Chapter X 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.
2 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.
3 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.
4 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.
5 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.
6 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.
7 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.
8 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.
9 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.
10 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.
11 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.
12 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.
13 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.
14 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.
15 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.
16 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.
17 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.
18 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.
19 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.
20 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.
21 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.
22 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.
23 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.
24 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.
25 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.
26 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.
27 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.
28 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.
29 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.
30 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.
31 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, f8, 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.
32 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.
33 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.
34 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.
35 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.
36 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.
37 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.
38 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.
39 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.
40 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.
41 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.
42 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.
43 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.
44 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.
45 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.
46 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.
47 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.
48 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.
49 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.
50 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.
51 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.