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 spoken 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, &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. 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
palæozoic period. No fixed law seems to determine the length of time
during which any single species or any single genus endures. There is
reason to believe that the 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 palæozoic period, and
of Ammonites at the close of the secondary period, we must remember
what has been already said on the probable wide intervals of time
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 palæontological discovery is more striking than the fact
that the forms of life change almost simultaneously throughout the
world. Thus our European Chalk formation can be recognised in many
distant 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 palæozoic formations of Russia, Western
Europe and North America, a similar parallelism in the forms of life
has been observed by several authors; so it is, according to Lyell,
with the 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 palæozoic 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 palæozoic forms of life in various parts of
Europe, they add, “If struck by this strange sequence, we turn our
attention to North America, and there discover a series of analogous
phenomena, it will appear certain that all these modifications of
species, their extinction, and the introduction of new ones, cannot be
owing to mere changes in marine currents or other causes more or less
local and temporary, but depend on general laws which govern the whole
animal kingdom.” M. Barrande has made forcible remarks to precisely the
same effect. It is, indeed, quite futile to look to changes of
currents, climate, or other physical conditions, as the cause of these
great mutations in the forms of life throughout the world, under the
most different climates. We must, as Barrande has remarked, look to
some special law. We shall see this more clearly when we treat of the
present distribution of organic beings, and find how slight is the
relation between the physical conditions of various countries and the
nature of their inhabitants.
This great fact of the parallel succession of the forms of life
throughout the world, is explicable on the theory of natural selection.
New species are formed by 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 palæontologist, 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 palæozoic 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, _a_14, _q_14, _p_14, will form a small family; _b_14
and _f_14, a closely allied family or subfamily; and _o_14, _e_14,
_m_14, a third family. These three families, together with the many
extinct genera on the several lines of descent diverging from the
parent form (A) will form an order; for all will have inherited
something in common from their ancient 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 _a_1, _a_5, _a_10, _f_8, _m_3, _m_6, _m_9,
were disinterred, these three families would be so closely linked
together that they probably would have to be united into one great
family, in nearly the same manner as has occurred with ruminants and
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 _a_14, &c., and _b_14, &c.) 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
(_a_14 to _m_14), 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
palæontologists 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 palæontologists 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 palæontologists, that fossils from two
consecutive formations are far more closely related to each other, than
are the fossils from two remote formations. Pictet gives as a
well-known instance, the general resemblance of the organic remains
from the several stages of the Chalk formation, though the species are
distinct in each stage. This fact alone, from its generality, seems to
have shaken Professor Pictet in his 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 palæozoic 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
palæontologists 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
palæontology 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
palæontologists, 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 palæontology plainly proclaim, as it seems to me, that species
have been produced by ordinary generation: old forms having been
supplanted by new and improved forms of life, the products of variation
and the survival of the fittest.