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 {313} 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.
{314}
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,
{315} 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 {316} 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 {317} 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 {318} groups of species last for very unequal periods; some groups,
as we have seen, having endured from the earliest known dawn of life to the
present day; some having disappeared before the close of the palæozoic
period. No fixed law seems to determine the length of time during which any
single species or any single genus endures. There is reason to believe that
the complete extinction of the species of a group is generally a slower
process than their production: if the appearance and disappearance of a
group of species be represented, as before, by a vertical line of varying
thickness, the line is found to taper more gradually at its upper end,
which marks the progress of extermination, than at its lower end, which
marks the first appearance and increase in numbers of the species. In some
cases, however, the extermination of whole groups of beings, as of
ammonites towards the close of the secondary period, has been wonderfully
sudden.
The whole subject of the extinction of species has been involved in the
most gratuitous mystery. Some authors have even supposed that as the
individual has a definite length of life, so have species a definite
duration. No one I think can have marvelled more at the extinction of
species, than I have done. When I found in La Plata the tooth of a horse
embedded with the remains of Mastodon, Megatherium, Toxodon, and other
extinct monsters, which all co-existed with still living shells at a very
late geological period, I was filled with astonishment; for seeing that the
horse, since its introduction by the Spaniards into South America, has run
wild over the whole country and has increased in numbers at an unparalleled
rate, I asked myself what could so recently have exterminated the former
horse under conditions of life apparently so favourable. But how utterly
groundless was my astonishment! {319} 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 {320} 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 specific 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. {321} Hence the improved and modified descendants of a
species will generally cause the extermination of the parent-species; and
if many new forms have been developed from any one species, the nearest
allies of that species, _i.e._ the species of the same genus, will be the
most liable to extermination. Thus, as I believe, a number of new species
descended from one species, that is a new genus, comes to supplant an old
genus, belonging to the same family. But it must often have happened that a
new species belonging to some one group will have seized on the place
occupied by a species belonging to a distinct group, and thus caused its
extermination; and if many allied forms be developed from the successful
intruder, many will have to yield their places; and it will generally be
allied forms, which will suffer from some inherited inferiority in common.
But whether it be species belonging to the same or to a distinct class,
which yield their places to other species which have been modified and
improved, a few of the sufferers may often long be preserved, from being
fitted to some peculiar line of life, or from inhabiting some distant and
isolated station, where they have escaped severe competition. For instance,
a single species of Trigonia, a great genus of shells in the secondary
formations, survives in the Australian seas; and a few members of the great
and almost extinct group of Ganoid fishes still inhabit our fresh waters.
Therefore the utter extinction of a group is generally, as we have seen, a
slower process than its production.
With respect to the apparently sudden extermination of whole families or
orders, as of Trilobites at the close of the palæozoic period and of
Ammonites at the close of the secondary period, we must remember what has
been already said on the probable wide intervals of time {322} between our
consecutive formations; and in these intervals there may have been much
slow extermination. Moreover, when by sudden immigration or by unusually
rapid development, many species of a new group have taken possession of a
new area, they will have exterminated in a correspondingly rapid manner
many of the old inhabitants; and the forms which thus yield their places
will commonly be allied, for they will partake of some inferiority in
common.
Thus, as it seems to me, the manner in which single species and whole
groups of species become extinct, accords well with the theory of natural
selection. We need not marvel at extinction; if we must marvel, let it be
at our presumption in imagining for a moment that we understand the many
complex contingencies, on which the existence of each species depends. If
we forget for an instant, that each species tends to increase inordinately,
and that some check is always in action, yet seldom perceived by us, the
whole economy of nature will be utterly obscured. Whenever we can precisely
say why this species is more abundant in individuals than that; why this
species and not another can be naturalised in a given country; then, and
not till then, we may justly feel surprise why we cannot account for the
extinction of this particular species or group of species.
_On the Forms of Life changing almost simultaneously throughout the
World._--Scarcely any palæontological discovery is more striking than the
fact, that the forms of life change almost simultaneously throughout the
world. Thus our European Chalk formation can be recognised in many distant
parts of the world, under the most different climates, where not a fragment
of the mineral chalk itself can be found; namely, in North {323} America,
in equatorial South America, in Tierra del Fuego, at the Cape of Good Hope,
and in the peninsula of India. For at these distant points, the organic
remains in certain beds present an unmistakeable degree of resemblance to
those of the Chalk. It is not that the same species are met with; for in
some cases not one species is identically the same, but they belong to the
same families, genera, and sections of genera, and sometimes are similarly
characterised in such trifling points as mere superficial sculpture.
Moreover other forms, which are not found in the Chalk of Europe, but which
occur in the formations either above or below, are similarly absent at
these distant points of the world. In the several successive palæozoic
formations of Russia, Western Europe and North America, a similar
parallelism in the forms of life has been observed by several authors: so
it is, according to Lyell, with the several European and North American
tertiary deposits. Even if the few fossil species which are common to the
Old and New Worlds be kept wholly out of view, the general parallelism in
the successive forms of life, in the stages of the widely separated
palæozoic and tertiary periods, would still be manifest, and the several
formations could be easily correlated.
These observations, however, relate to the marine inhabitants of distant
parts of the world: we have not sufficient data to judge whether the
productions of the land and of fresh water change at distant points in the
same parallel manner. We may doubt whether they have thus changed: if the
Megatherium, Mylodon, Macrauchenia, and Toxodon had been brought to Europe
from La Plata, without any information in regard to their geological
position, no one would have suspected that they had co-existed with still
living sea-shells; but as these anomalous monsters co-existed with the
{324} Mastodon and Horse, it might at least have been inferred that they
had lived during one of the later 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. {325} de Verneuil and d'Archiac. After referring to the
parallelism of the palæozoic forms of life in various parts of Europe, they
add, "If struck by this strange sequence, we turn our attention to North
America, and there discover a series of analogous phenomena, it will appear
certain that all these modifications of species, their extinction, and the
introduction of new ones, cannot be owing to mere changes in marine
currents or other causes more or less local and temporary, but depend on
general laws which govern the whole animal kingdom." M. Barrande has made
forcible remarks to precisely the same effect. It is, indeed, quite futile
to look to changes of currents, climate, or other physical conditions, as
the cause of these great mutations in the forms of life throughout the
world, under the most different climates. We must, as Barrande has
remarked, look to some special law. We shall see this more clearly when we
treat of the present distribution of organic beings, and find how slight is
the relation between the physical conditions of various countries, and the
nature of their inhabitants.
This great fact of the parallel succession of the forms of life throughout
the world, is explicable on the theory of natural selection. New species
are formed by new varieties arising, which have some advantage over older
forms; and those forms, which are already dominant, or have some advantage
over the other forms in their own country, would naturally oftenest give
rise to new varieties or incipient species; for these latter must be
victorious in a still higher degree in order to be preserved and to
survive. We have distinct evidence on this head, in the plants which are
dominant, that is, which are commonest in their own homes, and are most
widely diffused, having produced the greatest number of new varieties. It
is also natural that the {326} 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 {327} 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 {328} 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 occur in Europe. Mr. Prestwich, in his
admirable Memoirs on the eocene deposits of England and France, is able to
draw a close general parallelism between the successive stages in the two
countries; but when he compares certain stages in England with those in
France, although he finds in both a curious accordance in the numbers of
the species belonging to the same genera, yet the species themselves differ
in a manner very difficult to account for, considering the proximity of the
two areas,--unless, indeed, it be assumed that an isthmus separated two
seas 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 {329}
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 {330} 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 {331} Batrachians, the older Fish, the older
Cephalopods, and the eocene Mammals, with the more recent members of the
same classes, we must admit that there is some truth in the remark.
Let us see how far these several facts and inferences accord with the
theory of descent with modification. As the subject is somewhat complex, I
must request the reader to turn to the diagram in the fourth chapter. We
may suppose that the numbered letters represent genera, and the dotted
lines diverging from them the species in each genus. The diagram is much
too simple, too few genera and too few species being given, but this is
unimportant for us. The horizontal lines may represent successive
geological formations, and all the forms beneath the uppermost line may be
considered as extinct. The three existing genera, a^{14}, q^{14}, p^{14},
will form a small family; b^{14} and f^{14} a closely allied family or
sub-family; and o^{14}, e^{14}, m^{14}, a third family. These three
families, together with the many extinct genera on the several lines of
descent diverging from the parent-form (A), will form an order; for all
will have inherited something in common from their ancient and common
progenitor. On the principle of the continued tendency to divergence of
character, which was formerly illustrated by this diagram, the more recent
any form is, the more it will generally differ from its ancient progenitor.
Hence we can understand the rule that the most ancient fossils differ most
from existing forms. We must not, however, assume that divergence of
character is a necessary contingency; it depends solely on the descendants
from a species being thus enabled to seize on many and different places in
the economy of nature. Therefore it is quite possible, as we have seen in
the case of some Silurian forms, that a species might go on being slightly
modified in relation to its slightly altered conditions of {332} life, and
yet retain throughout a vast period the same general characteristics. This
is represented in the diagram by the letter F^{14}.
All the many forms, extinct and recent, descended from (A), make, as before
remarked, one order; and this order, from the continued effects of
extinction and divergence of character, has become divided into several
sub-families and families, some of which are supposed to have perished at
different periods, and some to have endured to the present day.
By looking at the diagram we can see that if many of the extinct forms,
supposed to be embedded in the successive formations, were discovered at
several points low down in the series, the three existing families on the
uppermost line would be rendered less distinct from each other. If, for
instance, the genera a^1, a^5, a^{10}, f^8, m^3, m^6, m^9, were
disinterred, these three families would be so closely linked together that
they probably would have to be united into one great family, in nearly the
same manner as has occurred with ruminants and pachyderms. Yet he who
objected to call the extinct genera, which thus linked the living genera of
three families together, intermediate in character, would be justified, as
they are intermediate, not directly, but only by a long and circuitous
course through many widely different forms. If many extinct forms were to
be discovered above one of the middle horizontal lines or geological
formations --for instance, above No. VI.--but none from beneath this line,
then only the two families on the left hand (namely, a^{14}, &c., and
b^{14}, &c.) would have to be united into one family; and the two other
families (namely, a^{14} to f^{14} now including five genera, and o^{14} to
m^{14}) would yet remain distinct. These two families, however, would be
less distinct from each other than they were before the discovery of the
fossils. If, for instance, we suppose the existing genera of the two
families to differ from each {333} other by a dozen characters, in this
case the genera, at the early period marked VI., would differ by a lesser
number of characters; for at this early stage of descent they have not
diverged in character from the common progenitor of the order, nearly so
much as they subsequently diverged. Thus it comes that ancient and extinct
genera are often in some slight degree intermediate in character between
their modified descendants, or between their collateral relations.
In nature the case will be far more complicated than is represented in the
diagram; for the groups will have been more numerous, they will have
endured for extremely unequal lengths of time, and will have been modified
in various degrees. As we possess only the last volume of the geological
record, and that in a very broken condition, we have no right to expect,
except in very rare cases, to fill up wide intervals in the natural system,
and thus unite distinct families or orders. All that we have a right to
expect, is that those groups, which have within known geological periods
undergone much modification, should in the older formations make some
slight approach to each other; so that the older members should differ less
from each other in some of their characters than do the existing members of
the same groups; and this by the concurrent evidence of our best
palæontologists seems frequently to be the case.
Thus, on the theory of descent with modification, the main facts with
respect to the mutual affinities of the extinct forms of life to each other
and to living forms, seem to me explained in a satisfactory manner. And
they are wholly inexplicable on any other view.
On this same theory, it is evident that the fauna of any great period in
the earth's history will be intermediate in general character between that
which preceded and that which succeeded it. Thus, the species which lived
at the sixth great stage of descent in the {334} 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 in any one region for the immigration of new
forms from other regions, and for a large amount of modification, during
the long and blank intervals between the successive formations. Subject to
these allowances, the fauna of each geological period undoubtedly is
intermediate in character, between the preceding and succeeding faunas. I
need give only one instance, namely, the manner in which the fossils of the
Devonian system, when this system was first discovered, were at once
recognised by palæontologists as intermediate in character between those of
the overlying carboniferous, and underlying Silurian system. But each fauna
is not necessarily exactly intermediate, as unequal intervals of time have
elapsed between consecutive formations.
It is no real objection to the truth of the statement, that the fauna of
each period as a whole is nearly intermediate in character between the
preceding and succeeding faunas, that certain genera offer exceptions to
the rule. For instance, mastodons and elephants, when arranged by Dr.
Falconer in two series, first according to their mutual affinities and then
according to their periods of existence, do not accord in arrangement. The
species extreme in character are not the oldest, or the most recent; nor
are those which are intermediate in character, intermediate in age. But
supposing for an instant, in this and other such cases, that the record of
the first appearance and disappearance of the species was perfect, we have
no reason to believe that forms successively produced necessarily endure
for {335} corresponding lengths of time: a very ancient form might
occasionally last much longer than a form elsewhere subsequently produced,
especially in the case of terrestrial productions inhabiting separated
districts. To compare small things with great: if the principal living and
extinct races of the domestic pigeon were arranged as well as they could be
in serial affinity, this arrangement would not closely accord with the
order in time of their production, and still less with the order of their
disappearance; for the parent rock-pigeon now lives; and many varieties
between the rock-pigeon and the carrier have become extinct; and carriers
which are extreme in the important character of length of beak originated
earlier than short-beaked tumblers, which are at the opposite end of the
series in this same respect.
Closely connected with the statement, that the organic remains from an
intermediate formation are in some degree intermediate in character, is the
fact, insisted on by all palæontologists, that fossils from two consecutive
formations are far more closely related to each other, than are the fossils
from two remote formations. Pictet gives as a well-known instance, the
general resemblance of the organic remains from the several stages of the
Chalk formation, though the species are distinct in each stage. This fact
alone, from its generality, seems to have shaken Professor Pictet in his
firm belief in the immutability of species. He who is acquainted with the
distribution of existing species over the globe, will not attempt to
account for the close resemblance of the distinct species in closely
consecutive formations, by the physical conditions of the ancient areas
having remained nearly the same. Let it be remembered that the forms of
life, at least those inhabiting the sea, have changed almost simultaneously
throughout the world, and therefore under the most different climates and
conditions. Consider the {336} 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.
The best definition probably is, that the higher forms have their organs
more distinctly specialised for different functions; and as such division
of physiological labour seems to be an advantage to each being, natural
selection will constantly tend in so far to make the later and more
modified forms higher than their early progenitors, or than the slightly
modified descendants of such progenitors. In a more general sense the {337}
more recent forms must, on my theory, be higher than the more ancient; for
each new species is formed by having had some advantage in the struggle for
life over other and preceding forms. If under a nearly similar climate, the
eocene inhabitants of one quarter of the world were put into competition
with the existing inhabitants of the same or some other quarter, the eocene
fauna or flora would certainly be beaten and exterminated; as would a
secondary fauna by an eocene, and a palæozoic fauna by a secondary fauna. I
do not doubt that this process of improvement has affected in a marked and
sensible manner the organisation of the more recent and victorious forms of
life, in comparison with the ancient and beaten forms; but I can see no way
of testing this sort of progress. Crustaceans, for instance, not the
highest in their own class, may have beaten the highest molluscs. From the
extraordinary manner in which European productions have recently spread
over New Zealand, and have seized on places which must have been previously
occupied, we may believe, if all the animals and plants of Great Britain
were set free in New Zealand, that in the course of time a multitude of
British forms would become thoroughly naturalized there, and would
exterminate many of the natives. On the other hand, from what we see now
occurring in New Zealand, and from hardly a single inhabitant of the
southern hemisphere having become wild in any part of Europe, we may doubt,
if all the productions of New Zealand were set free in Great Britain,
whether any considerable number would be enabled to seize on places now
occupied by our native plants and animals. Under this point of view, the
productions of Great Britain may be said to be higher than those of New
Zealand. Yet the most skilful naturalist from an examination of the {338}
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 {339} 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, {340} 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. {341}
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 old 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 {342}
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 {343} may
overlook how important a part migration must have played, when the
formations of any one great region alone, as that of Europe, are
considered; he may urge the apparent, but often falsely apparent, sudden
coming in of whole groups of species. He may ask where are the remains of
those infinitely numerous organisms which must have existed long before the
first bed of the Silurian system was deposited: I can answer this latter
question only hypothetically, by saying that as far as we can see, where
our oceans now extend they have for an enormous period extended, and where
our oscillating continents now stand they have stood ever since the
Silurian epoch; but that long before that period, the world may have
presented a wholly different aspect; and that the older continents, formed
of formations older than any known to us, may now all be in a metamorphosed
condition, or may lie buried under the ocean.
Passing from these difficulties, all the other great leading facts in
palæontology seem to me simply to follow on the theory of descent with
modification through natural selection. We can thus understand how it is
that new species come in slowly and successively; how species of different
classes do not necessarily change together, or at the same rate, or in the
same degree; yet in the long run that all undergo modification to some
extent. The extinction of old forms is the almost inevitable consequence of
the production of new forms. We can understand why when a species has once
disappeared it never reappears. Groups of species increase in numbers
slowly, and endure for unequal periods of time; for the process of
modification is necessarily slow, and depends on many complex
contingencies. The dominant species of the larger dominant groups tend to
leave many modified {344} 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 {345} become widely divergent. Extinct forms are seldom directly
intermediate between existing forms; but are intermediate only by a long
and circuitous course through many extinct and very different forms. We can
clearly see why the organic remains of closely consecutive formations are
more closely allied to each other, than are those of remote formations; for
the forms are more closely linked together by generation: we can clearly
see why the remains of an intermediate formation are intermediate in
character.
The inhabitants of each successive period in the world's history have
beaten their predecessors in the race for life, and are, in so far, higher
in the scale of nature; and this may account for that vague yet ill-defined
sentiment, felt by many palæontologists, that organisation on the whole has
progressed. If it should hereafter be proved that ancient animals resemble
to a certain extent the embryos of more recent animals of the same class,
the fact will be intelligible. The succession of the same types of
structure within the same areas during the later geological periods ceases
to be mysterious, and is simply explained by inheritance.
If then the geological record be as imperfect as I believe it to be, and it
may at least be asserted that the record cannot be proved to be much more
perfect, the main objections to the theory of natural selection are greatly
diminished or disappear. On the other hand, all the chief laws of
palæontology plainly proclaim, as it seems to me, that species have been
produced by ordinary generation: old forms having been supplanted by new
and improved forms of life, produced by the laws of variation still acting
round us, and preserved by Natural Selection.
* * * * *
{346}