- 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. |