diffusion would often be
very slow, depending on climatal and geographical changes, on strange ac-
cidents, and on the gradual acclimatization of new species to the various
climates through which they might have to pass, but in the course of time
the dominant forms would generally succeed in spreading and would ulti-
mately prevail. The diffusion would, it is probable, be slower with the ter-
restrial inhabitants of distinct continents than with the marine inhabitants
of the continuous sea. We might therefore expect to find, as we do find, a less
strict degree of parallelism in the succession of the productions of the land
than with those of the sea.

Thus, as it seems to me, the parallel, and, taken in a large sense, simultane-
ous, succession of the same forms of life throughout the world, accords well
with the principle of new species having been formed by dominant species
spreading widely and varying; the new species thus produced being them-
selves dominant, owing to their having had some advantage over their
already dominant parents, as well as over other species, and again spread-
ing, varying, and producing new forms. The old forms which are beaten
and which yield their places to the new and victorious forms, will generally
be allied in groups, from inheriting some inferiority in common; and, there-
fore, as new and improved groups spread throughout the world, old groups
disappear from the world; and the succession of forms everywhere tends to
correspond both in their first appearance and final disappearance.

There is one other remark connected with this subject worth making. I
have given my reasons for believing that most of our great formations, rich
in fossils, were deposited during periods of subsidence ; and that blank inter-
vals of vast duration, as far as fossils are concerned, occurred during the
periods when the bed of the sea was either stationary or rising, and likewise
when sediment was not thrown down quickly enough to embed and preserve
organic remains. During these long and blank intervals I suppose that the
inhabitants of each region underwent a considerable amount of modification
and extinction, and that there was much migration from other parts of the
world. As we have reason to believe that large areas are affected by the same
movement, it is probable that strictly contemporaneous formations have
often been accumulated over very wide spaces in the same quarter of the
world; but we are very far from having any right to conclude that this has
invariably been the case, and that large areas have invariably been affected
by the same movements. When two formations have been deposited in two
regions during nearly, but not exactly, the same period, we should find in
both, from the causes explained in the foregoing paragraphs, the same gen-
eral succession in the forms of life; but the species would not exactly cor-
respond; for there will have been little more time in the one region than in
the other for modification, extinction, and immigration.

I suspect that cases of this nature occur in Europe. Mr. Prestwich, in his


admirable Memoirs on the eocene deposits of England and France, is able
to draw a close general parallelism between the successive stages in the two
countries; but when he compares certain stages in England with those in
France, although he finds in both a curious accordance in the numbers of
the species belonging to the same genera, yet the species themselves differ in
a manner very difficult to account for considering the proximity of the two
areas, unless, indeed, it be assumed that an isthmus separated two seas in-
habited by distinct but contemporaneous faunas. Lyell has made similar
observations on some of the later tertiary formations. Barrande, also, shows
that there is a striking general parallelism in the successive Silurian deposits
of Bohemia and Scandinavia; nevertheless he finds a surprising amount of
difference in the species. If the several formations in these regions have not
been deposited during the same exact periods — a formation in one region
often corresponding with a blank interval in the other — and if in both
regions the species have gone on slowly changing during the accumulation
of the several formations and during the long intervals of time between
them; in this case the several formations in the two regions could be ar-
ranged in the same order, in accordance with the general succession of the
forms of life, and the order would falsely appear to be strictly parallel; nev-
ertheless the species would not be all the same in the apparently correspond-
ing stages in the two regions.



Let US now look to the mutual affinities of extinct and living species. All
fall into a few grand classes; and this fact is at once explained on the prin-
ciple of descent. The more ancient any form is, the more, as a general rule, it
differs from living forms. But, as Buckland long ago remarked, extinct
species can all be classed either in still existing groups, or between them.
That the extinct forms of life help to fill up the intervals between existing
genera, families, and orders, is certainly true ; but as this statement has often
been ignored or even denied, it may be well to make some remarks on this
subject, and to give some instances. If we confine our attention either to the
living or to the extinct species of the same class, the series is far less perfect
than if we combine both into one general system. In the writings of Professor
Owen we continually meet with the expression of generalized forms, as ap-
plied to extinct animals; and in the writings of Agassiz, of prophetic or syn-
thetic types ; and these terms imply that such forms are, in fact, intermediate
or connecting links. Another distinguished palaeontologist, M. Gaudry, has
shown in the most striking manner that many of the fossil mammals discov-
ered by him in Attica serve to break down the intervals between existing
genera. Cuvier ranked the Ruminants and Pachyderms as two of the most
distinct orders of mammals; but so many fossil links have been disentombed
that Owen has had to alter the whole classification, and has placed certain
Pachyderms in the same sub-order with Ruminants ; for example, he dissolves


by gradations the apparently wide interval between the pig and the camel.
The Ungulata or hoofed quadrupeds are now divided into the even-toed or
odd-toed divisions; but the Macrauchenia of South America connects to a
certain extent these two grand divisions. No one will deny that the Hipparion
is intermediate between the existing horse and certain other ungulate forms.
What a wonderful connecting link in the chain of mammals is the Typo-
therium from South America, as the name given to it by Professor Gervais
expresses, and which cannot be placed in any existing order. The Sirenia
form a very distinct group of the mammals, and one of the most remarkable
peculiarities in existing dugong and lamentin is the entire absence of hind
limbs, without even a rudiment being left; but the extinct Halitherium had,
according to Professor Flower, an ossified thigh-bone "articulated to a well-
defined acetabulum in the pelvis," and it thus makes some approach to
ordinary hoofed quadrupeds, to which the Sirenia are in other respects
allied. The cetaceans or whales are widely different from all other mammals,
but the tertiary Zeuglodon and Squalodon, which have been placed by some
naturalists in an order by themselves, are considered by Professor Huxley to
be undoubtedly cetaceans, "and to constitute connecting links with the
aquatic carnivora."

Even the wide interval between birds and reptiles has been shown by the
naturalists just quoted to be partially bridged over in the most unexpected
manner, on the one hand, by the ostrich and extinct Archeopteryx, and on
the other hand by the Compsognathus, one of the Dinosaurians — that group
which includes the most gigantic of all terrestrial reptiles. Turning to the
Invertebrata, Barrande asserts (a higher authority could not be named) that
he is every day taught that, although palaeozoic animals can certainly be
classed under existing groups, yet that at this ancient period the groups were
not so distinctly separated from each other as they now are.

Some writers have objected to any extinct species, or group of species,
being considered as intermediate between any two living species, or groups of
species. If by this term it is meant that an extinct form is directly inter-
mediate in all its characters between two living forms or groups, the objec-
tion is probably valid. But in a natural classification many fossil species cer-
tainly stand between living species, and some extinct genera between living
genera, even between genera belonging to distinct families. The most com-
mon case, especially with respect to very distinct groups, such as fish and
reptiles, seems to be that, supposing them to be distinguished at the present
day by a score of characters, the ancient members are separated by a some-
what lesser number of characters, so that the two groups formerly made a
somewhat nearer approach to each other than they now do.

It is a common belief, that the more ancient a form is, by so much the
more it tends to connect by some of its characters groups now widely sepa-
rated from each other. This remark no doubt must be restricted to those
groups which have undergone much change in the course of geological ages;
and it would be difficult to prove the truth of the proposition, for every now
and then even a living animal, as the Lepidosiren, is discovered having affini-


ties directed toward very distinct groups. Yet if we compare the older reptiles
and Batrachians, the older fish, the older cephalopods, and the eocene mam-
mals, with the recent members of the same classes, we must admit that there
is truth in the remark.

Let us see how far these several facts and inferences accord with the
theory of descent with modification. As the subject is somewhat complex, I
must request the reader to turn to the diagram in the fourth chapter. We
may suppose that the numbered letters in Italics represent genera, and the
dotted lines diverging from them the species in each genus. The diagram is
much too simple, too few genera and too few species being given, but this is
unimportant for us. The horizontal lines may represent successive geological
formations, and all the forms beneath the uppermost line may be considered
as extinct. The three existing genera a^*, q^^, p^^, will form a small family;
6^*, and /^*, a closely allied family or sub-family; and o^*, e^^, w}^, a third
family. These three families, together with the many extinct genera on the
several lines of descent diverging from the parent form (A), will form an
order, for all will have inherited something in common from their ancient
progenitor. On the principle of the continued tendency to divergence of char-
acter, which was formerly illustrated by this diagram, the more recent any
form is, the more it will generally differ from its ancient progenitor. Hence,
we can understand the rule that the most ancient fossils differ most from
existing forms. We must not, however, assume that divergence of character
is a necessary contingency; it depends solely on the descendants from a species
being thus enabled to seize on many and different places in the economy of
nature. Therefore it is quite possible, as we have seen in the case of some
Silurian forms, that a species might go on being slightly modified in relation
to its slightly altered conditions of life, and yet retain throughout a vast
period the same general characteristics. This is represented in the diagram
by the letter f *.

All the many forms, extinct and recent, descended from (A), make, as
before remarked, one order; and this order, from the continued effects of ex-
tinction and divergence of character, has become divided into several sub-
families and families, some of which are supposed to have perished at differ-
ent periods, and some to have endured to the present day.

By looking at the diagram we can see that if many of the extinct forms
supposed to be embedded in the successive formations, were discovered at
several points low down in the series, the three existing families on the upper-
most fine would be rendered less distinct from each other. If, for instance,
the genera a}-, a°, a^^, f, m^, m^, m^, were disinterred, these three families
would be so closely linked together that they probably would have to be
united into one great family, in nearly the same manner as has occurred with
Ruminants and certain Pachyderms. Yet he who objected to consider as inter-
mediate the extinct genera, which thus link together the living genera of
three families, would be partly justified, for they are intermediate, not
directly, but only by a long and circuitous course through many widely dif-
ferent forms. If many extinct forms were to be discovered above one of the


middle horizontal lines or geological formations — for instance, above No.
VI. — ^but none from beneath this line, then only two of the families (those
on the left hand, «^*, etc., and b^^, etc.) would have to be united into one;
and there would remain two families, which would be less distinct from each
other than they were before the discovery of the fossils. So, again, if the three
families formed of eight genera {a^^ to m^*), on the uppermost line, be sup-
posed to differ from each other by half-a-dozen important characters, then
the families which existed at a period marked VI. would certainly have dif-
fered from each other by a less number of characters ; for they would at this
early stage of descent have diverged in a less degree from their common
progenitor. Thus it comes that ancient and extinct genera are often in a
greater or lesser degree intermediate in character between their modified
descendants, or between their collateral relations.

Under nature the process will be far more complicated than is represented
in the diagram ; for the groups will have been more numerous ; they will have
endured for extremely unequal lengths of time, and will have been modified
in various degrees. As we possess only the last volume of the geological record,'
and that in a very broken condition, we have no right to expect, except in
rare cases, to fill up the wide intervals in the natural system, and thus to unite
distinct families or orders. All that we have a right to expect is, that those
groups which have, within known geological periods, undergone much modi-
fication, should in the older formations make some slight approach to each
other; so that the older members should differ less from each other in some
of their characters than do the existing members of the same group ; and this
by the concurrent evidence of our best palaeontologists is frequently the case.

Thus, on the theory of descent with modification, the main facts with re-
spect to the mutual aflfinities of the extinct forms of life to each other and to
living forms, are explained in a satisfactory manner. And they are wholly
inexplicable on any other view.

On this same theory, it is evident that the fauna during any one great
period in the earth's history will be intermediate in general character between
that which preceded and that which succeeded it. Thus the species which
lived at the sixth great stage of descent in the diagram are the modified off-
spring of those which lived at the fifth stage, and are the parents of those

Online LibraryCharles DarwinThe origin of species → online text (page 34 of 50)