ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS.
On the slow and successive appearance of new species. On their
different rates of change. Species once lost do not reappear. Groups of
species follow the same general rules in their appearance and
disappearance as do single species. On Extinction. On simultaneous
changes in the forms of life throughout the world. On the affinities of
extinct species to each other and to living species. On the state of
development of ancient forms. On the succession of the same types
within the same areas. Summary of preceding and present chapters.
Let us now see whether the several facts and rules relating to the
geological succession of organic beings, better accord with the common
view of the immutability of species, or with that of their slow and
gradual modification, through descent and natural selection.
New species have appeared very slowly, one after another, both on the
land and in the waters. Lyell has shown that it is hardly possible to
resist the evidence on this head in the case of the several tertiary
stages; and every year tends to fill up the blanks between them, and to
make the percentage system of lost and new forms more gradual. In some
of the most recent beds, though undoubtedly of high antiquity if
measured by years, only one or two species are lost forms, and only one
or two are new forms, having here appeared for the first time, either
locally, or, as far as we know, on the face of the earth. If we may
trust the observations of Philippi in Sicily, the successive changes in
the marine inhabitants of that island have been many and most gradual.
The secondary formations are more broken; but, as Bronn has remarked,
neither the appearance
nor disappearance of their many now extinct species has been
simultaneous in each separate formation.
Species of different genera and classes have not changed at the same
rate, or in the same degree. In the oldest tertiary beds a few living
shells may still be found in the midst of a multitude of extinct forms.
Falconer has given a striking instance of a similar fact, in an
existing crocodile associated with many strange and lost mammals and
reptiles in the sub-Himalayan deposits. The Silurian Lingula differs
but little from the living species of this genus; whereas most of the
other Silurian Molluscs and all the Crustaceans have changed greatly.
The productions of the land seem to change at a quicker rate than those
of the sea, of which a striking instance has lately been observed in
Switzerland. There is some reason to believe that organisms, considered
high in the scale of nature, change more quickly than those that are
low: though there are exceptions to this rule. The amount of organic
change, as Pictet has remarked, does not strictly correspond with the
succession of our geological formations; so that between each two
consecutive formations, the forms of life have seldom changed in
exactly the same degree. Yet if we compare any but the most closely
related formations, all the species will be found to have undergone
some change. When a species has once disappeared from the face of the
earth, we have reason to believe that the same identical form never
reappears. The strongest apparent exception to this latter rule, is
that of the so-called “colonies” of M. Barrande, which intrude for a
period in the midst of an older formation, and then allow the
pre-existing fauna to reappear; but Lyell’s explanation, namely, that
it is a case of temporary migration from a distinct geographical
province, seems to me satisfactory.
These several facts accord well with my theory. I believe in no fixed
law of development, causing all the inhabitants of a country to change
abruptly, or simultaneously, or to an equal degree. The process of
modification must be extremely slow. The variability of each species is
quite independent of that of all others. Whether such variability be
taken advantage of by natural selection, and whether the variations be
accumulated to a greater or lesser amount, thus causing a greater or
lesser amount of modification in the varying species, depends on many
complex contingencies,—on the variability being of a beneficial nature,
on the power of intercrossing, on the rate of breeding, on the slowly
changing physical conditions of the country, and more especially on the
nature of the other inhabitants with which the varying species comes
into competition. Hence it is by no means surprising that one species
should retain the same identical form much longer than others; or, if
changing, that it should change less. We see the same fact in
geographical distribution; for instance, in the land-shells and
coleopterous insects of Madeira having come to differ considerably from
their nearest allies on the continent of Europe, whereas the marine
shells and birds have remained unaltered. We can perhaps understand the
apparently quicker rate of change in terrestrial and in more highly
organised productions compared with marine and lower productions, by
the more complex relations of the higher beings to their organic and
inorganic conditions of life, as explained in a former chapter. When
many of the inhabitants of a country have become modified and improved,
we can understand, on the principle of competition, and on that of the
many all-important relations of organism to organism, that any form
which does not become in some degree modified and improved,
will be liable to be exterminated. Hence we can see why all the species
in the same region do at last, if we look to wide enough intervals of
time, become modified; for those which do not change will become
extinct.
In members of the same class the average amount of change, during long
and equal periods of time, may, perhaps, be nearly the same; but as the
accumulation of long-enduring fossiliferous formations depends on great
masses of sediment having been deposited on areas whilst subsiding, our
formations have been almost necessarily accumulated at wide and
irregularly intermittent intervals; consequently the amount of organic
change exhibited by the fossils embedded in consecutive formations is
not equal. Each formation, on this view, does not mark a new and
complete act of creation, but only an occasional scene, taken almost at
hazard, in a slowly changing drama.
We can clearly understand why a species when once lost should never
reappear, even if the very same conditions of life, organic and
inorganic, should recur. For though the offspring of one species might
be adapted (and no doubt this has occurred in innumerable instances) to
fill the exact place of another species in the economy of nature, and
thus supplant it; yet the two forms—the old and the new—would not be
identically the same; for both would almost certainly inherit different
characters from their distinct progenitors. For instance, it is just
possible, if our fantail-pigeons were all destroyed, that fanciers, by
striving during long ages for the same object, might make a new breed
hardly distinguishable from our present fantail; but if the parent
rock-pigeon were also destroyed, and in nature we have every reason to
believe that the parent-form will generally be supplanted and
exterminated by its improved offspring, it is quite incredible that a
fantail, identical with the existing breed, could be raised from any
other species of pigeon, or even from the other well-established races
of the domestic pigeon, for the newly-formed fantail would be almost
sure to inherit from its new progenitor some slight characteristic
differences.
Groups of species, that is, genera and families, follow the same
general rules in their appearance and disappearance as do single
species, changing more or less quickly, and in a greater or lesser
degree. A group does not reappear after it has once disappeared; or its
existence, as long as it lasts, is continuous. I am aware that there
are some apparent exceptions to this rule, but the exceptions are
surprisingly few, so few, that E. Forbes, Pictet, and Woodward (though
all strongly opposed to such views as I maintain) admit its truth; and
the rule strictly accords with my theory. For as all the species of the
same group have descended from some one species, it is clear that as
long as any species of the group have appeared in the long succession
of ages, so long must its members have continuously existed, in order
to have generated either new and modified or the same old and
unmodified forms. Species of the genus Lingula, for instance, must have
continuously existed by an unbroken succession of generations, from the
lowest Silurian stratum to the present day.
We have seen in the last chapter that the species of a group sometimes
falsely appear to have come in abruptly; and I have attempted to give
an explanation of this fact, which if true would have been fatal to my
views. But such cases are certainly exceptional; the general rule being
a gradual increase in number, till the group reaches its maximum, and
then, sooner or later, it gradually decreases. If the
number of the species of a genus, or the number of the genera of a
family, be represented by a vertical line of varying thickness,
crossing the successive geological formations in which the species are
found, the line will sometimes falsely appear to begin at its lower
end, not in a sharp point, but abruptly; it then gradually thickens
upwards, sometimes keeping for a space of equal thickness, and
ultimately thins out in the upper beds, marking the decrease and final
extinction of the species. This gradual increase in number of the
species of a group is strictly conformable with my theory; as the
species of the same genus, and the genera of the same family, can
increase only slowly and progressively; for the process of modification
and the production of a number of allied forms must be slow and
gradual,—one species giving rise first to two or three varieties, these
being slowly converted into species, which in their turn produce by
equally slow steps other species, and so on, like the branching of a
great tree from a single stem, till the group becomes large.
_On Extinction_.—We have as yet spoken only incidentally of the
disappearance of species and of groups of species. On the theory of
natural selection the extinction of old forms and the production of new
and improved forms are intimately connected together. The old notion of
all the inhabitants of the earth having been swept away at successive
periods by catastrophes, is very generally given up, even by those
geologists, as Elie de Beaumont, Murchison, Barrande, etc., whose
general views would naturally lead them to this conclusion. On the
contrary, we have every reason to believe, from the study of the
tertiary formations, that species and groups of species gradually
disappear, one after another, first from one spot, then from another,
and
finally from the world. Both single species and whole groups of species
last for very unequal periods; some groups, as we have seen, having
endured from the earliest known dawn of life to the present day; some
having disappeared before the close of the palæozoic period. No fixed
law seems to determine the length of time during which any single
species or any single genus endures. There is reason to believe that
the complete extinction of the species of a group is generally a slower
process than their production: if the appearance and disappearance of a
group of species be represented, as before, by a vertical line of
varying thickness, the line is found to taper more gradually at its
upper end, which marks the progress of extermination, than at its lower
end, which marks the first appearance and increase in numbers of the
species. In some cases, however, the extermination of whole groups of
beings, as of ammonites towards the close of the secondary period, has
been wonderfully sudden.
The whole subject of the extinction of species has been involved in the
most gratuitous mystery. Some authors have even supposed that as the
individual has a definite length of life, so have species a definite
duration. No one I think can have marvelled more at the extinction of
species, than I have done. When I found in La Plata the tooth of a
horse embedded with the remains of Mastodon, Megatherium, Toxodon, and
other extinct monsters, which all co-existed with still living shells
at a very late geological period, I was filled with astonishment; for
seeing that the horse, since its introduction by the Spaniards into
South America, has run wild over the whole country and has increased in
numbers at an unparalleled rate, I asked myself what could so recently
have exterminated the former horse under conditions of life apparently
so favourable. But
how utterly groundless was my astonishment! Professor Owen soon
perceived that the tooth, though so like that of the existing horse,
belonged to an extinct species. Had this horse been still living, but
in some degree rare, no naturalist would have felt the least surprise
at its rarity; for rarity is the attribute of a vast number of species
of all classes, in all countries. If we ask ourselves why this or that
species is rare, we answer that something is unfavourable in its
conditions of life; but what that something is, we can hardly ever
tell. On the supposition of the fossil horse still existing as a rare
species, we might have felt certain from the analogy of all other
mammals, even of the slow-breeding elephant, and from the history of
the naturalisation of the domestic horse in South America, that under
more favourable conditions it would in a very few years have stocked
the whole continent. But we could not have told what the unfavourable
conditions were which checked its increase, whether some one or several
contingencies, and at what period of the horse’s life, and in what
degree, they severally acted. If the conditions had gone on, however
slowly, becoming less and less favourable, we assuredly should not have
perceived the fact, yet the fossil horse would certainly have become
rarer and rarer, and finally extinct;—its place being seized on by some
more successful competitor.
It is most difficult always to remember that the increase of every
living being is constantly being checked by unperceived injurious
agencies; and that these same unperceived agencies are amply sufficient
to cause rarity, and finally extinction. 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 it ceases to
exist, is much the same as to admit that sickness in the individual 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 unknown
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 less-favoured 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 neighbourhood; 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 natural and
artificial, are bound together. In certain flourishing groups, the
number of new specific forms which have been produced within a given
time is probably greater than that of the old forms which have been
exterminated; but we know that the number of species has not gone on
indefinitely increasing, at least during the later geological periods,
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 believe, a number of new species descended
from one species, that is a new genus, comes to supplant an old genus,
belonging to the same family. But it must often have happened that a
new species belonging to some one group will have seized on the place
occupied by a species belonging to a distinct group, and thus caused
its extermination; and if many allied forms be developed from the
successful intruder, many will have to yield their places; and it will
generally be 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 yield their places to other species which
have been modified and improved, a few of the sufferers may often long
be preserved, from being fitted to some peculiar line of life, or from
inhabiting some distant and isolated station, where they have escaped
severe competition. For instance, a single species of Trigonia, a great
genus of shells in the secondary formations, survives 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 families
or orders, as of Trilobites at the close of the palæozoic period and of
Ammonites at the close of the secondary period, we must remember what
has been already said on the probable wide intervals of time
between our consecutive 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 a new area, they will have exterminated in a
correspondingly rapid manner many of the old inhabitants; and the forms
which thus yield their places will commonly be allied, for they will
partake of some inferiority in common.
Thus, as it seems to me, the manner in which single species and whole
groups of species become extinct, accords well with the theory of
natural selection. We need not marvel at extinction; if we must marvel,
let it be at our presumption in imagining for a moment that we
understand the 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 precisely say why this species is
more abundant in individuals than that; why this species and not
another can be naturalised in a given country; then, and not till then,
we may justly feel surprise why we cannot account for the extinction of
this particular species or group of species.
_On the Forms of Life changing almost simultaneously throughout the
World_.—Scarcely any palæontological 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
recognised in many distant parts of the world, 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 peninsula of India. For at these distant
points, the organic remains in certain beds present an unmistakeable
degree of 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 sections of
genera, and sometimes are similarly characterised 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, are similarly absent at these distant points of
the world. In the several successive palæozoic formations of Russia,
Western Europe and North America, a similar parallelism in the forms of
life has been observed by several authors: so it is, according to
Lyell, with the several European and North American tertiary deposits.
Even if the few fossil species which are common to the Old and New
Worlds be kept wholly out of view, the general parallelism in the
successive forms of life, in the stages of the widely separated
palæozoic and tertiary periods, would still be manifest, and the
several formations could be easily correlated.
These observations, however, relate to the marine inhabitants of
distant parts of the world: we have not sufficient data to judge
whether the productions of the land and of fresh water change at
distant points 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 La Plata, without any
information in regard to their geological position, no one would have
suspected that they had coexisted with still living sea-shells; but as
these anomalous monsters coexisted with the Mastodon
and Horse, it might at least have been inferred that they had lived
during one of the latter tertiary stages.
When the marine forms of life are spoken of as having changed
simultaneously throughout the world, it must not be supposed that this
expression relates to the same thousandth or hundred-thousandth year,
or even that it has a very strict geological sense; for if all the
marine animals which live at the present day in Europe, and all those
that lived in Europe during the pleistocene period (an enormously
remote period as measured by years, including the whole glacial epoch),
were to be compared with those now living in South America or in
Australia, the most skilful naturalist would hardly be able to say
whether the existing or the pleistocene inhabitants of Europe resembled
most closely those of the southern hemisphere. So, again, several
highly competent observers believe that the existing productions of the
United States are more closely related to those which lived in Europe
during certain later tertiary stages, than to those which now live
here; and if this be so, it is evident that fossiliferous beds
deposited at the present day on the shores of North America would
hereafter be liable to be classed with somewhat older European beds.
Nevertheless, looking to a remotely future epoch, there can, I think,
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 only found 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 observers, MM.
de Verneuil and d’Archiac. After referring to the parallelism of the
palæozoic 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 introduction 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 new varieties arising, which have some
advantage over older forms; and those forms, which are already
dominant, or have some advantage over the other forms in their own
country, would naturally oftenest give rise to new varieties or
incipient species; for these latter must be victorious in a still
higher degree in order to be preserved and to survive. We have distinct
evidence on this head, in the plants which are dominant, that is, which
are commonest in their own homes, and are most widely diffused, having
produced the greatest number of new varieties. It is also natural that
the dominant,
varying, and far-spreading species, which already have invaded to a
certain extent the territories of other species, should be those which
would have the best chance of spreading still further, and of giving
rise in new countries to new varieties and species. The process of
diffusion may often be very slow, being dependent on climatal and
geographical changes, or on strange accidents, but in the long run the
dominant forms will generally succeed in spreading. The diffusion
would, it is probable, be slower with the terrestrial inhabitants of
distinct continents than with the marine inhabitants of the continuous
sea. We might therefore expect to find, as we apparently do find, a
less strict degree of parallel succession in the productions of the
land than of the sea.
Dominant species spreading from any region might encounter still more
dominant species, and then their triumphant course, or even their
existence, would cease. We know not at all precisely what are all the
conditions most favourable for the multiplication of new and dominant
species; but we can, I think, clearly see that a number of individuals,
from giving a better chance of the appearance of favourable variations,
and that severe competition with many already existing forms, would be
highly favourable, as would be the power of spreading into new
territories. A certain amount of isolation, recurring at long intervals
of time, would probably be also favourable, as before explained. One
quarter of the world may have been most favourable for the production
of new and dominant species on the land, and another for those in the
waters of the sea. If two great regions had been for a long period
favourably circumstanced in an equal degree, whenever their inhabitants
met, the battle would be prolonged and severe; and some from one
birthplace and some from the other might be victorious. But in the
course of time, the
forms dominant in the highest degree, wherever produced, would tend
everywhere to prevail. As they prevailed, they would cause the
extinction of other and inferior forms; and as these inferior forms
would be allied in groups by inheritance, whole groups would tend
slowly to disappear; though here and there a single member might long
be enabled to survive.
Thus, as it seems to me, the parallel, and, taken in a large sense,
simultaneous, 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 themselves dominant owing to inheritance,
and to having already had some advantage over their parents or over
other species; these again spreading, varying, and producing new
species. The 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 therefore as new and
improved groups spread throughout the world, old groups will disappear
from the world; and the succession of forms in both ways will
everywhere tend to correspond.
There is one other remark connected with this subject worth making. I
have given my reasons for believing that all our greater fossiliferous
formations were deposited during periods of subsidence; and that blank
intervals of vast duration 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 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 general succession in
the forms of life; but the species would not exactly correspond; for
there will have been a little more time in the one region than in the
other for modification, extinction, and immigration.
I suspect that cases of this nature have occurred 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 inhabited 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 arranged in the same
order, in accordance with the general succession of the form of life,
and the order would falsely appear to be strictly parallel;
nevertheless the species would not all be the same in the apparently
corresponding stages in the two regions.
_On the Affinities of extinct Species to each other, and to living
forms_.—Let us now look to the mutual affinities of extinct and living
species. They all fall into one grand natural system; and this fact is
at once explained on the principle 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, all fossils can be classed either
in still existing groups, or between them. That the extinct forms of
life help to fill up the wide intervals between existing genera,
families, and orders, cannot be disputed. For if we confine our
attention either to the living or to the extinct alone, the series is
far less perfect than if we combine both into one general system. With
respect to the Vertebrata, whole pages could be filled with striking
illustrations from our great palæontologist, Owen, showing how extinct
animals fall in between existing groups. Cuvier ranked the Ruminants
and Pachyderms, as the two most distinct orders of mammals; but Owen
has discovered so many fossil links, that he has had to alter the whole
classification of these two orders; and has placed certain pachyderms
in the same sub-order with ruminants: for example, he dissolves by fine
gradations the apparently
wide difference between the pig and the camel. In regard to the
Invertebrata, Barrande, and a higher authority could not be named,
asserts that he is every day taught that palæozoic animals, though
belonging to the same orders, families, or genera with those living at
the present day, were not at this early epoch limited in such distinct
groups as they now are.
Some writers have objected to any extinct species or group of species
being considered as intermediate between living species or groups. If
by this term it is meant that an extinct form is directly intermediate
in all its characters between two living forms, the objection is
probably valid. But I apprehend that in a perfectly natural
classification many fossil species would have to stand between living
species, and some extinct genera between living genera, even between
genera belonging to distinct families. The most common 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 from
each other by a dozen characters, the ancient members of the same two
groups would be distinguished by a somewhat lesser number of
characters, so that the two groups, though formerly quite distinct, at
that period made some small approach to each other.
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
separated 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 affinities directed towards very
distinct groups. Yet if we compare the older Reptiles and
Batrachians, the older Fish, the older Cephalopods, and the eocene
Mammals, with the more recent members of the same classes, we must
admit that there is some 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 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_14, _q_14, _p_14, will form a small family; _b_14 and _f_14
a closely allied family or sub-family; and _o_14, _e_14, _m_14, 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 and common progenitor. On the principle of the continued
tendency to divergence of character, 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 F14.
All the many forms, extinct and recent, descended from A, make, as
before remarked, one order; and this order, from the continued effects
of extinction and divergence of character, has become divided into
several sub-families and families, some of which are supposed to have
perished at different 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 uppermost line would be rendered less distinct from each other.
If, for instance, the genera _a_1, _a_5, _a_10, _f_8, _m_3, _m_6, _m_9
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
pachyderms. Yet he who objected to call the extinct genera, which thus
linked the living genera of three families together, intermediate in
character, would be justified, as they are intermediate, not directly,
but only by a long and circuitous course through many widely different
forms. If many extinct forms were to be discovered above one of the
middle horizontal lines or geological formations—for instance, above
Number VI.—but none from beneath this line, then only the two families
on the left hand (namely, _a_14, etc., and _b_14, etc.) would have to
be united into one family; and the two other families (namely, _a_14 to
_f_14 now including five genera, and _o_14 to _m_14) would yet remain
distinct. These two families, however, would be less distinct from each
other than they were before the
discovery of the fossils. If, for instance, we suppose the existing
genera of the two families to differ from each other by a dozen
characters, in this case the genera, at the early period marked VI.,
would differ by a lesser number of characters; for at this early stage
of descent they have not diverged in character from the common
progenitor of the order, nearly so much as they subsequently diverged.
Thus it comes that ancient and extinct genera are often in some slight
degree intermediate in character between their modified descendants, or
between their collateral relations.
In nature the case 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 very rare cases, to fill up wide intervals
in the natural system, and thus 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 modification, 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 groups; and this by
the concurrent evidence of our best palæontologists seems frequently to
be the case.
Thus, on the theory of descent with modification, the main facts with
respect to the mutual affinities of the extinct forms of life to each
other and to living forms, seem to me explained in a satisfactory
manner. And they are wholly inexplicable on any other view.
On this same theory, it is evident that the fauna of any 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 offspring of those which lived
at the fifth stage, and are the parents of those which became still
more modified at the seventh stage; hence they could hardly fail to be
nearly intermediate in character between the forms of life above and
below. We must, however, allow for the entire extinction of some
preceding forms, and for the coming in of quite new forms by
immigration, and for a large amount of modification, during the long
and blank intervals between the successive formations. Subject to these
allowances, the fauna of each geological period undoubtedly is
intermediate in character, between the preceding and succeeding faunas.
I need give only one instance, namely, the manner in which the fossils
of the Devonian system, when this system was first discovered, were at
once recognised by palæontologists as intermediate in character between
those of the overlying carboniferous, and underlying Silurian system.
But each fauna is not necessarily exactly intermediate, as unequal
intervals of time have elapsed between consecutive formations.
It is no real objection to the truth of the statement, that the fauna
of each period as a whole is nearly intermediate in character between
the preceding and succeeding faunas, that certain genera offer
exceptions to the rule. For instance, mastodons and elephants, when
arranged by Dr. Falconer in two series, first according to their mutual
affinities and then according to their periods of existence, do not
accord in arrangement. The species extreme in character are not the
oldest, or the most recent; nor are those which are intermediate in
character, intermediate in age. But
supposing for an instant, in this and other such cases, that the record
of the first appearance and disappearance of the species was perfect,
we have no reason to believe that forms successively produced
necessarily endure for corresponding lengths of time: a very ancient
form might occasionally last much longer than a form elsewhere
subsequently produced, especially in the case of terrestrial
productions inhabiting separated districts. To compare small things
with great: if the principal living and extinct races of the domestic
pigeon were arranged as well as they could be in serial affinity, this
arrangement would not closely accord with the order in time of their
production, and still less with the order of their disappearance; for
the parent rock-pigeon now lives; and many varieties between the
rock-pigeon and the carrier have become extinct; and carriers which are
extreme in the important character of length of beak originated earlier
than short-beaked tumblers, which are at the opposite end of the series
in this same respect.
Closely connected with the statement, that the organic remains from an
intermediate formation are in some degree intermediate in character, is
the fact, insisted on by all palæontologists, that fossils from two
consecutive formations are far more closely related to each other, than
are the fossils from two remote formations. Pictet gives as a
well-known instance, the general resemblance of the organic remains
from the several stages of the chalk formation, though the species are
distinct in each stage. This fact alone, from its generality, seems to
have shaken Professor Pictet in his firm belief in the immutability of
species. He who is acquainted with the distribution of existing species
over the globe, will not attempt to account for the close resemblance
of the distinct species in closely consecutive
formations, by the physical conditions of the ancient areas having
remained nearly the same. Let it be remembered that the forms of life,
at least those inhabiting the sea, have changed almost simultaneously
throughout the world, and therefore under the most different climates
and conditions. Consider the prodigious vicissitudes of climate during
the pleistocene period, which includes the whole glacial period, and
note how little the specific forms of the inhabitants of the sea have
been affected.
On the theory of descent, the full meaning of the fact of fossil
remains from closely consecutive formations, though ranked as distinct
species, being closely related, is obvious. As the accumulation of each
formation has often been interrupted, and as long blank intervals have
intervened between successive formations, we ought not to expect to
find, as I attempted to show in the last chapter, in any one or two
formations all the intermediate varieties between the species which
appeared at the commencement and close of these periods; but we ought
to find after intervals, very long as measured by years, but only
moderately long as measured geologically, closely allied forms, or, as
they have been called by some authors, representative species; and
these we assuredly do find. We find, in short, such evidence of the
slow and scarcely sensible mutation of specific forms, as we have a
just right to expect to find.
_On the state of Development of Ancient Forms_.—There has been much
discussion whether recent forms are more highly developed than ancient.
I will not here enter on this subject, for naturalists have not as yet
defined to each other’s satisfaction what is meant by high and low
forms. But in one particular sense the
more recent forms must, on my theory, be higher than the more ancient;
for each new species is formed by having had some advantage in the
struggle for life over other and preceding forms. If under a nearly
similar climate, the eocene inhabitants of one quarter of the world
were put into competition with the existing inhabitants of the same or
some other quarter, the eocene fauna or flora would certainly be beaten
and exterminated; as would a secondary fauna by an eocene, and a
palæozoic fauna by a secondary fauna. I do not doubt that this process
of improvement has affected in a marked and sensible manner the
organisation of the more recent and victorious forms of life, in
comparison with the ancient and beaten forms; but I can see no way of
testing this sort of progress. Crustaceans, for instance, not the
highest in their own class, may have beaten the highest molluscs. From
the extraordinary manner in which European productions have recently
spread over New Zealand, and have seized on places which must have been
previously occupied, we may believe, if all the animals and plants of
Great Britain were set free in New Zealand, that in the course of time
a multitude of British forms would become thoroughly naturalized there,
and would exterminate many of the natives. On the other hand, from what
we see now occurring in New Zealand, and from hardly a single
inhabitant of the southern hemisphere having become wild in any part of
Europe, we may doubt, if all the productions of New Zealand were set
free in Great Britain, whether any considerable number would be enabled
to seize on places now occupied by our native plants and animals. Under
this point of view, the productions of Great Britain may be said to be
higher than those of New Zealand. Yet the most skilful naturalist from
an examination of the species
of the two countries could not have foreseen this result.
Agassiz insists that ancient animals resemble to a certain extent the
embryos of recent animals of the same classes; or that the geological
succession of extinct forms is in some degree parallel to the
embryological development of recent forms. I must follow Pictet and
Huxley in thinking that the truth of this doctrine is very far from
proved. Yet I fully expect to see it hereafter confirmed, at least in
regard to subordinate groups, which have branched off from each other
within comparatively recent times. For this doctrine of Agassiz accords
well with the theory of natural selection. In a future chapter I shall
attempt to show that the adult differs from its embryo, owing to
variations supervening at a not early age, and being inherited at a
corresponding age. This process, whilst it leaves the embryo almost
unaltered, continually adds, in the course of successive generations,
more and more difference to the adult.
Thus the embryo comes to be left as a sort of picture, preserved by
nature, of the ancient and less modified condition of each animal. This
view may be true, and yet it may never be capable of full proof.
Seeing, for instance, that the oldest known mammals, reptiles, and fish
strictly belong to their own proper classes, though some of these old
forms are in a slight degree less distinct from each other than are the
typical members of the same groups at the present day, it would be vain
to look for animals having the common embryological character of the
Vertebrata, until beds far beneath the lowest Silurian strata are
discovered—a discovery of which the chance is very small.
_On the Succession of the same Types within the same
areas, during the later tertiary periods_.—Mr. Clift many years ago
showed that the fossil mammals from the Australian caves were closely
allied to the living marsupials of that continent. In South America, a
similar relationship is manifest, even to an uneducated eye, in the
gigantic pieces of armour like those of the armadillo, found in several
parts of La Plata; and Professor Owen has shown in the most striking
manner that most of the fossil mammals, buried there in such numbers,
are related to South American types. This relationship is even more
clearly seen in the wonderful collection of fossil bones made by MM.
Lund and Clausen in the caves of Brazil. I was so much impressed with
these facts that I strongly insisted, in 1839 and 1845, on this “law of
the succession of types,”—on “this wonderful relationship in the same
continent between the dead and the living.” Professor Owen has
subsequently extended the same generalisation to the mammals of the Old
World. We see the same law in this author’s restorations of the extinct
and gigantic birds of New Zealand. We see it also in the birds of the
caves of Brazil. Mr. Woodward has shown that the same law holds good
with sea-shells, but from the wide distribution of most genera of
molluscs, it is not well displayed by them. Other cases could be added,
as the relation between the extinct and living land-shells of Madeira;
and between the extinct and living brackish-water shells of the
Aralo-Caspian Sea.
Now what does this remarkable law of the succession of the same types
within the same areas mean? He would be a bold man, who after comparing
the present climate of Australia and of parts of South America under
the same latitude, would attempt to account, on the one hand, by
dissimilar physical conditions for the dissimilarity of the inhabitants
of these two continents,
and, on the other hand, by similarity of conditions, for the uniformity
of the same types in each during the later tertiary periods. Nor can it
be pretended that it is an immutable law that marsupials should have
been chiefly or solely produced in Australia; or that Edentata and
other American types should have been solely produced in South America.
For we know that Europe in ancient times was peopled by numerous
marsupials; and I have shown in the publications above alluded to, that
in America the law of distribution of terrestrial mammals was formerly
different from what it now is. North America formerly partook strongly
of the present character of the southern half of the continent; and the
southern half was formerly more closely allied, than it is at present,
to the northern half. In a similar manner we know from Falconer and
Cautley’s discoveries, that northern India was formerly more closely
related in its mammals to Africa than it is at the present time.
Analogous facts could be given in relation to the distribution of
marine animals.
On the theory of descent with modification, the great law of the long
enduring, but not immutable, succession of the same types within the
same areas, is at once explained; for the inhabitants of each quarter
of the world will obviously tend to leave in that quarter, during the
next succeeding period of time, closely allied though in some degree
modified descendants. If the inhabitants of one continent formerly
differed greatly from those of another continent, so will their
modified descendants still differ in nearly the same manner and degree.
But after very long intervals of time and after great geographical
changes, permitting much inter-migration, the feebler will yield to the
more dominant forms, and there will be nothing immutable in the laws of
past and present distribution.
It may be asked in ridicule, whether I suppose that the megatherium and
other allied huge monsters have left behind them in South America the
sloth, armadillo, and anteater, as their degenerate descendants. This
cannot for an instant be admitted. These huge animals have become
wholly extinct, and have left no progeny. But in the caves of Brazil,
there are many extinct species which are closely allied in size and in
other characters to the species still living in South America; and some
of these fossils may be the actual progenitors of living species. It
must not be forgotten that, on my theory, all the species of the same
genus have descended from some one species; so that if six genera, each
having eight species, be found in one geological formation, and in the
next succeeding formation there be six other allied or representative
genera with the same number of species, then we may conclude that only
one species of each of the six older genera has left modified
descendants, constituting the six new genera. The other seven species
of the old genera have all died out and have left no progeny. Or, which
would probably be a far commoner case, two or three species of two or
three alone of the six older genera will have been the parents of the
six new genera; the other old species and the other whole genera having
become utterly extinct. In failing orders, with the genera and species
decreasing in numbers, as apparently is the case of the Edentata of
South America, still fewer genera and species will have left modified
blood-descendants.
_Summary of the preceding and present Chapters_.—I have attempted to
show that the geological record is extremely imperfect; that only a
small portion of the globe has been geologically explored with care;
that only
certain classes of organic beings have been largely preserved in a
fossil state; that the number both of specimens and of species,
preserved in our museums, is absolutely as nothing compared with the
incalculable number of generations which must have passed away even
during a single formation; that, owing to subsidence being necessary
for the accumulation of fossiliferous deposits thick enough to resist
future degradation, enormous intervals of time have elapsed between the
successive formations; that there has probably been more extinction
during the periods of subsidence, and more variation during the periods
of elevation, and during the latter the record will have been least
perfectly kept; that each single formation has not been continuously
deposited; that the duration of each formation is, perhaps, short
compared with the average duration of specific forms; that migration
has played an important part in the first appearance of new forms in
any one area and formation; that widely ranging species are those which
have varied most, and have oftenest given rise to new species; and that
varieties have at first often been local. All these causes taken
conjointly, must have tended to make the geological record extremely
imperfect, and will to a large extent explain why we do not find
interminable varieties, connecting together all the extinct and
existing forms of life by the finest graduated steps.
He who rejects these views on the nature of the geological record, will
rightly reject my whole theory. For he may ask in vain where are the
numberless transitional links which must formerly have connected the
closely allied or representative species, found in the several stages
of the same great formation. He may disbelieve in the enormous
intervals of time which have elapsed between our consecutive
formations; he
may overlook how important a part migration must have played, when the
formations of any one great region alone, as that of Europe, are
considered; he may urge the apparent, but often falsely apparent,
sudden coming in of whole groups of species. He may ask where are the
remains of those infinitely numerous organisms which must have existed
long before the first bed of the Silurian system was deposited: I can
answer this latter question only hypothetically, by saying that as far
as we can see, where our oceans now extend they have for an enormous
period extended, and where our oscillating continents now stand they
have stood ever since the Silurian epoch; but that long before that
period, the world may have presented a wholly different aspect; and
that the older continents, formed of formations older than any known to
us, may now all be in a metamorphosed condition, or may lie buried
under the ocean.
Passing from these difficulties, all the other great leading facts in
palæontology seem to me simply to follow on the theory of descent with
modification through natural selection. We can thus understand how it
is that new species come in slowly and successively; how species of
different classes do not necessarily change together, or at the same
rate, or in the same degree; yet in the long run that all undergo
modification to some extent. The extinction of old forms is the almost
inevitable consequence of the production of new forms. We can
understand why when a species has once disappeared it never reappears.
Groups of species increase in numbers slowly, and endure for unequal
periods of time; for the process of modification is necessarily slow,
and depends on many complex contingencies. The dominant species of the
larger dominant groups tend to leave many modified
descendants, and thus new sub-groups and groups are formed. As these
are formed, the species of the less vigorous groups, from their
inferiority inherited from a common progenitor, tend to become extinct
together, and to leave no modified offspring on the face of the earth.
But the utter extinction of a whole group of species may often be a
very slow process, from the survival of a few descendants, lingering in
protected and isolated situations. When a group has once wholly
disappeared, it does not reappear; for the link of generation has been
broken.
We can understand how the spreading of the dominant forms of life,
which are those that oftenest vary, will in the long run tend to people
the world with allied, but modified, descendants; and these will
generally succeed in taking the places of those groups of species which
are their inferiors in the struggle for existence. Hence, after long
intervals of time, the productions of the world will appear to have
changed simultaneously.
We can understand how it is that all the forms of life, ancient and
recent, make together one grand system; for all are connected by
generation. We can understand, from the continued tendency to
divergence of character, why the more ancient a form is, the more it
generally differs from those now living. Why ancient and extinct forms
often tend to fill up gaps between existing forms, sometimes blending
two groups previously classed as distinct into one; but more commonly
only bringing them a little closer together. The more ancient a form
is, the more often, apparently, it displays characters in some degree
intermediate between groups now distinct; for the more ancient a form
is, the more nearly it will be related to, and consequently resemble,
the common progenitor of groups, since become
widely divergent. Extinct forms are seldom directly intermediate
between existing forms; but are intermediate only by a long and
circuitous course through many extinct and very different forms. We can
clearly see why the organic remains of closely consecutive formations
are more closely allied to each other, than are those of remote
formations; for the forms are more closely linked together by
generation: we can clearly see why the remains of an intermediate
formation are intermediate in character.
The inhabitants of each successive period in the world’s history have
beaten their predecessors in the race for life, and are, in so far,
higher in the scale of nature; and this may account for that vague yet
ill-defined sentiment, felt by many palæontologists, that organisation
on the whole has progressed. If it should hereafter be proved that
ancient animals resemble to a certain extent the embryos of more recent
animals of the same class, the fact will be intelligible. The
succession of the same types of structure within the same areas during
the later geological periods ceases to be mysterious, and is simply
explained by inheritance.
If then the geological record be as imperfect as I believe it to be,
and it may at least be asserted that the record cannot be proved to be
much more perfect, the main objections to the theory of natural
selection are greatly diminished or disappear. On the other hand, all
the chief laws of palæontology plainly proclaim, as it seems to me,
that species have been produced by ordinary generation: old forms
having been supplanted by new and improved forms of life, produced by
the laws of variation still acting round us, and preserved by Natural
Selection.