Chapter X

On the Geological Succession of Organic Beings

On the slow and successive appearance of new species -- On their different

rates of change -- Species once lost do not reappear -- Groups of species

follow the same general rules in their appearance and disappearance as do

single species -- On Extinction -- On simultaneous changes in the forms of

life throughout the world -- On the affinities of extinct species to each

other and to living species -- On the state of development of ancient forms

-- On the succession of the same types within the same areas -- Summary of

preceding and present chapters.

Let us now see whether the several facts and rules relating to the

geological succession of organic beings, better accord with the common view

of the immutability of species, or with that of their slow and gradual

modification, through descent and natural selection.

New species have appeared very slowly, one after another, both on the land

and in the waters. Lyell has shown that it is hardly possible to resist

the evidence on this head in the case of the several tertiary stages; and

every year tends to fill up the blanks between them, and to make the

percentage system of lost and new forms more gradual. In some of the most

recent beds, though undoubtedly of high antiquity if measured by years,

only one or two species are lost forms, and only one or two are new forms,

having here appeared for the first time, either locally, or, as far as we

know, on the face of the earth. If we may trust the observations of

Philippi in Sicily, the successive changes in the marine inhabitants of

that island have been many and most gradual. The secondary formations are

more broken; but, as Bronn has remarked, neither the appearance nor

disappearance of their many now extinct species has been simultaneous in

each separate formation.

Species of different genera and classes have not changed at the same rate,

or in the same degree. In the oldest tertiary beds a few living shells may

still be found in the midst of a multitude of extinct forms. Falconer has

given a striking instance of a similar fact, in an existing crocodile

associated with many strange and lost mammals and reptiles in the

sub-Himalayan deposits. The Silurian Lingula differs but little from the

living species of this genus; whereas most of the other Silurian Molluscs

and all the Crustaceans have changed greatly. The productions of the land

seem to change at a quicker rate than those of the sea, of which a striking

instance has lately been observed in Switzerland. There is some reason to

believe that organisms, considered high in the scale of nature, change more

quickly than those that are low: though there are exceptions to this rule.

The amount of organic change, as Pictet has remarked, does not strictly

correspond with the succession of our geological formations; so that

between each two consecutive formations, the forms of life have seldom

changed in exactly the same degree. Yet if we compare any but the most

closely related formations, all the species will be found to have undergone

some change. When a species has once disappeared from the face of the

earth, we have reason to believe that the same identical form never

reappears. The strongest apparent exception to this latter rule, is that

of the so-called 'colonies' of M. Barrande, which intrude for a period in

the midst of an older formation, and then allow the pre-existing fauna to

reappear; but Lyell's explanation, namely, that it is a case of temporary

migration from a distinct geographical province, seems to me satisfactory.

These several facts accord well with my theory. I believe in no fixed law

of development, causing all the inhabitants of a country to change

abruptly, or simultaneously, or to an equal degree. The process of

modification must be extremely slow. The variability of each species is

quite independent of that of all others. Whether such variability be taken

advantage of by natural selection, and whether the variations be

accumulated to a greater or lesser amount, thus causing a greater or lesser

amount of modification in the varying species, depends on many complex

contingencies,--on the variability being of a beneficial nature, on the

power of intercrossing, on the rate of breeding, on the slowly changing

physical conditions of the country, and more especially on the nature of

the other inhabitants with which the varying species comes into

competition. Hence it is by no means surprising that one species should

retain the same identical form much longer than others; or, if changing,

that it should change less. We see the same fact in geographical

distribution; for instance, in the land-shells and coleopterous insects of

Madeira having come to differ considerably from their nearest allies on the

continent of Europe, whereas the marine shells and birds have remained

unaltered. We can perhaps understand the apparently quicker rate of change

in terrestrial and in more highly organised productions compared with

marine and lower productions, by the more complex relations of the higher

beings to their organic and inorganic conditions of life, as explained in a

former chapter. When many of the inhabitants of a country have become

modified and improved, we can understand, on the principle of competition,

and on that of the many all-important relations of organism to organism,

that any form which does not become in some degree modified and improved,

will be liable to be exterminated. Hence we can see why all the species in

the same region do at last, if we look to wide enough intervals of time,

become modified; for those which do not change will become extinct.

In members of the same class the average amount of change, during long and

equal periods of time, may, perhaps, be nearly the same; but as the

accumulation of long-enduring fossiliferous formations depends on great

masses of sediment having been deposited on areas whilst subsiding, our

formations have been almost necessarily accumulated at wide and irregularly

intermittent intervals; consequently the amount of organic change exhibited

by the fossils embedded in consecutive formations is not equal. Each

formation, on this view, does not mark a new and complete act of creation,

but only an occasional scene, taken almost at hazard, in a slowly changing

drama.

We can clearly understand why a species when once lost should never

reappear, even if the very same conditions of life, organic and inorganic,

should recur. For though the offspring of one species might be adapted

(and no doubt this has occurred in innumerable instances) to fill the exact

place of another species in the economy of nature, and thus supplant it;

yet the two forms--the old and the new--would not be identically the same;

for both would almost certainly inherit different characters from their

distinct progenitors. For instance, it is just possible, if our

fantail-pigeons were all destroyed, that fanciers, by striving during long

ages for the same object, might make a new breed hardly distinguishable

from our present fantail; but if the parent rock-pigeon were also

destroyed, and in nature we have every reason to believe that the

parent-form will generally be supplanted and exterminated by its improved

offspring, it is quite incredible that a fantail, identical with the

existing breed, could be raised from any other species of pigeon, or even

from the other well-established races of the domestic pigeon, for the

newly-formed fantail would be almost sure to inherit from its new

progenitor some slight characteristic differences.

Groups of species, that is, genera and families, follow the same general

rules in their appearance and disappearance as do single species, changing

more or less quickly, and in a greater or lesser degree. A group does not

reappear after it has once disappeared; or its existence, as long as it

lasts, is continuous. I am aware that there are some apparent exceptions

to this rule, but the exceptions are surprisingly few, so few, that E.

Forbes, Pictet, and Woodward (though all strongly opposed to such views as

I maintain) admit its truth; and the rule strictly accords with my theory.

For as all the species of the same group have descended from some one

species, it is clear that as long as any species of the group have appeared

in the long succession of ages, so long must its members have continuously

existed, in order to have generated either new and modified or the same old

and unmodified forms. Species of the genus Lingula, for instance, must

have continuously existed by an unbroken succession of generations, from

the lowest Silurian stratum to the present day.

We have seen in the last chapter that the species of a group sometimes

falsely appear to have come in abruptly; and I have attempted to give an

explanation of this fact, which if true would have been fatal to my views.

But such cases are certainly exceptional; the general rule being a gradual

increase in number, till the group reaches its maximum, and then, sooner or

later, it gradually decreases. If the number of the species of a genus, or

the number of the genera of a family, be represented by a vertical line of

varying thickness, crossing the successive geological formations in which

the species are found, the line will sometimes falsely appear to begin at

its lower end, not in a sharp point, but abruptly; it then gradually

thickens upwards, sometimes keeping for a space of equal thickness, and

ultimately thins out in the upper beds, marking the decrease and final

extinction of the species. This gradual increase in number of the species

of a group is strictly conformable with my theory; as the species of the

same genus, and the genera of the same family, can increase only slowly and

progressively; for the process of modification and the production of a

number of allied forms must be slow and gradual,--one species giving rise

first to two or three varieties, these being slowly converted into species,

which in their turn produce by equally slow steps other species, and so on,

like the branching of a great tree from a single stem, till the group

becomes large.

On Extinction. -- We have as yet spoken only incidentally of the

disappearance of species and of groups of species. On the theory of

natural selection the extinction of old forms and the production of new and

improved forms are intimately connected together. The old notion of all

the inhabitants of the earth having been swept away at successive periods

by catastrophes, is very generally given up, even by those geologists, as

Elie de Beaumont, Murchison, Barrande, &c., whose general views would

naturally lead them to this conclusion. On the contrary, we have every

reason to believe, from the study of the tertiary formations, that species

and groups of species gradually disappear, one after another, first from

one spot, then from another, and finally from the world. Both single

species and whole groups of species last for very unequal periods; some

groups, as we have seen, having endured from the earliest known dawn of

life to the present day; some having disappeared before the close of the

palaeozoic period. No fixed law seems to determine the length of time

during which any single species or any single genus endures. There is

reason to believe that the complete extinction of the species of a group is

generally a slower process than their production: if the appearance and

disappearance of a group of species be represented, as before, by a

vertical line of varying thickness, the line is found to taper more

gradually at its upper end, which marks the progress of extermination, than

at its lower end, which marks the first appearance and increase in numbers

of the species. In some cases, however, the extermination of whole groups

of beings, as of ammonites towards the close of the secondary period, has

been wonderfully sudden.

The whole subject of the extinction of species has been involved in the

most gratuitous mystery. Some authors have even supposed that as the

individual has a definite length of life, so have species a definite

duration. No one I think can have marvelled more at the extinction of

species, than I have done. When I found in La Plata the tooth of a horse

embedded with the remains of Mastodon, Megatherium, Toxodon, and other

extinct monsters, which all co-existed with still living shells at a very

late geological period, I was filled with astonishment; for seeing that the

horse, since its introduction by the Spaniards into South America, has run

wild over the whole country and has increased in numbers at an unparalleled

rate, I asked myself what could so recently have exterminated the former

horse under conditions of life apparently so favourable. But how utterly

groundless was my astonishment! Professor Owen soon perceived that the

tooth, though so like that of the existing horse, belonged to an extinct

species. Had this horse been still living, but in some degree rare, no

naturalist would have felt the least surprise at its rarity; for rarity is

the attribute of a vast number of species of all classes, in all countries.

If we ask ourselves why this or that species is rare, we answer that

something is unfavourable in its conditions of life; but what that

something is, we can hardly ever tell. On the supposition of the fossil

horse still existing as a rare species, we might have felt certain from the

analogy of all other mammals, even of the slow-breeding elephant, and from

the history of the naturalisation of the domestic horse in South America,

that under more favourable conditions it would in a very few years have

stocked the whole continent. But we could not have told what the

unfavourable conditions were which checked its increase, whether some one

or several contingencies, and at what period of the horse's life, and in

what degree, they severally acted. If the conditions had gone on, however

slowly, becoming less and less favourable, we assuredly should not have

perceived the fact, yet the fossil horse would certainly have become rarer

and rarer, and finally extinct;--its place being seized on by some more

successful competitor.

It is most difficult always to remember that the increase of every living

being is constantly being checked by unperceived injurious agencies; and

that these same unperceived agencies are amply sufficient to cause rarity,

and finally extinction. We see in many cases in the more recent tertiary

formations, that rarity precedes extinction; and we know that this has been

the progress of events with those animals which have been exterminated,

either locally or wholly, through man's agency. I may repeat what I

published in 1845, namely, that to admit that species generally become rare

before they become extinct--to feel no surprise at the rarity of a species,

and yet to marvel greatly when it ceases to exist, is much the same as to

admit that sickness in the individual is the forerunner of death--to feel

no surprise at sickness, but when the sick man dies, to wonder and to

suspect that he died by some unknown deed of violence.

The theory of natural selection is grounded on the belief that each new

variety, and ultimately each new species, is produced and maintained by

having some advantage over those with which it comes into competition; and

the consequent extinction of less-favoured forms almost inevitably follows.

It is the same with our domestic productions: when a new and slightly

improved variety has been raised, it at first supplants the less improved

varieties in the same neighbourhood; when much improved it is transported

far and near, like our short-horn cattle, and takes the place of other

breeds in other countries. Thus the appearance of new forms and the

disappearance of old forms, both natural and artificial, are bound

together. In certain flourishing groups, the number of new specific forms

which have been produced within a given time is probably greater than that

of the old forms which have been exterminated; but we know that the number

of species has not gone on indefinitely increasing, at least during the

later geological periods, so that looking to later times we may believe

that the production of new forms has caused the extinction of about the

same number of old forms.

The competition will generally be most severe, as formerly explained and

illustrated by examples, between the forms which are most like each other

in all respects. Hence the improved and modified descendants of a species

will generally cause the extermination of the parent-species; and if many

new forms have been developed from any one species, the nearest allies of

that species, i.e. the species of the same genus, will be the most liable

to extermination. Thus, as I believe, a number of new species descended

from one species, that is a new genus, comes to supplant an old genus,

belonging to the same family. But it must often have happened that a new

species belonging to some one group will have seized on the place occupied

by a species belonging to a distinct group, and thus caused its

extermination; and if many allied forms be developed from the successful

intruder, many will have to yield their places; and it will generally be

allied forms, which will suffer from some inherited inferiority in common.

But whether it be species belonging to the same or to a distinct class,

which yield their places to other species which have been modified and

improved, a few of the sufferers may often long be preserved, from being

fitted to some peculiar line of life, or from inhabiting some distant and

isolated station, where they have escaped severe competition. For

instance, a single species of Trigonia, a great genus of shells in the

secondary formations, survives in the Australian seas; and a few members of

the great and almost extinct group of Ganoid fishes still inhabit our fresh

waters. Therefore the utter extinction of a group is generally, as we have

seen, a slower process than its production.

With respect to the apparently sudden extermination of whole families or

orders, as of Trilobites at the close of the palaeozoic period and of

Ammonites at the close of the secondary period, we must remember what has

been already said on the probable wide intervals of time between our

consecutive formations; and in these intervals there may have been much

slow extermination. Moreover, when by sudden immigration or by unusually

rapid development, many species of a new group have taken possession of a

new area, they will have exterminated in a correspondingly rapid manner

many of the old inhabitants; and the forms which thus yield their places

will commonly be allied, for they will partake of some inferiority in

common.

Thus, as it seems to me, the manner in which single species and whole

groups of species become extinct, accords well with the theory of natural

selection. We need not marvel at extinction; if we must marvel, let it be

at our presumption in imagining for a moment that we understand the many

complex contingencies, on which the existence of each species depends. If

we forget for an instant, that each species tends to increase inordinately,

and that some check is always in action, yet seldom perceived by us, the

whole economy of nature will be utterly obscured. Whenever we can

precisely say why this species is more abundant in individuals than that;

why this species and not another can be naturalised in a given country;

then, and not till then, we may justly feel surprise why we cannot account

for the extinction of this particular species or group of species.

On the Forms of Life changing almost simultaneously throughout the World. -

- Scarcely any palaeontological discovery is more striking than the fact,

that the forms of life change almost simultaneously throughout the world.

Thus our European Chalk formation can be recognised in many distant parts

of the world, under the most different climates, where not a fragment of

the mineral chalk itself can be found; namely, in North America, in

equatorial South America, in Tierra del Fuego, at the Cape of Good Hope,

and in the peninsula of India. For at these distant points, the organic

remains in certain beds present an unmistakeable degree of resemblance to

those of the Chalk. It is not that the same species are met with; for in

some cases not one species is identically the same, but they belong to the

same families, genera, and sections of genera, and sometimes are similarly

characterised in such trifling points as mere superficial sculpture.

Moreover other forms, which are not found in the Chalk of Europe, but which

occur in the formations either above or below, are similarly absent at

these distant points of the world. In the several successive palaeozoic

formations of Russia, Western Europe and North America, a similar

parallelism in the forms of life has been observed by several authors: so

it is, according to Lyell, with the several European and North American

tertiary deposits. Even if the few fossil species which are common to the

Old and New Worlds be kept wholly out of view, the general parallelism in

the successive forms of life, in the stages of the widely separated

palaeozoic and tertiary periods, would still be manifest, and the several

formations could be easily correlated.

These observations, however, relate to the marine inhabitants of distant

parts of the world: we have not sufficient data to judge whether the

productions of the land and of fresh water change at distant points in the

same parallel manner. We may doubt whether they have thus changed: if the

Megatherium, Mylodon, Macrauchenia, and Toxodon had been brought to Europe

from La Plata, without any information in regard to their geological

position, no one would have suspected that they had coexisted with still

living sea-shells; but as these anomalous monsters coexisted with the

Mastodon and Horse, it might at least have been inferred that they had

lived during one of the latter tertiary stages.

When the marine forms of life are spoken of as having changed

simultaneously throughout the world, it must not be supposed that this

expression relates to the same thousandth or hundred-thousandth year, or

even that it has a very strict geological sense; for if all the marine

animals which live at the present day in Europe, and all those that lived

in Europe during the pleistocene period (an enormously remote period as

measured by years, including the whole glacial epoch), were to be compared

with those now living in South America or in Australia, the most skilful

naturalist would hardly be able to say whether the existing or the

pleistocene inhabitants of Europe resembled most closely those of the

southern hemisphere. So, again, several highly competent observers believe

that the existing productions of the United States are more closely related

to those which lived in Europe during certain later tertiary stages, than

to those which now live here; and if this be so, it is evident that

fossiliferous beds deposited at the present day on the shores of North

America would hereafter be liable to be classed with somewhat older

European beds. Nevertheless, looking to a remotely future epoch, there

can, I think, be little doubt that all the more modern marine formations,

namely, the upper pliocene, the pleistocene and strictly modern beds, of

Europe, North and South America, and Australia, from containing fossil

remains in some degree allied, and from not including those forms which are

only found in the older underlying deposits, would be correctly ranked as

simultaneous in a geological sense.

The fact of the forms of life changing simultaneously, in the above large

sense, at distant parts of the world, has greatly struck those admirable

observers, MM. de Verneuil and d'Archiac. After referring to the

parallelism of the palaeozoic forms of life in various parts of Europe,

they add, 'If struck by this strange sequence, we turn our attention to

North America, and there discover a series of analogous phenomena, it will

appear certain that all these modifications of species, their extinction,

and the introduction of new ones, cannot be owing to mere changes in marine

currents or other causes more or less local and temporary, but depend on

general laws which govern the whole animal kingdom.' M. Barrande has made

forcible remarks to precisely the same effect. It is, indeed, quite futile

to look to changes of currents, climate, or other physical conditions, as

the cause of these great mutations in the forms of life throughout the

world, under the most different climates. We must, as Barrande has

remarked, look to some special law. We shall see this more clearly when we

treat of the present distribution of organic beings, and find how slight is

the relation between the physical conditions of various countries, and the

nature of their inhabitants.

This great fact of the parallel succession of the forms of life throughout

the world, is explicable on the theory of natural selection. New species

are formed by new varieties arising, which have some advantage over older

forms; and those forms, which are already dominant, or have some advantage

over the other forms in their own country, would naturally oftenest give

rise to new varieties or incipient species; for these latter must be

victorious in a still higher degree in order to be preserved and to

survive. We have distinct evidence on this head, in the plants which are

dominant, that is, which are commonest in their own homes, and are most

widely diffused, having produced the greatest number of new varieties. It

is also natural that the dominant, varying, and far-spreading species,

which already have invaded to a certain extent the territories of other

species, should be those which would have the best chance of spreading

still further, and of giving rise in new countries to new varieties and

species. The process of diffusion may often be very slow, being dependent

on climatal and geographical changes, or on strange accidents, but in the

long run the dominant forms will generally succeed in spreading. The

diffusion would, it is probable, be slower with the terrestrial inhabitants

of distinct continents than with the marine inhabitants of the continuous

sea. We might therefore expect to find, as we apparently do find, a less

strict degree of parallel succession in the productions of the land than of

the sea.

Dominant species spreading from any region might encounter still more

dominant species, and then their triumphant course, or even their

existence, would cease. We know not at all precisely what are all the

conditions most favourable for the multiplication of new and dominant

species; but we can, I think, clearly see that a number of individuals,

from giving a better chance of the appearance of favourable variations, and

that severe competition with many already existing forms, would be highly

favourable, as would be the power of spreading into new territories. A

certain amount of isolation, recurring at long intervals of time, would

probably be also favourable, as before explained. One quarter of the world

may have been most favourable for the production of new and dominant

species on the land, and another for those in the waters of the sea. If

two great regions had been for a long period favourably circumstanced in an

equal degree, whenever their inhabitants met, the battle would be prolonged

and severe; and some from one birthplace and some from the other might be

victorious. But in the course of time, the forms dominant in the highest

degree, wherever produced, would tend everywhere to prevail. As they

prevailed, they would cause the extinction of other and inferior forms; and

as these inferior forms would be allied in groups by inheritance, whole

groups would tend slowly to disappear; though here and there a single

member might long be enabled to survive.

Thus, as it seems to me, the parallel, and, taken in a large sense,

simultaneous, succession of the same forms of life throughout the world,

accords well with the principle of new species having been formed by

dominant species spreading widely and varying; the new species thus

produced being themselves dominant owing to inheritance, and to having

already had some advantage over their parents or over other species; these

again spreading, varying, and producing new species. The forms which are

beaten and which yield their places to the new and victorious forms, will

generally be allied in groups, from inheriting some inferiority in common;

and therefore as new and improved groups spread throughout the world, old

groups will disappear from the world; and the succession of forms in both

ways will everywhere tend to correspond.

There is one other remark connected with this subject worth making. I have

given my reasons for believing that all our greater fossiliferous

formations were deposited during periods of subsidence; and that blank

intervals of vast duration occurred during the periods when the bed of the

sea was either stationary or rising, and likewise when sediment was not

thrown down quickly enough to embed and preserve organic remains. During

these long and blank intervals I suppose that the inhabitants of each

region underwent a considerable amount of modification and extinction, and

that there was much migration from other parts of the world. As we have

reason to believe that large areas are affected by the same movement, it is

probable that strictly contemporaneous formations have often been

accumulated over very wide spaces in the same quarter of the world; but we

are far from having any right to conclude that this has invariably been the

case, and that large areas have invariably been affected by the same

movements. When two formations have been deposited in two regions during

nearly, but not exactly the same period, we should find in both, from the

causes explained in the foregoing paragraphs, the same general succession

in the forms of life; but the species would not exactly correspond; for

there will have been a little more time in the one region than in the other

for modification, extinction, and immigration.

I suspect that cases of this nature have occurred in Europe. Mr.

Prestwich, in his admirable Memoirs on the eocene deposits of England and

France, is able to draw a close general parallelism between the successive

stages in the two countries; but when he compares certain stages in England

with those in France, although he finds in both a curious accordance in the

numbers of the species belonging to the same genera, yet the species

themselves differ in a manner very difficult to account for, considering

the proximity of the two areas,--unless, indeed, it be assumed that an

isthmus separated two seas inhabited by distinct, but contemporaneous,

faunas. Lyell has made similar observations on some of the later tertiary

formations. Barrande, also, shows that there is a striking general

parallelism in the successive Silurian deposits of Bohemia and Scandinavia;

nevertheless he finds a surprising amount of difference in the species. If

the several formations in these regions have not been deposited during the

same exact periods,--a formation in one region often corresponding with a

blank interval in the other,--and if in both regions the species have gone

on slowly changing during the accumulation of the several formations and

during the long intervals of time between them; in this case, the several

formations in the two regions could be arranged in the same order, in

accordance with the general succession of the form of life, and the order

would falsely appear to be strictly parallel; nevertheless the species

would not all be the same in the apparently corresponding stages in the two

regions.

On the Affinities of extinct Species to each other, and to living forms. --

Let us now look to the mutual affinities of extinct and living species.

They all fall into one grand natural system; and this fact is at once

explained on the principle of descent. The more ancient any form is, the

more, as a general rule, it differs from living forms. But, as Buckland

long ago remarked, all fossils can be classed either in still existing

groups, or between them. That the extinct forms of life help to fill up

the wide intervals between existing genera, families, and orders, cannot be

disputed. For if we confine our attention either to the living or to the

extinct alone, the series is far less perfect than if we combine both into

one general system. With respect to the Vertebrata, whole pages could be

filled with striking illustrations from our great palaeontologist, Owen,

showing how extinct animals fall in between existing groups. Cuvier ranked

the Ruminants and Pachyderms, as the two most distinct orders of mammals;

but Owen has discovered so many fossil links, that he has had to alter the

whole classification of these two orders; and has placed certain pachyderms

in the same sub-order with ruminants: for example, he dissolves by fine

gradations the apparently wide difference between the pig and the camel.

In regard to the Invertebrata, Barrande, and a higher authority could not

be named, asserts that he is every day taught that palaeozoic animals,

though belonging to the same orders, families, or genera with those living

at the present day, were not at this early epoch limited in such distinct

groups as they now are.

Some writers have objected to any extinct species or group of species being

considered as intermediate between living species or groups. If by this

term it is meant that an extinct form is directly intermediate in all its

characters between two living forms, the objection is probably valid. But

I apprehend that in a perfectly natural classification many fossil species

would have to stand between living species, and some extinct genera between

living genera, even between genera belonging to distinct families. The

most common case, especially with respect to very distinct groups, such as

fish and reptiles, seems to be, that supposing them to be distinguished at

the present day from each other by a dozen characters, the ancient members

of the same two groups would be distinguished by a somewhat lesser number

of characters, so that the two groups, though formerly quite distinct, at

that period made some small approach to each other.

It is a common belief that the more ancient a form is, by so much the more

it tends to connect by some of its characters groups now widely separated

from each other. This remark no doubt must be restricted to those groups

which have undergone much change in the course of geological ages; and it

would be difficult to prove the truth of the proposition, for every now and

then even a living animal, as the Lepidosiren, is discovered having

affinities directed towards very distinct groups. Yet if we compare the

older Reptiles and Batrachians, the older Fish, the older Cephalopods, and

the eocene Mammals, with the more recent members of the same classes, we

must admit that there is some truth in the remark.

Let us see how far these several facts and inferences accord with the

theory of descent with modification. As the subject is somewhat complex, I

must request the reader to turn to the diagram in the fourth chapter. We

may suppose that the numbered letters represent genera, and the dotted

lines diverging from them the species in each genus. The diagram is much

too simple, too few genera and too few species being given, but this is

unimportant for us. The horizontal lines may represent successive

geological formations, and all the forms beneath the uppermost line may be

considered as extinct. The three existing genera, a14, q14, p14, will form

a small family; b14 and f14 a closely allied family or sub-family; and o14,

e14, m14, a third family. These three families, together with the many

extinct genera on the several lines of descent diverging from the

parent-form A, will form an order; for all will have inherited something in

common from their ancient and common progenitor. On the principle of the

continued tendency to divergence of character, which was formerly

illustrated by this diagram, the more recent any form is, the more it will

generally differ from its ancient progenitor. Hence we can understand the

rule that the most ancient fossils differ most from existing forms. We

must not, however, assume that divergence of character is a necessary

contingency; it depends solely on the descendants from a species being thus

enabled to seize on many and different places in the economy of nature.

Therefore it is quite possible, as we have seen in the case of some

Silurian forms, that a species might go on being slightly modified in

relation to its slightly altered conditions of life, and yet retain

throughout a vast period the same general characteristics. This is

represented in the diagram by the letter F14.

All the many forms, extinct and recent, descended from A, make, as before

remarked, one order; and this order, from the continued effects of

extinction and divergence of character, has become divided into several

sub-families and families, some of which are supposed to have perished at

different periods, and some to have endured to the present day.

By looking at the diagram we can see that if many of the extinct forms,

supposed to be embedded in the successive formations, were discovered at

several points low down in the series, the three existing families on the

uppermost line would be rendered less distinct from each other. If, for

instance, the genera a1, a5, a10, f8, m3, m6, m9 were disinterred, these

three families would be so closely linked together that they probably would

have to be united into one great family, in nearly the same manner as has

occurred with ruminants and pachyderms. Yet he who objected to call the

extinct genera, which thus linked the living genera of three families

together, intermediate in character, would be justified, as they are

intermediate, not directly, but only by a long and circuitous course

through many widely different forms. If many extinct forms were to be

discovered above one of the middle horizontal lines or geological

formations--for instance, above No. VI.--but none from beneath this line,

then only the two families on the left hand (namely, a14, &c., and b14,

&c.) would have to be united into one family; and the two other families

(namely, a14 to f14 now including five genera, and o14 to m14) would yet

remain distinct. These two families, however, would be less distinct from

each other than they were before the discovery of the fossils. If, for

instance, we suppose the existing genera of the two families to differ from

each other by a dozen characters, in this case the genera, at the early

period marked VI., would differ by a lesser number of characters; for at

this early stage of descent they have not diverged in character from the

common progenitor of the order, nearly so much as they subsequently

diverged. Thus it comes that ancient and extinct genera are often in some

slight degree intermediate in character between their modified descendants,

or between their collateral relations.

In nature the case will be far more complicated than is represented in the

diagram; for the groups will have been more numerous, they will have

endured for extremely unequal lengths of time, and will have been modified

in various degrees. As we possess only the last volume of the geological

record, and that in a very broken condition, we have no right to expect,

except in very rare cases, to fill up wide intervals in the natural system,

and thus unite distinct families or orders. All that we have a right to

expect, is that those groups, which have within known geological periods

undergone much modification, should in the older formations make some

slight approach to each other; so that the older members should differ less

from each other in some of their characters than do the existing members of

the same groups; and this by the concurrent evidence of our best

palaeontologists seems frequently to be the case.

Thus, on the theory of descent with modification, the main facts with

respect to the mutual affinities of the extinct forms of life to each other

and to living forms, seem to me explained in a satisfactory manner. And

they are wholly inexplicable on any other view.

On this same theory, it is evident that the fauna of any great period in

the earth's history will be intermediate in general character between that

which preceded and that which succeeded it. Thus, the species which lived

at the sixth great stage of descent in the diagram are the modified

offspring of those which lived at the fifth stage, and are the parents of

those which became still more modified at the seventh stage; hence they

could hardly fail to be nearly intermediate in character between the forms

of life above and below. We must, however, allow for the entire extinction

of some preceding forms, and for the coming in of quite new forms by

immigration, and for a large amount of modification, during the long and

blank intervals between the successive formations. Subject to these

allowances, the fauna of each geological period undoubtedly is intermediate

in character, between the preceding and succeeding faunas. I need give

only one instance, namely, the manner in which the fossils of the Devonian

system, when this system was first discovered, were at once recognised by

palaeontologists as intermediate in character between those of the

overlying carboniferous, and underlying Silurian system. But each fauna is

not necessarily exactly intermediate, as unequal intervals of time have

elapsed between consecutive formations.

It is no real objection to the truth of the statement, that the fauna of

each period as a whole is nearly intermediate in character between the

preceding and succeeding faunas, that certain genera offer exceptions to

the rule. For instance, mastodons and elephants, when arranged by Dr.

Falconer in two series, first according to their mutual affinities and then

according to their periods of existence, do not accord in arrangement. The

species extreme in character are not the oldest, or the most recent; nor

are those which are intermediate in character, intermediate in age. But

supposing for an instant, in this and other such cases, that the record of

the first appearance and disappearance of the species was perfect, we have

no reason to believe that forms successively produced necessarily endure

for corresponding lengths of time: a very ancient form might occasionally

last much longer than a form elsewhere subsequently produced, especially in

the case of terrestrial productions inhabiting separated districts. To

compare small things with great: if the principal living and extinct races

of the domestic pigeon were arranged as well as they could be in serial

affinity, this arrangement would not closely accord with the order in time

of their production, and still less with the order of their disappearance;

for the parent rock-pigeon now lives; and many varieties between the

rock-pigeon and the carrier have become extinct; and carriers which are

extreme in the important character of length of beak originated earlier

than short-beaked tumblers, which are at the opposite end of the series in

this same respect.

Closely connected with the statement, that the organic remains from an

intermediate formation are in some degree intermediate in character, is the

fact, insisted on by all palaeontologists, that fossils from two

consecutive formations are far more closely related to each other, than are

the fossils from two remote formations. Pictet gives as a well-known

instance, the general resemblance of the organic remains from the several

stages of the chalk formation, though the species are distinct in each

stage. This fact alone, from its generality, seems to have shaken

Professor Pictet in his firm belief in the immutability of species. He who

is acquainted with the distribution of existing species over the globe,

will not attempt to account for the close resemblance of the distinct

species in closely consecutive formations, by the physical conditions of

the ancient areas having remained nearly the same. Let it be remembered

that the forms of life, at least those inhabiting the sea, have changed

almost simultaneously throughout the world, and therefore under the most

different climates and conditions. Consider the prodigious vicissitudes of

climate during the pleistocene period, which includes the whole glacial

period, and note how little the specific forms of the inhabitants of the

sea have been affected.

On the theory of descent, the full meaning of the fact of fossil remains

from closely consecutive formations, though ranked as distinct species,

being closely related, is obvious. As the accumulation of each formation

has often been interrupted, and as long blank intervals have intervened

between successive formations, we ought not to expect to find, as I

attempted to show in the last chapter, in any one or two formations all the

intermediate varieties between the species which appeared at the

commencement and close of these periods; but we ought to find after

intervals, very long as measured by years, but only moderately long as

measured geologically, closely allied forms, or, as they have been called

by some authors, representative species; and these we assuredly do find.

We find, in short, such evidence of the slow and scarcely sensible mutation

of specific forms, as we have a just right to expect to find.

On the state of Development of Ancient Forms. -- There has been much

discussion whether recent forms are more highly developed than ancient. I

will not here enter on this subject, for naturalists have not as yet

defined to each other's satisfaction what is meant by high and low forms.

But in one particular sense the more recent forms must, on my theory, be

higher than the more ancient; for each new species is formed by having had

some advantage in the struggle for life over other and preceding forms. If

under a nearly similar climate, the eocene inhabitants of one quarter of

the world were put into competition with the existing inhabitants of the

same or some other quarter, the eocene fauna or flora would certainly be

beaten and exterminated; as would a secondary fauna by an eocene, and a

palaeozoic fauna by a secondary fauna. I do not doubt that this process of

improvement has affected in a marked and sensible manner the organisation

of the more recent and victorious forms of life, in comparison with the

ancient and beaten forms; but I can see no way of testing this sort of

progress. Crustaceans, for instance, not the highest in their own class,

may have beaten the highest molluscs. From the extraordinary manner in

which European productions have recently spread over New Zealand, and have

seized on places which must have been previously occupied, we may believe,

if all the animals and plants of Great Britain were set free in New

Zealand, that in the course of time a multitude of British forms would

become thoroughly naturalized there, and would exterminate many of the

natives. On the other hand, from what we see now occurring in New Zealand,

and from hardly a single inhabitant of the southern hemisphere having

become wild in any part of Europe, we may doubt, if all the productions of

New Zealand were set free in Great Britain, whether any considerable number

would be enabled to seize on places now occupied by our native plants and

animals. Under this point of view, the productions of Great Britain may be

said to be higher than those of New Zealand. Yet the most skilful

naturalist from an examination of the species of the two countries could

not have foreseen this result.

Agassiz insists that ancient animals resemble to a certain extent the

embryos of recent animals of the same classes; or that the geological

succession of extinct forms is in some degree parallel to the embryological

development of recent forms. I must follow Pictet and Huxley in thinking

that the truth of this doctrine is very far from proved. Yet I fully

expect to see it hereafter confirmed, at least in regard to subordinate

groups, which have branched off from each other within comparatively recent

times. For this doctrine of Agassiz accords well with the theory of

natural selection. In a future chapter I shall attempt to show that the

adult differs from its embryo, owing to variations supervening at a not

early age, and being inherited at a corresponding age. This process,

whilst it leaves the embryo almost unaltered, continually adds, in the

course of successive generations, more and more difference to the adult.

Thus the embryo comes to be left as a sort of picture, preserved by nature,

of the ancient and less modified condition of each animal. This view may

be true, and yet it may never be capable of full proof. Seeing, for

instance, that the oldest known mammals, reptiles, and fish strictly belong

to their own proper classes, though some of these old forms are in a slight

degree less distinct from each other than are the typical members of the

same groups at the present day, it would be vain to look for animals having

the common embryological character of the Vertebrata, until beds far

beneath the lowest Silurian strata are discovered--a discovery of which the

chance is very small.

On the Succession of the same Types within the same areas, during the later

tertiary periods. -- Mr. Clift many years ago showed that the fossil

mammals from the Australian caves were closely allied to the living

marsupials of that continent. In South America, a similar relationship is

manifest, even to an uneducated eye, in the gigantic pieces of armour like

those of the armadillo, found in several parts of La Plata; and Professor

Owen has shown in the most striking manner that most of the fossil mammals,

buried there in such numbers, are related to South American types. This

relationship is even more clearly seen in the wonderful collection of

fossil bones made by MM. Lund and Clausen in the caves of Brazil. I was so

much impressed with these facts that I strongly insisted, in 1839 and 1845,

on this 'law of the succession of types,'--on 'this wonderful relationship

in the same continent between the dead and the living.' Professor Owen has

subsequently extended the same generalisation to the mammals of the Old

World. We see the same law in this author's restorations of the extinct

and gigantic birds of New Zealand. We see it also in the birds of the

caves of Brazil. Mr. Woodward has shown that the same law holds good with

sea-shells, but from the wide distribution of most genera of molluscs, it

is not well displayed by them. Other cases could be added, as the relation

between the extinct and living land-shells of Madeira; and between the

extinct and living brackish-water shells of the Aralo-Caspian Sea.

Now what does this remarkable law of the succession of the same types

within the same areas mean? He would be a bold man, who after comparing

the present climate of Australia and of parts of South America under the

same latitude, would attempt to account, on the one hand, by dissimilar

physical conditions for the dissimilarity of the inhabitants of these two

continents, and, on the other hand, by similarity of conditions, for the

uniformity of the same types in each during the later tertiary periods.

Nor can it be pretended that it is an immutable law that marsupials should

have been chiefly or solely produced in Australia; or that Edentata and

other American types should have been solely produced in South America.

For we know that Europe in ancient times was peopled by numerous

marsupials; and I have shown in the publications above alluded to, that in

America the law of distribution of terrestrial mammals was formerly

different from what it now is. North America formerly partook strongly of

the present character of the southern half of the continent; and the

southern half was formerly more closely allied, than it is at present, to

the northern half. In a similar manner we know from Falconer and Cautley's

discoveries, that northern India was formerly more closely related in its

mammals to Africa than it is at the present time. Analogous facts could be

given in relation to the distribution of marine animals.

On the theory of descent with modification, the great law of the long

enduring, but not immutable, succession of the same types within the same

areas, is at once explained; for the inhabitants of each quarter of the

world will obviously tend to leave in that quarter, during the next

succeeding period of time, closely allied though in some degree modified

descendants. If the inhabitants of one continent formerly differed greatly

from those of another continent, so will their modified descendants still

differ in nearly the same manner and degree. But after very long intervals

of time and after great geographical changes, permitting much

inter-migration, the feebler will yield to the more dominant forms, and

there will be nothing immutable in the laws of past and present

distribution.

It may be asked in ridicule, whether I suppose that the megatherium and

other allied huge monsters have left behind them in South America the

sloth, armadillo, and anteater, as their degenerate descendants. This

cannot for an instant be admitted. These huge animals have become wholly

extinct, and have left no progeny. But in the caves of Brazil, there are

many extinct species which are closely allied in size and in other

characters to the species still living in South America; and some of these

fossils may be the actual progenitors of living species. It must not be

forgotten that, on my theory, all the species of the same genus have

descended from some one species; so that if six genera, each having eight

species, be found in one geological formation, and in the next succeeding

formation there be six other allied or representative genera with the same

number of species, then we may conclude that only one species of each of

the six older genera has left modified descendants, constituting the six

new genera. The other seven species of the old genera have all died out

and have left no progeny. Or, which would probably be a far commoner case,

two or three species of two or three alone of the six older genera will

have been the parents of the six new genera; the other old species and the

other whole genera having become utterly extinct. In failing orders, with

the genera and species decreasing in numbers, as apparently is the case of

the Edentata of South America, still fewer genera and species will have

left modified blood-descendants.

Summary of the preceding and present Chapters -- I have attempted to show

that the geological record is extremely imperfect; that only a small

portion of the globe has been geologically explored with care; that only

certain classes of organic beings have been largely preserved in a fossil

state; that the number both of specimens and of species, preserved in our

museums, is absolutely as nothing compared with the incalculable number of

generations which must have passed away even during a single formation;

that, owing to subsidence being necessary for the accumulation of

fossiliferous deposits thick enough to resist future degradation, enormous

intervals of time have elapsed between the successive formations; that

there has probably been more extinction during the periods of subsidence,

and more variation during the periods of elevation, and during the latter

the record will have been least perfectly kept; that each single formation

has not been continuously deposited; that the duration of each formation

is, perhaps, short compared with the average duration of specific forms;

that migration has played an important part in the first appearance of new

forms in any one area and formation; that widely ranging species are those

which have varied most, and have oftenest given rise to new species; and

that varieties have at first often been local. All these causes taken

conjointly, must have tended to make the geological record extremely

imperfect, and will to a large extent explain why we do not find

interminable varieties, connecting together all the extinct and existing

forms of life by the finest graduated steps.

He who rejects these views on the nature of the geological record, will

rightly reject my whole theory. For he may ask in vain where are the

numberless transitional links which must formerly have connected the

closely allied or representative species, found in the several stages of

the same great formation. He may disbelieve in the enormous intervals of

time which have elapsed between our consecutive formations; he may overlook

how important a part migration must have played, when the formations of any

one great region alone, as that of Europe, are considered; he may urge the

apparent, but often falsely apparent, sudden coming in of whole groups of

species. He may ask where are the remains of those infinitely numerous

organisms which must have existed long before the first bed of the Silurian

system was deposited: I can answer this latter question only

hypothetically, by saying that as far as we can see, where our oceans now

extend they have for an enormous period extended, and where our oscillating

continents now stand they have stood ever since the Silurian epoch; but

that long before that period, the world may have presented a wholly

different aspect; and that the older continents, formed of formations older

than any known to us, may now all be in a metamorphosed condition, or may

lie buried under the ocean.

Passing from these difficulties, all the other great leading facts in

palaeontology seem to me simply to follow on the theory of descent with

modification through natural selection. We can thus understand how it is

that new species come in slowly and successively; how species of different

classes do not necessarily change together, or at the same rate, or in the

same degree; yet in the long run that all undergo modification to some

extent. The extinction of old forms is the almost inevitable consequence

of the production of new forms. We can understand why when a species has

once disappeared it never reappears. Groups of species increase in numbers

slowly, and endure for unequal periods of time; for the process of

modification is necessarily slow, and depends on many complex

contingencies. The dominant species of the larger dominant groups tend to

leave many modified descendants, and thus new sub-groups and groups are

formed. As these are formed, the species of the less vigorous groups, from

their inferiority inherited from a common progenitor, tend to become

extinct together, and to leave no modified offspring on the face of the

earth. But the utter extinction of a whole group of species may often be a

very slow process, from the survival of a few descendants, lingering in

protected and isolated situations. When a group has once wholly

disappeared, it does not reappear; for the link of generation has been

broken.

We can understand how the spreading of the dominant forms of life, which

are those that oftenest vary, will in the long run tend to people the world

with allied, but modified, descendants; and these will generally succeed in

taking the places of those groups of species which are their inferiors in

the struggle for existence. Hence, after long intervals of time, the

productions of the world will appear to have changed simultaneously.

We can understand how it is that all the forms of life, ancient and recent,

make together one grand system; for all are connected by generation. We

can understand, from the continued tendency to divergence of character, why

the more ancient a form is, the more it generally differs from those now

living. Why ancient and extinct forms often tend to fill up gaps between

existing forms, sometimes blending two groups previously classed as

distinct into one; but more commonly only bringing them a little closer

together. The more ancient a form is, the more often, apparently, it

displays characters in some degree intermediate between groups now

distinct; for the more ancient a form is, the more nearly it will be

related to, and consequently resemble, the common progenitor of groups,

since become widely divergent. Extinct forms are seldom directly

intermediate between existing forms; but are intermediate only by a long

and circuitous course through many extinct and very different forms. We

can clearly see why the organic remains of closely consecutive formations

are more closely allied to each other, than are those of remote formations;

for the forms are more closely linked together by generation: we can

clearly see why the remains of an intermediate formation are intermediate

in character.

The inhabitants of each successive period in the world's history have

beaten their predecessors in the race for life, and are, in so far, higher

in the scale of nature; and this may account for that vague yet ill-defined

sentiment, felt by many palaeontologists, that organisation on the whole

has progressed. If it should hereafter be proved that ancient animals

resemble to a certain extent the embryos of more recent animals of the same

class, the fact will be intelligible. The succession of the same types of

structure within the same areas during the later geological periods ceases

to be mysterious, and is simply explained by inheritance.

If then the geological record be as imperfect as I believe it to be, and it

may at least be asserted that the record cannot be proved to be much more

perfect, the main objections to the theory of natural selection are greatly

diminished or disappear. On the other hand, all the chief laws of

palaeontology plainly proclaim, as it seems to me, that species have been

produced by ordinary generation: old forms having been supplanted by new

and improved forms of life, produced by the laws of variation still acting

round us, and preserved by Natural Selection.