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Charles Darwin: The origin of species
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CHAPTER XI.
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 laws relating to the
geological succession of organic beings accord best with the common view of
the immutability of species, or with that of their slow and gradual
modification, through variation 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 the stages, and to make the
proportion between the lost and existing 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 extinct, and only one or two are new,
having appeared there for the first time, either locally, or, as far as we
know, on the face of the earth. The secondary formations are more broken;
but, as Bronn has remarked, neither the appearance nor disappearance of the
many species embedded in each formation has been simultaneous.
Species belonging to different genera and classes have not changed at the
same rate, or in the same degree. In the older 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, for an existing
crocodile is associated with many 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 have changed at a quicker rate than those of the sea, of which a
striking instance has been observed in Switzerland. There is some reason
to believe that organisms high in the scale, change more quickly than those
that are low: though there are exceptions to this rule. The amount of
organic change, as Pictet has remarked, is not the same in each successive
so-called formation. 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 no
reason to believe that the same identical form ever 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 satisfactory.
These several facts accord well with our theory, which includes no fixed
law of development, causing all the inhabitants of an area to change
abruptly, or simultaneously, or to an equal degree. The process of
modification must be slow, and will generally affect only a few species at
the same time; for the variability of each species is independent of that
of all others. Whether such variations or individual differences as may
arise will be accumulated through natural selection in a greater or less
degree, thus causing a greater or less amount of permanent modification,
will depend on many complex contingencies--on the variations being of a
beneficial nature, on the freedom of intercrossing, on the slowly changing
physical conditions of the country, on the immigration of new colonists,
and on the nature of the other inhabitants with which the varying species
come 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, should change in a less degree. We find similar relations
between the existing inhabitants of distinct countries; for instance, the
land-shells and coleopterous insects of Madeira have 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 any area have become modified and improved, we can
understand, on the principle of competition, and from the all-important
relations of organism to organism in the struggle for life, that any form
which did not become in some degree modified and improved, would be liable
to extermination. Hence, we see why all the species in the same region do
at last, if we look to long enough intervals of time, become modified; for
otherwise they would 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 enduring formations, rich in fossils, depends on great
masses of sediment being deposited on subsiding areas, our formations have
been almost necessarily accumulated at wide and irregularly intermittent
intervals of time; 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 an ever 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 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; and organisms already differing would vary in a different
manner. For instance, it is possible, if all our fantail-pigeons were
destroyed, that fanciers might make a new breed hardly distinguishable from
the present breed; but if the parent rock-pigeon were likewise destroyed,
and under nature we have every reason to believe that parent forms are
generally supplanted and exterminated by their improved offspring, it is
incredible that a fantail, identical with the existing breed, could be
raised from any other species of pigeon, or even from any other well
established race of the domestic pigeon, for the successive variations
would almost certainly be in some degree different, and the newly-formed
variety would probably inherit from its progenitor some 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, when it
has once disappeared, never reappears; that is, 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
the theory. For all the species of the same group, however long it may
have lasted, are the modified descendants one from the other, and all from
a common progenitor. In the genus Lingula, for instance, the species which
have successively appeared at all ages must have been connected by an
unbroken series of generations, from the lowest Silurian stratum to the
present day.
We have seen in the last chapter that whole groups of species sometimes
falsely appear to have been abruptly developed; and I have attempted to
give an explanation of this fact, which if true would be fatal to my views.
But such cases are certainly exceptional; the general rule being a gradual
increase in number, until the group reaches its maximum, and then, sooner
or later, a gradual decrease. If the number of the species included within
a genus, or the number of the genera within a family, be represented by a
vertical line of varying thickness, ascending through 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, often keeping of equal
thickness for a space, 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 the theory;
for the species of the same genus, and the genera of the same family, can
increase only slowly and progressively; the process of modification and the
production of a number of allied forms necessarily being a slow and gradual
process, one species first giving rise to two or three varieties, these
being slowly converted into species, which in their turn produce by equally
slow steps other varieties and 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 only spokesn 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 by catastrophes at successive periods is
very generally given up, even by those geologists, as Elie de Beaumont,
Murchison, Barrande, etc., whose general views would naturally lead them to
this conclusion. On the contrary, we have every reason to believe, from
the study of the tertiary formations, that species and groups of species
gradually disappear, one after another, first from one spot, then from
another, and finally from the world. In some few cases, however, as by the
breaking of an isthmus and the consequent irruption of a multitude of new
inhabitants into an adjoining sea, or by the final subsidence of an island,
the process of extinction may have been rapid. Both single species and
whole groups of species last for very unequal periods; some groups, as we
have seen, have endured from the earliest known dawn of life to the present
day; some have 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
extinction of a whole group of species is generally a slower process than
their production: if their appearance and disappearance 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 the early
increase in number of the species. In some cases, however, the
extermination of whole groups, as of ammonites, towards the close of the
secondary period, has been wonderfully sudden.
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 can have
marvelled more than I have done at the extinction of species. 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 my astonishment was groundless. 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
creature is constantly being checked by unperceived hostile agencies; and
that these same unperceived agencies are amply sufficient to cause rarity,
and finally extinction. So little is this subject understood, that I have
heard surprise repeatedly expressed at such great monsters as the Mastodon
and the more ancient Dinosaurians having become extinct; as if mere bodily
strength gave victory in the battle of life. Mere size, on the contrary,
would in some cases determine, as has been remarked by Owen, quicker
extermination, from the greater amount of requisite food. Before man
inhabited India or Africa, some cause must have checked the continued
increase of the existing elephant. A highly capable judge, Dr. Falconer,
believes that it is chiefly insects which, from incessantly harassing and
weakening the elephant in India, check its increase; and this was Bruce's
conclusion with respect to the African elephant in Abyssinia. It is
certain that insects and blood-sucking bats determine the existence of the
larger naturalised quadrupeds in several parts of South America.
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
the species 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
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 those naturally and artificially produced,
are bound together. In flourishing groups, the number of new specific
forms which have been produced within a given time has at some periods
probably been greater than the number of the old specific forms which have
been exterminated; but we know that species have not gone on indefinitely
increasing, at least during the later geological epochs, so that, looking
to later times, we may believe that the production of new forms has caused
the extinction of about the same number of old forms.
The competition will generally be most severe, as formerly explained and
illustrated by examples, between the forms which are most like each other
in all respects. Hence the improved and modified descendants of a species
will generally cause the extermination of the parent-species; and if many
new forms have been developed from any one species, the nearest allies of
that species, i.e. the species of the same genus, will be the most liable
to extermination. Thus, as I believe, a number of new species descended
from one species, that is a new genus, comes to supplant an old genus,
belonging to the same family. But it must often have happened that a new
species belonging to some one group has seized on the place occupied by a
species belonging to a distinct group, and thus have caused its
extermination. If many allied forms be developed from the successful
intruder, many will have to yield their places; and it will generally be
the 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 have yielded their places to other modified and improved
species, a few of the sufferers may often be preserved for a long time,
from being fitted to some peculiar line of life, or from inhabiting some
distant and isolated station, where they will have escaped severe
competition. For instance, some species of Trigonia, a great genus of
shells in the secondary formations, survive in the Australian seas; and a
few members of the great and almost extinct group of Ganoid fishes still
inhabit our fresh waters. Therefore, the utter extinction of a group is
generally, as we have seen, a slower process than its production.
With respect to the apparently sudden extermination of whole families or
orders, as of Trilobites at the close of the palaeozoic period, and of
Ammonites at the close of the secondary period, we must remember what has
been already said on the probable wide intervals of time between our
consecutive formations; and in these intervals there may have been much
slow extermination. Moreover, when, by sudden immigration or by unusually
rapid development, many species of a new group have taken possession of an
area, many of the older species will have been exterminated in a
correspondingly rapid manner; and the forms which thus yield their places
will commonly be allied, for they will partake of the same 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 until then, we may justly feel surprise why we cannot account
for the extinction of any particular species or group of species.
ON THE FORMS OF LIFE CHANGING ALMOST SIMULTANEOUSLY THROUGHOUT THE WORLD.
Scarcely any palaeontological discovery is more striking than the fact that
the forms of life change almost simultaneously throughout the world. Thus
our European Chalk formation can be recognised in many distant regions,
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 unmistakable 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, occur in the same order 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 European and North American tertiary deposits. Even if the
few fossil species which are common to the Old and New Worlds were kept
wholly out of view, the general parallelism in the successive forms of
life, in the palaeozoic and tertiary stages, would still be manifest, and
the several formations could be easily correlated.
These observations, however, relate to the marine inhabitants of the world:
we have not sufficient data to judge whether the productions of the land
and of fresh water at distant points change 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 sea-shells all still living; but as
these anomalous monsters co-existed with the 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 year, or even to the same century, or even
that it has a very strict geological sense; for if all the marine animals
now living in Europe, and all those that lived in Europe during the
pleistocene period (a very remote period as measured by years, including
the whole glacial epoch) were compared with those now existing in South
America or in Australia, the most skilful naturalist would hardly be able
to say whether the present or the pleistocene inhabitants of Europe
resembled most closely those of the southern hemisphere. So, again,
several highly competent observers maintain that the existing productions
of the United States are more closely related to those which lived in
Europe during certain late tertiary stages, than to the present inhabitants
of Europe; and if this be so, it is evident that fossiliferous beds now
deposited 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 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 found only in the older underlying deposits, would be
correctly ranked as simultaneous in a geological sense.
The fact of the forms of life changing simultaneously in the above large
sense, at distant parts of the world, has greatly struck those admirable
observers, MM. de Verneuil and d'Archiac. After referring to the
parallelism of the palaeozoic forms of life in various parts of Europe,
they add, "If struck by this strange sequence, we turn our attention to
North America, and there discover a series of analogous phenomena, it will
appear certain that all these modifications of species, their extinction,
and the introduction of new ones, cannot be owing to mere changes in marine
currents or other causes more or less local and temporary, but depend on
general laws which govern the whole animal kingdom." M. Barrande has made
forcible remarks to precisely the same effect. It is, indeed, quite futile
to look to changes of currents, climate, or other physical conditions, as
the cause of these great mutations in the forms of life throughout the
world, under the most different climates. We must, as Barrande has
remarked, look to some special law. We shall see this more clearly when we
treat of the present distribution of organic beings, and find how slight is
the relation between the physical conditions of various countries and the
nature of their inhabitants.
This great fact of the parallel succession of the forms of life throughout
the world, is explicable on the theory of natural selection. New species
are formed by having some advantage over older forms; and the forms, which
are already dominant, or have some advantage over the other forms in their
own country, give birth to the greatest number of new varieties or
incipient species. We have distinct evidence on this head, in the plants
which are dominant, that is, which are commonest and most widely diffused,
producing the greatest number of new varieties. It is also natural that
the dominant, varying and far-spreading species, which have already
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 other new varieties and species. The
process of diffusion would often be very slow, depending on climatal and
geographical changes, on strange accidents, and on the gradual
acclimatization of new species to the various climates through which they
might have to pass, but in the course of time the dominant forms would
generally succeed in spreading and would ultimately prevail. 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 do find, a less strict degree of
parallelism in the succession of the productions of the land than with
those of the sea.
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 their having had some
advantage over their already dominant parents, as well as over other
species; and again spreading, varying, and producing new forms. The old
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 disappear from the world; and the
succession of forms everywhere tends to correspond both in their first
appearance and final disappearance.
There is one other remark connected with this subject worth making. I have
given my reasons for believing that most of our great formations, rich in
fossils, were deposited during periods of subsidence; and that blank
intervals of vast duration, as far as fossils are concerned, 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 very 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 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 forms 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.
All fall into a few grand classes; 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, extinct species can all be classed either in still existing
groups, or between them. That the extinct forms of life help to fill up
the intervals between existing genera, families, and orders, is certainly
true; but as this statement has often been ignored or even denied, it may
be well to make some remarks on this subject, and to give some instances.
If we confine our attention either to the living or to the extinct species
of the same class, the series is far less perfect than if we combine both
into one general system. In the writings of Professor Owen we continually
meet with the expression of generalised forms, as applied to extinct
animals; and in the writings of Agassiz, of prophetic or synthetic types;
and these terms imply that such forms are, in fact, intermediate or
connecting links. Another distinguished palaeontologist, M. Gaudry, has
shown in the most striking manner that many of the fossil mammals
discovered by him in Attica serve to break down the intervals between
existing genera. Cuvier ranked the Ruminants and Pachyderms as two of the
most distinct orders of mammals; but so many fossil links have been
disentombed that Owen has had to alter the whole classification, and has
placed certain Pachyderms in the same sub-order with ruminants; for
example, he dissolves by gradations the apparently wide interval between
the pig and the camel. The Ungulata or hoofed quadrupeds are now divided
into the even-toed or odd-toed divisions; but the Macrauchenia of South
America connects to a certain extent these two grand divisions. No one
will deny that the Hipparion is intermediate between the existing horse and
certain other ungulate forms. What a wonderful connecting link in the
chain of mammals is the Typotherium from South America, as the name given
to it by Professor Gervais expresses, and which cannot be placed in any
existing order. The Sirenia form a very distinct group of the mammals, and
one of the most remarkable peculiarities in existing dugong and lamentin is
the entire absence of hind limbs, without even a rudiment being left; but
the extinct Halitherium had, according to Professor Flower, an ossified
thigh-bone "articulated to a well-defined acetabulum in the pelvis," and it
thus makes some approach to ordinary hoofed quadrupeds, to which the
Sirenia are in other respects allied. The cetaceans or whales are widely
different from all other mammals, but the tertiary Zeuglodon and Squalodon,
which have been placed by some naturalists in an order by themselves, are
considered by Professor Huxley to be undoubtedly cetaceans, "and to
constitute connecting links with the aquatic carnivora."
Even the wide interval between birds and reptiles has been shown by the
naturalist just quoted to be partially bridged over in the most unexpected
manner, on the one hand, by the ostrich and extinct Archeopteryx, and on
the other hand by the Compsognathus, one of the Dinosaurians--that group
which includes the most gigantic of all terrestrial reptiles. Turning to
the Invertebrata, Barrande asserts, a higher authority could not be named,
that he is every day taught that, although palaeozoic animals can certainly
be classed under existing groups, yet that at this ancient period the
groups were not so distinctly separated from each other as they now are.
Some writers have objected to any extinct species, or group of species,
being considered as intermediate between any two living species, or groups
of species. If by this term it is meant that an extinct form is directly
intermediate in all its characters between two living forms or groups, the
objection is probably valid. But in a natural classification many fossil
species certainly 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 by a score of characters, the ancient members are
separated by a somewhat lesser number of characters, so that the two groups
formerly made a somewhat nearer approach to each other than they now do.
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 recent members of the same classes, we must
admit that there is 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 in italics 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 subfamily; and o14,
i14, 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 progenitor. On the principle of the continued
tendency to divergence of character, which was formerly illustrated by this
diagram, the more recent any form is the more it will generally differ from
its ancient progenitor. Hence, we can understand the rule that the most
ancient fossils differ most from existing forms. We must not, however,
assume that divergence of character is a necessary contingency; it depends
solely on the descendants from a species being thus enabled to seize on
many and different places in the economy of nature. Therefore it is quite
possible, as we have seen in the case of some Silurian forms, that a
species might go on being slightly modified in relation to its slightly
altered conditions of life, and yet retain throughout a vast period the
same general characteristics. This is represented in the diagram by the
letter F14.
All the many forms, extinct and recent, descended from (A), make, as before
remarked, one order; and this order, from the continued effects of
extinction and divergence of character, has become divided into several
sub-families and families, some of which are supposed to have perished at
different periods, and some to have endured to the present day.
By looking at the diagram we can see that if many of the extinct forms
supposed to be embedded in the successive formations, were discovered at
several points low down in the series, the three existing families on the
uppermost line would be rendered less distinct from each other. If, for
instance, the genera a1, a5, a10, 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 certain pachyderms. Yet he who objected to
consider as intermediate the extinct genera, which thus link together the
living genera of three families, would be partly justified, for 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 two of the families (those on the left hand a14, etc., and b14,
etc.) would have to be united into one; and there would remain two families
which would be less distinct from each other than they were before the
discovery of the fossils. So again, if the three families formed of eight
genera (a14 to m14), on the uppermost line, be supposed to differ from each
other by half-a-dozen important characters, then the families which existed
at a period marked VI would certainly have differed from each other by a
less number of characters; for they would at this early stage of descent
have diverged in a less degree from their common progenitor. Thus it comes
that ancient and extinct genera are often in a greater or less degree
intermediate in character between their modified descendants, or between
their collateral relations.
Under nature the process 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 rare cases, to fill up the wide intervals in the natural system,
and thus to 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 is frequently 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, are explained in a satisfactory manner. And they are
wholly inexplicable on any other view.
On this same theory, it is evident that the fauna during any one 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 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 palaeontologists as intermediate in
character between those of the overlying carboniferous and underlying
Silurian systems. 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, the species of mastodons and elephants, when
arranged by Dr. Falconer in two series--in the first place according to
their mutual affinities, and in the second place 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 complete, which is far from
the case, we have no reason to believe that forms successively produced
necessarily endure for corresponding lengths of time. A very ancient form
may occasionally have lasted 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 in serial
affinity, this arrangement would not closely accord with the order in time
of their production, and even less with the order of their disappearance;
for the parent rock-pigeon still 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 respect.
Closely connected with the statement, that the organic remains from an
intermediate formation are in some degree intermediate in character, is the
fact, insisted on by all palaeontologists, that fossils from two
consecutive formations are far more closely related to each other, than are
the fossils from two remote formations. Pictet gives as a well-known
instance, the general resemblance of the organic remains from the several
stages of the Chalk formation, though the species are distinct in each
stage. This fact alone, from its generality, seems to have shaken
Professor Pictet in his 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 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 epoch, 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 fossil remains from
closely consecutive formations, being closely related, though ranked as
distinct species, 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 in any 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 assuredly we do find.
We find, in short, such evidence of the slow and scarcely sensible
mutations of specific forms, as we have the right to expect.
ON THE STATE OF DEVELOPMENT OF ANCIENT COMPARED WITH LIVING FORMS.
We have seen in the fourth chapter that the degree of differentiation and
specialisation of the parts in organic beings, when arrived at maturity, is
the best standard, as yet suggested, of their degree of perfection or
highness. We have also seen that, as the specialisation of parts is an
advantage to each being, so natural selection will tend to render the
organisation of each being more specialised and perfect, and in this sense
higher; not but that it may leave many creatures with simple and unimproved
structures fitted for simple conditions of life, and in some cases will
even degrade or simplify the organisation, yet leaving such degraded beings
better fitted for their new walks of life. In another and more general
manner, new species become superior to their predecessors; for they have to
beat in the struggle for life all the older forms, with which they come
into close competition. We may therefore conclude that if under a nearly
similar climate the eocene inhabitants of the world could be put into
competition with the existing inhabitants, the former would be beaten and
exterminated by the latter, as would the secondary by the eocene, and the
palaeozoic by the secondary forms. So that by this fundamental test of
victory in the battle for life, as well as by the standard of the
specialisation of organs, modern forms ought, on the theory of natural
selection, to stand higher than ancient forms. Is this the case? A large
majority of palaeontologists would answer in the affirmative; and it seems
that this answer must be admitted as true, though difficult of proof.
It is no valid objection to this conclusion, that certain Brachiopods have
been but slightly modified from an extremely remote geological epoch; and
that certain land and fresh-water shells have remained nearly the same,
from the time when, as far as is known, they first appeared. It is not an
insuperable difficulty that Foraminifera have not, as insisted on by Dr.
Carpenter, progressed in organisation since even the Laurentian epoch; for
some organisms would have to remain fitted for simple conditions of life,
and what could be better fitted for this end than these lowly organised
Protozoa? Such objections as the above would be fatal to my view, if it
included advance in organisation as a necessary contingent. They would
likewise be fatal, if the above Foraminifera, for instance, could be proved
to have first come into existence during the Laurentian epoch, or the above
Brachiopods during the Cambrian formation; for in this case, there would
not have been time sufficient for the development of these organisms up to
the standard which they had then reached. When advanced up to any given
point, there is no necessity, on the theory of natural selection, for their
further continued process; though they will, during each successive age,
have to be slightly modified, so as to hold their places in relation to
slight changes in their conditions. The foregoing objections hinge on the
question whether we really know how old the world is, and at what period
the various forms of life first appeared; and this may well be disputed.
The problem whether organisation on the whole has advanced is in many ways
excessively intricate. The geological record, at all times imperfect, does
not extend far enough back to show with unmistakable clearness that within
the known history of the world organisation has largely advanced. Even at
the present day, looking to members of the same class, naturalists are not
unanimous which forms ought to be ranked as highest: thus, some look at
the selaceans or sharks, from their approach in some important points of
structure to reptiles, as the highest fish; others look at the teleosteans
as the highest. The ganoids stand intermediate between the selaceans and
teleosteans; the latter at the present day are largely preponderant in
number; but formerly selaceans and ganoids alone existed; and in this case,
according to the standard of highness chosen, so will it be said that
fishes have advanced or retrograded in organisation. To attempt to compare
members of distinct types in the scale of highness seems hopeless; who will
decide whether a cuttle-fish be higher than a bee--that insect which the
great Von Baer believed to be "in fact more highly organised than a fish,
although upon another type?" In the complex struggle for life it is quite
credible that crustaceans, not very high in their own class, might beat
cephalopods, the highest molluscs; and such crustaceans, though not highly
developed, would stand very high in the scale of invertebrate animals, if
judged by the most decisive of all trials--the law of battle. Beside these
inherent difficulties in deciding which forms are the most advanced in
organisation, we ought not solely to compare the highest members of a class
at any two periods--though undoubtedly this is one and perhaps the most
important element in striking a balance--but we ought to compare all the
members, high and low, at two periods. At an ancient epoch the highest and
lowest molluscoidal animals, namely, cephalopods and brachiopods, swarmed
in numbers; at the present time both groups are greatly reduced, while
others, intermediate in organisation, have largely increased; consequently
some naturalists maintain that molluscs were formerly more highly developed
than at present; but a stronger case can be made out on the opposite side,
by considering the vast reduction of brachiopods, and the fact that our
existing cephalopods, though few in number, are more highly organised than
their ancient representatives. We ought also to compare the relative
proportional numbers, at any two periods, of the high and low classes
throughout the world: if, for instance, at the present day fifty thousand
kinds of vertebrate animals exist, and if we knew that at some former
period only ten thousand kinds existed, we ought to look at this increase
in number in the highest class, which implies a great displacement of lower
forms, as a decided advance in the organisation of the world. We thus see
how hopelessly difficult it is to compare with perfect fairness, under such
extremely complex relations, the standard of organisation of the
imperfectly-known faunas of successive periods.
We shall appreciate this difficulty more clearly by looking to certain
existing faunas and floras. 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 by the indigenes, we
must believe, that if all the animals and plants of Great Britain were set
free in New Zealand, a multitude of British forms would in the course of
time become thoroughly naturalized there, and would exterminate many of the
natives. On the other hand, from the fact that hardly a single inhabitant
of the southern hemisphere has become wild in any part of Europe, we may
well doubt whether, if all the productions of New Zealand were set free in
Great Britain, 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 stand much higher in the scale than those
of New Zealand. Yet the most skilful naturalist, from an examination of
the species of the two countries, could not have foreseen this result.
Agassiz and several other highly competent judges insist that ancient
animals resemble to a certain extent the embryos of recent animals
belonging to the same classes; and that the geological succession of
extinct forms is nearly parallel with the embryological development of
existing forms. This view accords admirably well with our theory. In a
future chapter I shall attempt to show that the adult differs from its
embryo, owing to variations having supervened at a not early age, and
having been 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
former and less modified condition of the species. This view may be true,
and yet may never be capable of proof. Seeing, for instance, that the
oldest known mammals, reptiles, and fishes strictly belong to their 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 rich in fossils are
discovered far beneath the lowest Cambrian strata--a discovery of which the
chance is small.
ON THE SUCCESSION OF THE SAME TYPES WITHIN THE SAME AREAS, DURING THE LATER
TERTIARY PERIODS.
Mr. Clift many years ago showed that the fossil mammals from the Australian
caves were closely allied to the living marsupials of that continent. In
South America, a similar relationship is manifest, even to an uneducated
eye, in the gigantic pieces of armour, like those of the armadillo, found
in several parts of La Plata; and Professor Owen has shown in the most
striking manner that most of the fossil mammals, buried there in such
numbers, are related to South American types. This relationship is even
more clearly seen in the wonderful collection of fossil bones made by MM.
Lund and Clausen in the caves of Brazil. I was so much impressed with
these facts that I strongly insisted, in 1839 and 1845, on this "law of the
succession of types,"--on "this wonderful relationship in the same
continent between the dead and the living." Professor Owen has
subsequently extended the same generalisation to the mammals of the Old
World. We see the same law in this author's restorations of the extinct
and gigantic birds of New Zealand. We see it also in the birds of the
caves of Brazil. Mr. Woodward has shown that the same law holds good with
sea-shells, but, from the wide distribution of most 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 through dissimilar
physical conditions, for the dissimilarity of the inhabitants of these two
continents; and, on the other hand through similarity of conditions, for
the uniformity of the same types in each continent 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
intermigration, the feebler will yield to the more dominant forms, and
there will be nothing immutable in the distribution of organic beings.
It may be asked in ridicule whether I suppose that the megatherium and
other allied huge monsters, which formerly lived in South America, have
left behind them 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 all other characters to the species still living in South America; and
some of these fossils may have been the actual progenitors of the living
species. It must not be forgotten that, on our theory, all the species of
the same genus are the descendants of some one species; so that, if six
genera, each having eight species, be found in one geological formation,
and in a succeeding formation there be six other allied or representative
genera, each with the same number of species, then we may conclude that
generally only one species of each of the older genera has left modified
descendants, which constitute the new genera containing the several
species; the other seven species of each old genus having died out and left
no progeny. Or, and this will be a far commoner case, two or three species
in two or three alone of the six older genera will be the parents of the
new genera: the other species and the other old genera having become
utterly extinct. In failing orders, with the genera and species decreasing
in numbers as is the case with the Edentata of South America, still fewer
genera and species will leave modified blood-descendants.
SUMMARY OF THE PRECEDING AND PRESENT CHAPTERS.
I have attempted to show that the geological record is extremely imperfect;
that only a small portion of the globe has been geologically explored with
care; that only certain classes of organic beings have been largely
preserved in a fossil state; that the number both of specimens and of
species, preserved in our museums, is absolutely as nothing compared with
the number of generations which must have passed away even during a single
formation; that, owing to subsidence being almost necessary for the
accumulation of deposits rich in fossil species of many kinds, and thick
enough to outlast future degradation, great intervals of time must have
elapsed between most of our 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 probably
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 frequently, and have oftenest given rise to new species; that
varieties have at first been local; and lastly, although each species must
have passed through numerous transitional stages, it is probable that the
periods, during which each underwent modification, though many and long as
measured by years, have been short in comparison with the periods during
which each remained in an unchanged condition. These causes, taken
conjointly, will to a large extent explain why--though we do find many
links--we do not find interminable varieties, connecting together all
extinct and existing forms by the finest graduated steps. It should also
be constantly borne in mind that any linking variety between two forms,
which might be found, would be ranked, unless the whole chain could be
perfectly restored, as a new and distinct species; for it is not pretended
that we have any sure criterion by which species and varieties can be
discriminated.
He who rejects this view of the imperfection of the geological record, will
rightly reject the 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 successive stages of
the same great formation? He may disbelieve in the immense intervals of
time which must have elapsed between our consecutive formations; he may
overlook how important a part migration has played, when the formations of
any one great region, as those 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 Cambrian system was
deposited? We now know that at least one animal did then exist; but I can
answer this last question only by supposing that where our oceans now
extend they have extended for an enormous period, and where our oscillating
continents now stand they have stood since the commencement of the Cambrian
system; but that, long before that epoch, the world presented a widely
different aspect; and that the older continents, formed of formations older
than any known to us, exist now only as remnants in a metamorphosed
condition, or lie still buried under the ocean.
Passing from these difficulties, the other great leading facts in
palaeontology agree admirably with the theory of descent with modification
through variation and 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 belonging to large and dominant groups
tend to leave many modified descendants, which form new sub-groups and
groups. 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 has sometimes
been a 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 it is that dominant forms which spread widely and
yield the greatest number of varieties tend to people the world with
allied, but modified, descendants; and these will generally succeed in
displacing the groups which are their inferiors in the struggle for
existence. Hence, after long intervals of time, the productions of the
world appear to have changed simultaneously.
We can understand how it is that all the forms of life, ancient and recent,
make together a few grand classes. 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
bringing them only a little closer together. The more ancient a form is,
the more often it stands 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 other extinct and different forms. We can
clearly see why the organic remains of closely consecutive formations are
closely allied; for they are closely linked together by generation. We can
clearly see why the remains of an intermediate formation are intermediate
in character.
The inhabitants of the world at each successive period in its history have
beaten their predecessors in the race for life, and are, in so far, higher
in the scale, and their structure has generally become more specialised;
and this may account for the common belief held by so many
palaeontologists, that organisation on the whole has progressed. Extinct
and ancient animals resemble to a certain extent the embryos of the more
recent animals belonging to the same classes, and this wonderful fact
receives a simple explanation according to our views. The succession of
the same types of structure within the same areas during the later
geological periods ceases to be mysterious, and is intelligible on the
principle of inheritance.
If, then, the geological record be as imperfect as many believe, 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, the products of variation and the survival of
the fittest.
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