 Chapter
10 - 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, &c., 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 palaeozoic 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 palaeozoic 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 palaeontological 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 palaeozoic 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 palaeozoic
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
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 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 palaeontologist, 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 palaeozoic 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, a14, q14,
p14, will form a small family; b14 and f14
a closely allied family or sub-family; and o14, e14,
m14, 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 a1, a5, a10,
m3, m6, m9 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 No. VI. but none from beneath this line,
then only the two families on the left hand (namely, a14,
&c., and b14, &c.) would have to be united
into one family; and the two other families (namely, a14
to f14 now including five genera, and o14
to m14) 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 palaeontologists 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 palaeontologists 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
palaeontologists, 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 palaeozoic 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 palaeontology 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 palaeontologists, 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 palaeontology
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.
|
