On the Origin of Species, 6th Edition
by
Charles Darwin
Part 7 out of 11
descendants. But we continually overrate the perfection of the geological
record, and falsely infer, because certain genera or families have not been
found beneath a certain stage, that they did not exist before that stage.
In all cases positive palaeontological evidence may be implicitly trusted;
negative evidence is worthless, as experience has so often shown. We
continually forget how large the world is, compared with the area over
which our geological formations have been carefully examined; we forget
that groups of species may elsewhere have long existed, and have slowly
multiplied, before they invaded the ancient archipelagoes of Europe and the
United States. We do not make due allowance for the enormous intervals of
time which have elapsed between our consecutive formations, longer perhaps
in many cases than the time required for the accumulation of each
formation. These intervals will have given time for the multiplication of
species from some one parent-form: and in the succeeding formation, such
groups or species will appear as if suddenly created.
I may here recall a remark formerly made, namely, that it might require a
long succession of ages to adapt an organism to some new and peculiar line
of life, for instance, to fly through the air; and consequently that the
transitional forms would often long remain confined to some one region; but
that, when this adaptation had once been effected, and a few species had
thus acquired a great advantage over other organisms, a comparatively short
time would be necessary to produce many divergent forms, which would spread
rapidly and widely throughout the world. Professor Pictet, in his
excellent Review of this work, in commenting on early transitional forms,
and taking birds as an illustration, cannot see how the successive
modifications of the anterior limbs of a supposed prototype could possibly
have been of any advantage. But look at the penguins of the Southern
Ocean; have not these birds their front limbs in this precise intermediate
state of "neither true arms nor true wings?" Yet these birds hold their
place victoriously in the battle for life; for they exist in infinite
numbers and of many kinds. I do not suppose that we here see the real
transitional grades through which the wings of birds have passed; but what
special difficulty is there in believing that it might profit the modified
descendants of the penguin, first to become enabled to flap along the
surface of the sea like the logger-headed duck, and ultimately to rise from
its surface and glide through the air?
I will now give a few examples to illustrate the foregoing remarks, and to
show how liable we are to error in supposing that whole groups of species
have suddenly been produced. Even in so short an interval as that between
the first and second editions of Pictet's great work on Palaeontology,
published in 1844-46 and in 1853-57, the conclusions on the first
appearance and disappearance of several groups of animals have been
considerably modified; and a third edition would require still further
changes. I may recall the well-known fact that in geological treatises,
published not many years ago, mammals were always spoken of as having
abruptly come in at the commencement of the tertiary series. And now one
of the richest known accumulations of fossil mammals belongs to the middle
of the secondary series; and true mammals have been discovered in the new
red sandstone at nearly the commencement of this great series. Cuvier used
to urge that no monkey occurred in any tertiary stratum; but now extinct
species have been discovered in India, South America and in Europe, as far
back as the miocene stage. Had it not been for the rare accident of the
preservation of footsteps in the new red sandstone of the United States,
who would have ventured to suppose that no less than at least thirty
different bird-like animals, some of gigantic size, existed during that
period? Not a fragment of bone has been discovered in these beds. Not
long ago, palaeontologists maintained that the whole class of birds came
suddenly into existence during the eocene period; but now we know, on the
authority of Professor Owen, that a bird certainly lived during the
deposition of the upper greensand; and still more recently, that strange
bird, the Archeopteryx, with a long lizard-like tail, bearing a pair of
feathers on each joint, and with its wings furnished with two free claws,
has been discovered in the oolitic slates of Solenhofen. Hardly any recent
discovery shows more forcibly than this how little we as yet know of the
former inhabitants of the world.
I may give another instance, which, from having passed under my own eyes
has much struck me. In a memoir on Fossil Sessile Cirripedes, I stated
that, from the large number of existing and extinct tertiary species; from
the extraordinary abundance of the individuals of many species all over the
world, from the Arctic regions to the equator, inhabiting various zones of
depths, from the upper tidal limits to fifty fathoms; from the perfect
manner in which specimens are preserved in the oldest tertiary beds; from
the ease with which even a fragment of a valve can be recognised; from all
these circumstances, I inferred that, had sessile cirripedes existed during
the secondary periods, they would certainly have been preserved and
discovered; and as not one species had then been discovered in beds of this
age, I concluded that this great group had been suddenly developed at the
commencement of the tertiary series. This was a sore trouble to me,
adding, as I then thought, one more instance of the abrupt appearance of a
great group of species. But my work had hardly been published, when a
skilful palaeontologist, M. Bosquet, sent me a drawing of a perfect
specimen of an unmistakable sessile cirripede, which he had himself
extracted from the chalk of Belgium. And, as if to make the case as
striking as possible, this cirripede was a Chthamalus, a very common,
large, and ubiquitous genus, of which not one species has as yet been found
even in any tertiary stratum. Still more recently, a Pyrgoma, a member of
a distinct subfamily of sessile cirripedes, has been discovered by Mr.
Woodward in the upper chalk; so that we now have abundant evidence of the
existence of this group of animals during the secondary period.
The case most frequently insisted on by palaeontologists of the apparently
sudden appearance of a whole group of species, is that of the teleostean
fishes, low down, according to Agassiz, in the Chalk period. This group
includes the large majority of existing species. But certain Jurassic and
Triassic forms are now commonly admitted to be teleostean; and even some
palaeozoic forms have thus been classed by one high authority. If the
teleosteans had really appeared suddenly in the northern hemisphere at the
commencement of the chalk formation, the fact would have been highly
remarkable; but it would not have formed an insuperable difficulty, unless
it could likewise have been shown that at the same period the species were
suddenly and simultaneously developed in other quarters of the world. It
is almost superfluous to remark that hardly any fossil-fish are known from
south of the equator; and by running through Pictet's Palaeontology it will
be seen that very few species are known from several formations in Europe.
Some few families of fish now have a confined range; the teleostean fishes
might formerly have had a similarly confined range, and after having been
largely developed in some one sea, have spread widely. Nor have we any
right to suppose that the seas of the world have always been so freely open
from south to north as they are at present. Even at this day, if the Malay
Archipelago were converted into land, the tropical parts of the Indian
Ocean would form a large and perfectly enclosed basin, in which any great
group of marine animals might be multiplied; and here they would remain
confined, until some of the species became adapted to a cooler climate, and
were enabled to double the southern capes of Africa or Australia, and thus
reach other and distant seas.
>From these considerations, from our ignorance of the geology of other
countries beyond the confines of Europe and the United States, and from the
revolution in our palaeontological knowledge effected by the discoveries of
the last dozen years, it seems to me to be about as rash to dogmatize on
the succession of organic forms throughout the world, as it would be for a
naturalist to land for five minutes on a barren point in Australia, and
then to discuss the number and range of its productions.
ON THE SUDDEN APPEARANCE OF GROUPS OF ALLIED SPECIES IN THE LOWEST KNOWN
FOSSILIFEROUS STRATA.
There is another and allied difficulty, which is much more serious. I
allude to the manner in which species belonging to several of the main
divisions of the animal kingdom suddenly appear in the lowest known
fossiliferous rocks. Most of the arguments which have convinced me that
all the existing species of the same group are descended from a single
progenitor, apply with equal force to the earliest known species. For
instance, it cannot be doubted that all the Cambrian and Silurian
trilobites are descended from some one crustacean, which must have lived
long before the Cambrian age, and which probably differed greatly from any
known animal. Some of the most ancient animals, as the Nautilus, Lingula,
etc., do not differ much from living species; and it cannot on our theory
be supposed, that these old species were the progenitors of all the species
belonging to the same groups which have subsequently appeared, for they are
not in any degree intermediate in character.
Consequently, if the theory be true, it is indisputable that before the
lowest Cambrian stratum was deposited long periods elapsed, as long as, or
probably far longer than, the whole interval from the Cambrian age to the
present day; and that during these vast periods the world swarmed with
living creatures. Here we encounter a formidable objection; for it seems
doubtful whether the earth, in a fit state for the habitation of living
creatures, has lasted long enough. Sir W. Thompson concludes that the
consolidation of the crust can hardly have occurred less than twenty or
more than four hundred million years ago, but probably not less than
ninety-eight or more than two hundred million years. These very wide
limits show how doubtful the data are; and other elements may have
hereafter to be introduced into the problem. Mr. Croll estimates that
about sixty million years have elapsed since the Cambrian period, but this,
judging from the small amount of organic change since the commencement of
the Glacial epoch, appears a very short time for the many and great
mutations of life, which have certainly occurred since the Cambrian
formation; and the previous one hundred and forty million years can hardly
be considered as sufficient for the development of the varied forms of life
which already existed during the Cambrian period. It is, however,
probable, as Sir William Thompson insists, that the world at a very early
period was subjected to more rapid and violent changes in its physical
conditions than those now occurring; and such changes would have tended to
induce changes at a corresponding rate in the organisms which then existed.
To the question why we do not find rich fossiliferous deposits belonging to
these assumed earliest periods prior to the Cambrian system, I can give no
satisfactory answer. Several eminent geologists, with Sir R. Murchison at
their head, were until recently convinced that we beheld in the organic
remains of the lowest Silurian stratum the first dawn of life. Other
highly competent judges, as Lyell and E. Forbes, have disputed this
conclusion. We should not forget that only a small portion of the world is
known with accuracy. Not very long ago M. Barrande added another and lower
stage, abounding with new and peculiar species, beneath the then known
Silurian system; and now, still lower down in the Lower Cambrian formation,
Mr Hicks has found South Wales beds rich in trilobites, and containing
various molluscs and annelids. The presence of phosphatic nodules and
bituminous matter, even in some of the lowest azotic rocks, probably
indicates life at these periods; and the existence of the Eozoon in the
Laurentian formation of Canada is generally admitted. There are three
great series of strata beneath the Silurian system in Canada, in the lowest
of which the Eozoon is found. Sir W. Logan states that their "united
thickness may possibly far surpass that of all the succeeding rocks, from
the base of the palaeozoic series to the present time. We are thus carried
back to a period so remote, that the appearance of the so-called primordial
fauna (of Barrande) may by some be considered as a comparatively modern
event." The Eozoon belongs to the most lowly organised of all classes of
animals, but is highly organised for its class; it existed in countless
numbers, and, as Dr. Dawson has remarked, certainly preyed on other minute
organic beings, which must have lived in great numbers. Thus the words,
which I wrote in 1859, about the existence of living beings long before the
Cambrian period, and which are almost the same with those since used by Sir
W. Logan, have proved true. Nevertheless, the difficulty of assigning any
good reason for the absence of vast piles of strata rich in fossils beneath
the Cambrian system is very great. It does not seem probable that the most
ancient beds have been quite worn away by denudation, or that their fossils
have been wholly obliterated by metamorphic action, for if this had been
the case we should have found only small remnants of the formations next
succeeding them in age, and these would always have existed in a partially
metamorphosed condition. But the descriptions which we possess of the
Silurian deposits over immense territories in Russia and in North America,
do not support the view that the older a formation is the more invariably
it has suffered extreme denudation and metamorphism.
The case at present must remain inexplicable; and may be truly urged as a
valid argument against the views here entertained. To show that it may
hereafter receive some explanation, I will give the following hypothesis.
>From the nature of the organic remains which do not appear to have
inhabited profound depths, in the several formations of Europe and of the
United States; and from the amount of sediment, miles in thickness, of
which the formations are composed, we may infer that from first to last
large islands or tracts of land, whence the sediment was derived, occurred
in the neighbourhood of the now existing continents of Europe and North
America. This same view has since been maintained by Agassiz and others.
But we do not know what was the state of things in the intervals between
the several successive formations; whether Europe and the United States
during these intervals existed as dry land, or as a submarine surface near
land, on which sediment was not deposited, or as the bed of an open and
unfathomable sea.
Looking to the existing oceans, which are thrice as extensive as the land,
we see them studded with many islands; but hardly one truly oceanic island
(with the exception of New Zealand, if this can be called a truly oceanic
island) is as yet known to afford even a remnant of any palaeozoic or
secondary formation. Hence, we may perhaps infer, that during the
palaeozoic and secondary periods, neither continents nor continental
islands existed where our oceans now extend; for had they existed,
palaeozoic and secondary formations would in all probability have been
accumulated from sediment derived from their wear and tear; and would have
been at least partially upheaved by the oscillations of level, which must
have intervened during these enormously long periods. If, then, we may
infer anything from these facts, we may infer that, where our oceans now
extend, oceans have extended from the remotest period of which we have any
record; and on the other hand, that where continents now exist, large
tracts of land have existed, subjected, no doubt, to great oscillations of
level, since the Cambrian period. The coloured map appended to my volume
on Coral Reefs, led me to conclude that the great oceans are still mainly
areas of subsidence, the great archipelagoes still areas of oscillations of
level, and the continents areas of elevation. But we have no reason to
assume that things have thus remained from the beginning of the world. Our
continents seem to have been formed by a preponderance, during many
oscillations of level, of the force of elevation. But may not the areas of
preponderant movement have changed in the lapse of ages? At a period long
antecedent to the Cambrian epoch, continents may have existed where oceans
are now spread out, and clear and open oceans may have existed where our
continents now stand. Nor should we be justified in assuming that if, for
instance, the bed of the Pacific Ocean were now converted into a continent
we should there find sedimentary formations, in recognisable condition,
older than the Cambrian strata, supposing such to have been formerly
deposited; for it might well happen that strata which had subsided some
miles nearer to the centre of the earth, and which had been pressed on by
an enormous weight of superincumbent water, might have undergone far more
metamorphic action than strata which have always remained nearer to the
surface. The immense areas in some parts of the world, for instance in
South America, of naked metamorphic rocks, which must have been heated
under great pressure, have always seemed to me to require some special
explanation; and we may perhaps believe that we see in these large areas
the many formations long anterior to the Cambrian epoch in a completely
metamorphosed and denuded condition.
The several difficulties here discussed, namely, that, though we find in
our geological formations many links between the species which now exist
and which formerly existed, we do not find infinitely numerous fine
transitional forms closely joining them all together. The sudden manner in
which several groups of species first appear in our European formations,
the almost entire absence, as at present known, of formations rich in
fossils beneath the Cambrian strata, are all undoubtedly of the most
serious nature. We see this in the fact that the most eminent
palaeontologists, namely, Cuvier, Agassiz, Barrande, Pictet, Falconer, E.
Forbes, etc., and all our greatest geologists, as Lyell, Murchison,
Sedgwick, etc., have unanimously, often vehemently, maintained the
immutability of species. But Sir Charles Lyell now gives the support of
his high authority to the opposite side, and most geologists and
palaeontologists are much shaken in their former belief. Those who believe
that the geological record is in any degree perfect, will undoubtedly at
once reject my theory. For my part, following out Lyell's metaphor, I look
at the geological record as a history of the world imperfectly kept and
written in a changing dialect. Of this history we possess the last volume
alone, relating only to two or three countries. Of this volume, only here
and there a short chapter has been preserved, and of each page, only here
and there a few lines. Each word of the slowly-changing language, more or
less different in the successive chapters, may represent the forms of life,
which are entombed in our consecutive formations, and which falsely appear
to have been abruptly introduced. On this view the difficulties above
discussed are greatly diminished or even disappear.
CHAPTER XI.
ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS.
On the slow and successive appearance of new species -- On their different
rates of change -- Species once lost do not reappear -- Groups of species
follow the same general rules in their appearance and disappearance as do
single species -- On extinction -- On simultaneous changes in the forms of
life throughout the world -- On the affinities of extinct species to each
other and to living species -- On the state of development of ancient forms
-- On the succession of the same types within the same areas -- Summary of
preceding and present chapters.
Let us now see whether the several facts and laws relating to the
geological succession of organic beings accord best with the common view of
the immutability of species, or with that of their slow and gradual
modification, through variation and natural selection.
New species have appeared very slowly, one after another, both on the land
and in the waters. Lyell has shown that it is hardly possible to resist
the evidence on this head in the case of the several tertiary stages; and
every year tends to fill up the blanks between the stages, and to make the
proportion between the lost and existing forms more gradual. In some of
the most recent beds, though undoubtedly of high antiquity if measured by
years, only one or two species are extinct, and only one or two are new,
having appeared there for the first time, either locally, or, as far as we
know, on the face of the earth. The secondary formations are more broken;
but, as Bronn has remarked, neither the appearance nor disappearance of the
many species embedded in each formation has been simultaneous.
Species belonging to different genera and classes have not changed at the
same rate, or in the same degree. In the older tertiary beds a few living
shells may still be found in the midst of a multitude of extinct forms.
Falconer has given a striking instance of a similar fact, for an existing
crocodile is associated with many lost mammals and reptiles in the
sub-Himalayan deposits. The Silurian Lingula differs but little from the
living species of this genus; whereas most of the other Silurian Molluscs
and all the Crustaceans have changed greatly. The productions of the land
seem to have changed at a quicker rate than those of the sea, of which a
striking instance has been observed in Switzerland. There is some reason
to believe that organisms high in the scale, change more quickly than those
that are low: though there are exceptions to this rule. The amount of
organic change, as Pictet has remarked, is not the same in each successive
so-called formation. Yet if we compare any but the most closely related
formations, all the species will be found to have undergone some change.
When a species has once disappeared from the face of the earth, we have no
reason to believe that the same identical form ever reappears. The
strongest apparent exception to this latter rule is that of the so-called
"colonies" of M. Barrande, which intrude for a period in the midst of an
older formation, and then allow the pre-existing fauna to reappear; but
Lyell's explanation, namely, that it is a case of temporary migration from
a distinct geographical province, seems satisfactory.
These several facts accord well with our theory, which includes no fixed
law of development, causing all the inhabitants of an area to change
abruptly, or simultaneously, or to an equal degree. The process of
modification must be slow, and will generally affect only a few species at
the same time; for the variability of each species is independent of that
of all others. Whether such variations or individual differences as may
arise will be accumulated through natural selection in a greater or less
degree, thus causing a greater or less amount of permanent modification,
will depend on many complex contingencies--on the variations being of a
beneficial nature, on the freedom of intercrossing, on the slowly changing
physical conditions of the country, on the immigration of new colonists,
and on the nature of the other inhabitants with which the varying species
come into competition. Hence it is by no means surprising that one species
should retain the same identical form much longer than others; or, if
changing, should change in a less degree. We find similar relations
between the existing inhabitants of distinct countries; for instance, the
land-shells and coleopterous insects of Madeira have come to differ
considerably from their nearest allies on the continent of Europe, whereas
the marine shells and birds have remained unaltered. We can perhaps
understand the apparently quicker rate of change in terrestrial and in more
highly organised productions compared with marine and lower productions, by
the more complex relations of the higher beings to their organic and
inorganic conditions of life, as explained in a former chapter. When many
of the inhabitants of any area have become modified and improved, we can
understand, on the principle of competition, and from the all-important
relations of organism to organism in the struggle for life, that any form
which did not become in some degree modified and improved, would be liable
to extermination. Hence, we see why all the species in the same region do
at last, if we look to long enough intervals of time, become modified; for
otherwise they would become extinct.
In members of the same class the average amount of change, during long and
equal periods of time, may, perhaps, be nearly the same; but as the
accumulation of enduring formations, rich in fossils, depends on great
masses of sediment being deposited on subsiding areas, our formations have
been almost necessarily accumulated at wide and irregularly intermittent
intervals of time; consequently the amount of organic change exhibited by
the fossils embedded in consecutive formations is not equal. Each
formation, on this view, does not mark a new and complete act of creation,
but only an occasional scene, taken almost at hazard, in an ever slowly
changing drama.
We can clearly understand why a species when once lost should never
reappear, even if the very same conditions of life, organic and inorganic,
should recur. For though the offspring of one species might be adapted
(and no doubt this has occurred in innumerable instances) to fill the place
of another species in the economy of nature, and thus supplant it; yet the
two forms--the old and the new--would not be identically the same; for both
would almost certainly inherit different characters from their distinct
progenitors; and organisms already differing would vary in a different
manner. For instance, it is possible, if all our fantail-pigeons were
destroyed, that fanciers might make a new breed hardly distinguishable from
the present breed; but if the parent rock-pigeon were likewise destroyed,
and under nature we have every reason to believe that parent forms are
generally supplanted and exterminated by their improved offspring, it is
incredible that a fantail, identical with the existing breed, could be
raised from any other species of pigeon, or even from any other well
established race of the domestic pigeon, for the successive variations
would almost certainly be in some degree different, and the newly-formed
variety would probably inherit from its progenitor some characteristic
differences.
Groups of species, that is, genera and families, follow the same general
rules in their appearance and disappearance as do single species, changing
more or less quickly, and in a greater or lesser degree. A group, when it
has once disappeared, never reappears; that is, its existence, as long as
it lasts, is continuous. I am aware that there are some apparent
exceptions to this rule, but the exceptions are surprisingly few, so few
that E. Forbes, Pictet, and Woodward (though all strongly opposed to such
views as I maintain) admit its truth; and the rule strictly accords with
the theory. For all the species of the same group, however long it may
have lasted, are the modified descendants one from the other, and all from
a common progenitor. In the genus Lingula, for instance, the species which
have successively appeared at all ages must have been connected by an
unbroken series of generations, from the lowest Silurian stratum to the
present day.
We have seen in the last chapter that whole groups of species sometimes
falsely appear to have been abruptly developed; and I have attempted to
give an explanation of this fact, which if true would be fatal to my views.
But such cases are certainly exceptional; the general rule being a gradual
increase in number, until the group reaches its maximum, and then, sooner
or later, a gradual decrease. If the number of the species included within
a genus, or the number of the genera within a family, be represented by a
vertical line of varying thickness, ascending through the successive
geological formations, in which the species are found, the line will
sometimes falsely appear to begin at its lower end, not in a sharp point,
but abruptly; it then gradually thickens upwards, often keeping of equal
thickness for a space, and ultimately thins out in the upper beds, marking
the decrease and final extinction of the species. This gradual increase in
number of the species of a group is strictly conformable with the theory;
for the species of the same genus, and the genera of the same family, can
increase only slowly and progressively; the process of modification and the
production of a number of allied forms necessarily being a slow and gradual
process, one species first giving rise to two or three varieties, these
being slowly converted into species, which in their turn produce by equally
slow steps other varieties and species, and so on, like the branching of a
great tree from a single stem, till the group becomes large.
ON EXTINCTION.
We have as yet only spokesn incidentally of the disappearance of species
and of groups of species. On the theory of natural selection, the
extinction of old forms and the production of new and improved forms are
intimately connected together. The old notion of all the inhabitants of
the earth having been swept away by catastrophes at successive periods is
very generally given up, even by those geologists, as Elie de Beaumont,
Murchison, Barrande, etc., whose general views would naturally lead them to
this conclusion. On the contrary, we have every reason to believe, from
the study of the tertiary formations, that species and groups of species
gradually disappear, one after another, first from one spot, then from
another, and finally from the world. In some few cases, however, as by the
breaking of an isthmus and the consequent irruption of a multitude of new
inhabitants into an adjoining sea, or by the final subsidence of an island,
the process of extinction may have been rapid. Both single species and
whole groups of species last for very unequal periods; some groups, as we
have seen, have endured from the earliest known dawn of life to the present
day; some have disappeared before the close of the palaeozoic period. No
fixed law seems to determine the length of time during which any single
species or any single genus endures. There is reason to believe that the
extinction of a whole group of species is generally a slower process than
their production: if their appearance and disappearance be represented, as
before, by a vertical line of varying thickness the line is found to taper
more gradually at its upper end, which marks the progress of extermination,
than at its lower end, which marks the first appearance and the early
increase in number of the species. In some cases, however, the
extermination of whole groups, as of ammonites, towards the close of the
secondary period, has been wonderfully sudden.
The extinction of species has been involved in the most gratuitous mystery.
Some authors have even supposed that, as the individual has a definite
length of life, so have species a definite duration. No one can have
marvelled more than I have done at the extinction of species. When I found
in La Plata the tooth of a horse embedded with the remains of Mastodon,
Megatherium, Toxodon and other extinct monsters, which all co-existed with
still living shells at a very late geological period, I was filled with
astonishment; for, seeing that the horse, since its introduction by the
Spaniards into South America, has run wild over the whole country and has
increased in numbers at an unparalleled rate, I asked myself what could so
recently have exterminated the former horse under conditions of life
apparently so favourable. But my astonishment was groundless. Professor
Owen soon perceived that the tooth, though so like that of the existing
horse, belonged to an extinct species. Had this horse been still living,
but in some degree rare, no naturalist would have felt the least surprise
at its rarity; for rarity is the attribute of a vast number of species of
all classes, in all countries. If we ask ourselves why this or that
species is rare, we answer that something is unfavourable in its conditions
of life; but what that something is, we can hardly ever tell. On the
supposition of the fossil horse still existing as a rare species, we might
have felt certain, from the analogy of all other mammals, even of the
slow-breeding elephant, and from the history of the naturalisation of the
domestic horse in South America, that under more favourable conditions it
would in a very few years have stocked the whole continent. But we could
not have told what the unfavourable conditions were which checked its
increase, whether some one or several contingencies, and at what period of
the horse's life, and in what degree they severally acted. If the
conditions had gone on, however slowly, becoming less and less favourable,
we assuredly should not have perceived the fact, yet the fossil horse would
certainly have become rarer and rarer, and finally extinct--its place being
seized on by some more successful competitor.
It is most difficult always to remember that the increase of every living
creature is constantly being checked by unperceived hostile agencies; and
that these same unperceived agencies are amply sufficient to cause rarity,
and finally extinction. So little is this subject understood, that I have
heard surprise repeatedly expressed at such great monsters as the Mastodon
and the more ancient Dinosaurians having become extinct; as if mere bodily
strength gave victory in the battle of life. Mere size, on the contrary,
would in some cases determine, as has been remarked by Owen, quicker
extermination, from the greater amount of requisite food. Before man
inhabited India or Africa, some cause must have checked the continued
increase of the existing elephant. A highly capable judge, Dr. Falconer,
believes that it is chiefly insects which, from incessantly harassing and
weakening the elephant in India, check its increase; and this was Bruce's
conclusion with respect to the African elephant in Abyssinia. It is
certain that insects and blood-sucking bats determine the existence of the
larger naturalised quadrupeds in several parts of South America.
We see in many cases in the more recent tertiary formations that rarity
precedes extinction; and we know that this has been the progress of events
with those animals which have been exterminated, either locally or wholly,
through man's agency. I may repeat what I published in 1845, namely, that
to admit that species generally become rare before they become extinct--to
feel no surprise at the rarity of a species, and yet to marvel greatly when
the species ceases to exist, is much the same as to admit that sickness in
the individual is the forerunner of death--to feel no surprise at sickness,
but, when the sick man dies, to wonder and to suspect that he died by some
deed of violence.
The theory of natural selection is grounded on the belief that each new
variety and ultimately each new species, is produced and maintained by
having some advantage over those with which it comes into competition; and
the consequent extinction of less-favoured forms almost inevitably follows.
It is the same with our domestic productions: when a new and slightly
improved variety has been raised, it at first supplants the less improved
varieties in the same neighbourhood; when much improved it is transported
far and near, like our short-horn cattle, and takes the place of other
breeds in other countries. Thus the appearance of new forms and the
disappearance of old forms, both those naturally and artificially produced,
are bound together. In flourishing groups, the number of new specific
forms which have been produced within a given time has at some periods
probably been greater than the number of the old specific forms which have
been exterminated; but we know that species have not gone on indefinitely
increasing, at least during the later geological epochs, so that, looking
to later times, we may believe that the production of new forms has caused
the extinction of about the same number of old forms.
The competition will generally be most severe, as formerly explained and
illustrated by examples, between the forms which are most like each other
in all respects. Hence the improved and modified descendants of a species
will generally cause the extermination of the parent-species; and if many
new forms have been developed from any one species, the nearest allies of
that species, i.e. the species of the same genus, will be the most liable
to extermination. Thus, as I believe, a number of new species descended
from one species, that is a new genus, comes to supplant an old genus,
belonging to the same family. But it must often have happened that a new
species belonging to some one group has seized on the place occupied by a
species belonging to a distinct group, and thus have caused its
extermination. If many allied forms be developed from the successful
intruder, many will have to yield their places; and it will generally be
the allied forms, which will suffer from some inherited inferiority in
common. But whether it be species belonging to the same or to a distinct
class, which have yielded their places to other modified and improved
species, a few of the sufferers may often be preserved for a long time,
from being fitted to some peculiar line of life, or from inhabiting some
distant and isolated station, where they will have escaped severe
competition. For instance, some species of Trigonia, a great genus of
shells in the secondary formations, survive in the Australian seas; and a
few members of the great and almost extinct group of Ganoid fishes still
inhabit our fresh waters. Therefore, the utter extinction of a group is
generally, as we have seen, a slower process than its production.
With respect to the apparently sudden extermination of whole families or
orders, as of Trilobites at the close of the palaeozoic period, and of
Ammonites at the close of the secondary period, we must remember what has
been already said on the probable wide intervals of time between our
consecutive formations; and in these intervals there may have been much
slow extermination. Moreover, when, by sudden immigration or by unusually
rapid development, many species of a new group have taken possession of an
area, many of the older species will have been exterminated in a
correspondingly rapid manner; and the forms which thus yield their places
will commonly be allied, for they will partake of the same inferiority in
common.
Thus, as it seems to me, the manner in which single species and whole
groups of species become extinct accords well with the theory of natural
selection. We need not marvel at extinction; if we must marvel, let it be
at our presumption in imagining for a moment that we understand the many
complex contingencies on which the existence of each species depends. If
we forget for an instant that each species tends to increase inordinately,
and that some check is always in action, yet seldom perceived by us, the
whole economy of nature will be utterly obscured. Whenever we can
precisely say why this species is more abundant in individuals than that;
why this species and not another can be naturalised in a given country;
then, and not until then, we may justly feel surprise why we cannot account
for the extinction of any particular species or group of species.
ON THE FORMS OF LIFE CHANGING ALMOST SIMULTANEOUSLY THROUGHOUT THE WORLD.
Scarcely any palaeontological discovery is more striking than the fact that
the forms of life change almost simultaneously throughout the world. Thus
our European Chalk formation can be recognised in many distant regions,
under the most different climates, where not a fragment of the mineral
chalk itself can be found; namely, in North America, in equatorial South
America, in Tierra del Fuego, at the Cape of Good Hope, and in the
peninsula of India. For at these distant points, the organic remains in
certain beds present an unmistakable resemblance to those of the Chalk. It
is not that the same species are met with; for in some cases not one
species is identically the same, but they belong to the same families,
genera, and sections of genera, and sometimes are similarly characterised
in such trifling points as mere superficial sculpture. Moreover, other
forms, which are not found in the Chalk of Europe, but which occur in the
formations either above or below, occur in the same order at these distant
points of the world. In the several successive palaeozoic formations of
Russia, Western Europe and North America, a similar parallelism in the
forms of life has been observed by several authors; so it is, according to
Lyell, with the European and North American tertiary deposits. Even if the
few fossil species which are common to the Old and New Worlds were kept
wholly out of view, the general parallelism in the successive forms of
life, in the palaeozoic and tertiary stages, would still be manifest, and
the several formations could be easily correlated.
These observations, however, relate to the marine inhabitants of the world:
we have not sufficient data to judge whether the productions of the land
and of fresh water at distant points change in the same parallel manner.
We may doubt whether they have thus changed: if the Megatherium, Mylodon,
Macrauchenia, and Toxodon had been brought to Europe from La Plata, without
any information in regard to their geological position, no one would have
suspected that they had co-existed with sea-shells all still living; but as
these anomalous monsters co-existed with the Mastodon and Horse, it might
at least have been inferred that they had lived during one of the later
tertiary stages.
When the marine forms of life are spoken of as having changed
simultaneously throughout the world, it must not be supposed that this
expression relates to the same year, or even to the same century, or even
that it has a very strict geological sense; for if all the marine animals
now living in Europe, and all those that lived in Europe during the
pleistocene period (a very remote period as measured by years, including
the whole glacial epoch) were compared with those now existing in South
America or in Australia, the most skilful naturalist would hardly be able
to say whether the present or the pleistocene inhabitants of Europe
resembled most closely those of the southern hemisphere. So, again,
several highly competent observers maintain that the existing productions
of the United States are more closely related to those which lived in
Europe during certain late tertiary stages, than to the present inhabitants
of Europe; and if this be so, it is evident that fossiliferous beds now
deposited on the shores of North America would hereafter be liable to be
classed with somewhat older European beds. Nevertheless, looking to a
remotely future epoch, there can be little doubt that all the more modern
MARINE formations, namely, the upper pliocene, the pleistocene and strictly
modern beds of Europe, North and South America, and Australia, from
containing fossil remains in some degree allied, and from not including
those forms which are found only in the older underlying deposits, would be
correctly ranked as simultaneous in a geological sense.
The fact of the forms of life changing simultaneously in the above large
sense, at distant parts of the world, has greatly struck those admirable
observers, MM. de Verneuil and d'Archiac. After referring to the
parallelism of the palaeozoic forms of life in various parts of Europe,
they add, "If struck by this strange sequence, we turn our attention to
North America, and there discover a series of analogous phenomena, it will
appear certain that all these modifications of species, their extinction,
and the introduction of new ones, cannot be owing to mere changes in marine
currents or other causes more or less local and temporary, but depend on
general laws which govern the whole animal kingdom." M. Barrande has made
forcible remarks to precisely the same effect. It is, indeed, quite futile
to look to changes of currents, climate, or other physical conditions, as
the cause of these great mutations in the forms of life throughout the
world, under the most different climates. We must, as Barrande has
remarked, look to some special law. We shall see this more clearly when we
treat of the present distribution of organic beings, and find how slight is
the relation between the physical conditions of various countries and the
nature of their inhabitants.
This great fact of the parallel succession of the forms of life throughout
the world, is explicable on the theory of natural selection. New species
are formed by having some advantage over older forms; and the forms, which
are already dominant, or have some advantage over the other forms in their
own country, give birth to the greatest number of new varieties or
incipient species. We have distinct evidence on this head, in the plants
which are dominant, that is, which are commonest and most widely diffused,
producing the greatest number of new varieties. It is also natural that
the dominant, varying and far-spreading species, which have already
invaded, to a certain extent, the territories of other species, should be
those which would have the best chance of spreading still further, and of
giving rise in new countries to other new varieties and species. The
process of diffusion would often be very slow, depending on climatal and
geographical changes, on strange accidents, and on the gradual
acclimatization of new species to the various climates through which they
might have to pass, but in the course of time the dominant forms would
generally succeed in spreading and would ultimately prevail. The diffusion
would, it is probable, be slower with the terrestrial inhabitants of
distinct continents than with the marine inhabitants of the continuous sea.
We might therefore expect to find, as we do find, a less strict degree of
parallelism in the succession of the productions of the land than with
those of the sea.
Thus, as it seems to me, the parallel, and, taken in a large sense,
simultaneous, succession of the same forms of life throughout the world,
accords well with the principle of new species having been formed by
dominant species spreading widely and varying; the new species thus
produced being themselves dominant, owing to their having had some
advantage over their already dominant parents, as well as over other
species; and again spreading, varying, and producing new forms. The old
forms which are beaten and which yield their places to the new and
victorious forms, will generally be allied in groups, from inheriting some
inferiority in common; and, therefore, as new and improved groups spread
throughout the world, old groups disappear from the world; and the
succession of forms everywhere tends to correspond both in their first
appearance and final disappearance.
There is one other remark connected with this subject worth making. I have
given my reasons for believing that most of our great formations, rich in
fossils, were deposited during periods of subsidence; and that blank
intervals of vast duration, as far as fossils are concerned, occurred
during the periods when the bed of the sea was either stationary or rising,
and likewise when sediment was not thrown down quickly enough to embed and
preserve organic remains. During these long and blank intervals I suppose
that the inhabitants of each region underwent a considerable amount of
modification and extinction, and that there was much migration from other
parts of the world. As we have reason to believe that large areas are
affected by the same movement, it is probable that strictly contemporaneous
formations have often been accumulated over very wide spaces in the same
quarter of the world; but we are very far from having any right to conclude
that this has invariably been the case, and that large areas have
invariably been affected by the same movements. When two formations have
been deposited in two regions during nearly, but not exactly, the same
period, we should find in both, from the causes explained in the foregoing
paragraphs, the same general succession in the forms of life; but the
species would not exactly correspond; for there will have been a little
more time in the one region than in the other for modification, extinction,
and immigration.
I suspect that cases of this nature occur in Europe. Mr. Prestwich, in his
admirable Memoirs on the eocene deposits of England and France, is able to
draw a close general parallelism between the successive stages in the two
countries; but when he compares certain stages in England with those in
France, although he finds in both a curious accordance in the numbers of
the species belonging to the same genera, yet the species themselves differ
in a manner very difficult to account for considering the proximity of the
two areas, unless, indeed, it be assumed that an isthmus separated two seas
inhabited by distinct, but contemporaneous faunas. Lyell has made similar
observations on some of the later tertiary formations. Barrande, also,
shows that there is a striking general parallelism in the successive
Silurian deposits of Bohemia and Scandinavia; nevertheless he finds a
surprising amount of difference in the species. If the several formations
in these regions have not been deposited during the same exact periods--a
formation in one region often corresponding with a blank interval in the
other--and if in both regions the species have gone on slowly changing
during the accumulation of the several formations and during the long
intervals of time between them; in this case the several formations in the
two regions could be arranged in the same order, in accordance with the
general succession of the forms of life, and the order would falsely appear
to be strictly parallel; nevertheless the species would not all be the same
in the apparently corresponding stages in the two regions.
ON THE AFFINITIES OF EXTINCT SPECIES TO EACH OTHER, AND TO LIVING FORMS.
Let us now look to the mutual affinities of extinct and living species.
All fall into a few grand classes; and this fact is at once explained on
the principle of descent. The more ancient any form is, the more, as a
general rule, it differs from living forms. But, as Buckland long ago
remarked, extinct species can all be classed either in still existing
groups, or between them. That the extinct forms of life help to fill up
the intervals between existing genera, families, and orders, is certainly
true; but as this statement has often been ignored or even denied, it may
be well to make some remarks on this subject, and to give some instances.
If we confine our attention either to the living or to the extinct species
of the same class, the series is far less perfect than if we combine both
into one general system. In the writings of Professor Owen we continually
meet with the expression of generalised forms, as applied to extinct
animals; and in the writings of Agassiz, of prophetic or synthetic types;
and these terms imply that such forms are, in fact, intermediate or
connecting links. Another distinguished palaeontologist, M. Gaudry, has
shown in the most striking manner that many of the fossil mammals
discovered by him in Attica serve to break down the intervals between
existing genera. Cuvier ranked the Ruminants and Pachyderms as two of the
most distinct orders of mammals; but so many fossil links have been
disentombed that Owen has had to alter the whole classification, and has
placed certain Pachyderms in the same sub-order with ruminants; for
example, he dissolves by gradations the apparently wide interval between
the pig and the camel. The Ungulata or hoofed quadrupeds are now divided
into the even-toed or odd-toed divisions; but the Macrauchenia of South
America connects to a certain extent these two grand divisions. No one
will deny that the Hipparion is intermediate between the existing horse and
certain other ungulate forms. What a wonderful connecting link in the
chain of mammals is the Typotherium from South America, as the name given
to it by Professor Gervais expresses, and which cannot be placed in any
existing order. The Sirenia form a very distinct group of the mammals, and
one of the most remarkable peculiarities in existing dugong and lamentin is
the entire absence of hind limbs, without even a rudiment being left; but
the extinct Halitherium had, according to Professor Flower, an ossified
thigh-bone "articulated to a well-defined acetabulum in the pelvis," and it
thus makes some approach to ordinary hoofed quadrupeds, to which the
Sirenia are in other respects allied. The cetaceans or whales are widely
different from all other mammals, but the tertiary Zeuglodon and Squalodon,
which have been placed by some naturalists in an order by themselves, are
considered by Professor Huxley to be undoubtedly cetaceans, "and to
constitute connecting links with the aquatic carnivora."
Even the wide interval between birds and reptiles has been shown by the
naturalist just quoted to be partially bridged over in the most unexpected
manner, on the one hand, by the ostrich and extinct Archeopteryx, and on
the other hand by the Compsognathus, one of the Dinosaurians--that group
which includes the most gigantic of all terrestrial reptiles. Turning to
the Invertebrata, Barrande asserts, a higher authority could not be named,
that he is every day taught that, although palaeozoic animals can certainly
be classed under existing groups, yet that at this ancient period the
groups were not so distinctly separated from each other as they now are.
Some writers have objected to any extinct species, or group of species,
being considered as intermediate between any two living species, or groups
of species. If by this term it is meant that an extinct form is directly
intermediate in all its characters between two living forms or groups, the
objection is probably valid. But in a natural classification many fossil
species certainly stand between living species, and some extinct genera
between living genera, even between genera belonging to distinct families.
The most common case, especially with respect to very distinct groups, such
as fish and reptiles, seems to be that, supposing them to be distinguished
at the present day by a score of characters, the ancient members are
separated by a somewhat lesser number of characters, so that the two groups
formerly made a somewhat nearer approach to each other than they now do.
It is a common belief that the more ancient a form is, by so much the more
it tends to connect by some of its characters groups now widely separated
from each other. This remark no doubt must be restricted to those groups
which have undergone much change in the course of geological ages; and it
would be difficult to prove the truth of the proposition, for every now and
then even a living animal, as the Lepidosiren, is discovered having
affinities directed towards very distinct groups. Yet if we compare the
older Reptiles and Batrachians, the older Fish, the older Cephalopods, and
the eocene Mammals, with the recent members of the same classes, we must
admit that there is truth in the remark.
Let us see how far these several facts and inferences accord with the
theory of descent with modification. As the subject is somewhat complex, I
must request the reader to turn to the diagram in the fourth chapter. We
may suppose that the numbered letters in italics represent genera, and the
dotted lines diverging from them the species in each genus. The diagram is
much too simple, too few genera and too few species being given, but this
is unimportant for us. The horizontal lines may represent successive
geological formations, and all the forms beneath the uppermost line may be
considered as extinct. The three existing genera, a14, q14, p14, will form
a small family; b14 and f14, a closely allied family or subfamily; and o14,
i14, m14, a third family. These three families, together with the many
extinct genera on the several lines of descent diverging from the parent
form (A) will form an order; for all will have inherited something in
common from their ancient progenitor. On the principle of the continued
tendency to divergence of character, which was formerly illustrated by this
diagram, the more recent any form is the more it will generally differ from
its ancient progenitor. Hence, we can understand the rule that the most
ancient fossils differ most from existing forms. We must not, however,
assume that divergence of character is a necessary contingency; it depends
solely on the descendants from a species being thus enabled to seize on
many and different places in the economy of nature. Therefore it is quite
possible, as we have seen in the case of some Silurian forms, that a
species might go on being slightly modified in relation to its slightly
altered conditions of life, and yet retain throughout a vast period the
same general characteristics. This is represented in the diagram by the
letter F14.
All the many forms, extinct and recent, descended from (A), make, as before
remarked, one order; and this order, from the continued effects of
extinction and divergence of character, has become divided into several
sub-families and families, some of which are supposed to have perished at
different periods, and some to have endured to the present day.
By looking at the diagram we can see that if many of the extinct forms
supposed to be embedded in the successive formations, were discovered at
several points low down in the series, the three existing families on the
uppermost line would be rendered less distinct from each other. If, for
instance, the genera a1, a5, a10, f8, m3, m6, m9, were disinterred, these
three families would be so closely linked together that they probably would
have to be united into one great family, in nearly the same manner as has
occurred with ruminants and certain pachyderms. Yet he who objected to
consider as intermediate the extinct genera, which thus link together the
living genera of three families, would be partly justified, for they are
intermediate, not directly, but only by a long and circuitous course
through many widely different forms. If many extinct forms were to be
discovered above one of the middle horizontal lines or geological
formations--for instance, above No. VI.--but none from beneath this line,
then only two of the families (those on the left hand a14, etc., and b14,
etc.) would have to be united into one; and there would remain two families
which would be less distinct from each other than they were before the
discovery of the fossils. So again, if the three families formed of eight
genera (a14 to m14), on the uppermost line, be supposed to differ from each
other by half-a-dozen important characters, then the families which existed
at a period marked VI would certainly have differed from each other by a
less number of characters; for they would at this early stage of descent
have diverged in a less degree from their common progenitor. Thus it comes
that ancient and extinct genera are often in a greater or less degree
intermediate in character between their modified descendants, or between
their collateral relations.
Under nature the process will be far more complicated than is represented
in the diagram; for the groups will have been more numerous; they will have
endured for extremely unequal lengths of time, and will have been modified
in various degrees. As we possess only the last volume of the geological
record, and that in a very broken condition, we have no right to expect,
except in rare cases, to fill up the wide intervals in the natural system,
and thus to unite distinct families or orders. All that we have a right to
expect is, that those groups which have, within known geological periods,
undergone much modification, should in the older formations make some
slight approach to each other; so that the older members should differ less
from each other in some of their characters than do the existing members of
the same groups; and this by the concurrent evidence of our best
palaeontologists is frequently the case.
Thus, on the theory of descent with modification, the main facts with
respect to the mutual affinities of the extinct forms of life to each other
and to living forms, are explained in a satisfactory manner. And they are
wholly inexplicable on any other view.
On this same theory, it is evident that the fauna during any one great
period in the earth's history will be intermediate in general character
between that which preceded and that which succeeded it. Thus the species
which lived at the sixth great stage of descent in the diagram are the
modified offspring of those which lived at the fifth stage, and are the
parents of those which became still more modified at the seventh stage;
hence they could hardly fail to be nearly intermediate in character between
the forms of life above and below. We must, however, allow for the entire
extinction of some preceding forms, and in any one region for the
immigration of new forms from other regions, and for a large amount of
modification during the long and blank intervals between the successive
formations. Subject to these allowances, the fauna of each geological
period undoubtedly is intermediate in character, between the preceding and
succeeding faunas. I need give only one instance, namely, the manner in
which the fossils of the Devonian system, when this system was first
discovered, were at once recognised by palaeontologists as intermediate in
character between those of the overlying carboniferous and underlying
Silurian systems. But each fauna is not necessarily exactly intermediate,
as unequal intervals of time have elapsed between consecutive formations.
It is no real objection to the truth of the statement that the fauna of
each period as a whole is nearly intermediate in character between the
preceding and succeeding faunas, that certain genera offer exceptions to
the rule. For instance, the species of mastodons and elephants, when
arranged by Dr. Falconer in two series--in the first place according to
their mutual affinities, and in the second place according to their periods
of existence--do not accord in arrangement. The species extreme in
character are not the oldest or the most recent; nor are those which are
intermediate in character, intermediate in age. But supposing for an
instant, in this and other such cases, that the record of the first
appearance and disappearance of the species was complete, which is far from
the case, we have no reason to believe that forms successively produced
necessarily endure for corresponding lengths of time. A very ancient form
may occasionally have lasted much longer than a form elsewhere subsequently
produced, especially in the case of terrestrial productions inhabiting
separated districts. To compare small things with great; if the principal
living and extinct races of the domestic pigeon were arranged in serial
affinity, this arrangement would not closely accord with the order in time
of their production, and even less with the order of their disappearance;
for the parent rock-pigeon still lives; and many varieties between the
rock-pigeon and the carrier have become extinct; and carriers which are
extreme in the important character of length of beak originated earlier
than short-beaked tumblers, which are at the opposite end of the series in
this respect.
Closely connected with the statement, that the organic remains from an
intermediate formation are in some degree intermediate in character, is the
fact, insisted on by all palaeontologists, that fossils from two
consecutive formations are far more closely related to each other, than are
the fossils from two remote formations. Pictet gives as a well-known
instance, the general resemblance of the organic remains from the several
stages of the Chalk formation, though the species are distinct in each
stage. This fact alone, from its generality, seems to have shaken
Professor Pictet in his belief in the immutability of species. He who is
acquainted with the distribution of existing species over the globe, will
not attempt to account for the close resemblance of distinct species in
closely consecutive formations, by the physical conditions of the ancient
areas having remained nearly the same. Let it be remembered that the forms
of life, at least those inhabiting the sea, have changed almost
simultaneously throughout the world, and therefore under the most different
climates and conditions. Consider the prodigious vicissitudes of climate
during the pleistocene period, which includes the whole glacial epoch, and
note how little the specific forms of the inhabitants of the sea have been
affected.
On the theory of descent, the full meaning of the fossil remains from
closely consecutive formations, being closely related, though ranked as
distinct species, is obvious. As the accumulation of each formation has
often been interrupted, and as long blank intervals have intervened between
successive formations, we ought not to expect to find, as I attempted to
show in the last chapter, in any one or in any two formations, all the
intermediate varieties between the species which appeared at the
commencement and close of these periods: but we ought to find after
intervals, very long as measured by years, but only moderately long as
measured geologically, closely allied forms, or, as they have been called
by some authors, representative species; and these assuredly we do find.
We find, in short, such evidence of the slow and scarcely sensible
mutations of specific forms, as we have the right to expect.
ON THE STATE OF DEVELOPMENT OF ANCIENT COMPARED WITH LIVING FORMS.
We have seen in the fourth chapter that the degree of differentiation and
specialisation of the parts in organic beings, when arrived at maturity, is
the best standard, as yet suggested, of their degree of perfection or
highness. We have also seen that, as the specialisation of parts is an
advantage to each being, so natural selection will tend to render the
organisation of each being more specialised and perfect, and in this sense
higher; not but that it may leave many creatures with simple and unimproved
structures fitted for simple conditions of life, and in some cases will
even degrade or simplify the organisation, yet leaving such degraded beings
better fitted for their new walks of life. In another and more general
manner, new species become superior to their predecessors; for they have to
beat in the struggle for life all the older forms, with which they come
into close competition. We may therefore conclude that if under a nearly
similar climate the eocene inhabitants of the world could be put into
competition with the existing inhabitants, the former would be beaten and
exterminated by the latter, as would the secondary by the eocene, and the
palaeozoic by the secondary forms. So that by this fundamental test of
victory in the battle for life, as well as by the standard of the
specialisation of organs, modern forms ought, on the theory of natural
selection, to stand higher than ancient forms. Is this the case? A large
majority of palaeontologists would answer in the affirmative; and it seems
that this answer must be admitted as true, though difficult of proof.
It is no valid objection to this conclusion, that certain Brachiopods have
been but slightly modified from an extremely remote geological epoch; and
that certain land and fresh-water shells have remained nearly the same,
from the time when, as far as is known, they first appeared. It is not an
insuperable difficulty that Foraminifera have not, as insisted on by Dr.
Carpenter, progressed in organisation since even the Laurentian epoch; for
some organisms would have to remain fitted for simple conditions of life,
and what could be better fitted for this end than these lowly organised
Protozoa? Such objections as the above would be fatal to my view, if it
included advance in organisation as a necessary contingent. They would
likewise be fatal, if the above Foraminifera, for instance, could be proved
to have first come into existence during the Laurentian epoch, or the above
Brachiopods during the Cambrian formation; for in this case, there would
not have been time sufficient for the development of these organisms up to
the standard which they had then reached. When advanced up to any given
point, there is no necessity, on the theory of natural selection, for their
further continued process; though they will, during each successive age,
have to be slightly modified, so as to hold their places in relation to
slight changes in their conditions. The foregoing objections hinge on the
question whether we really know how old the world is, and at what period
the various forms of life first appeared; and this may well be disputed.
The problem whether organisation on the whole has advanced is in many ways
excessively intricate. The geological record, at all times imperfect, does
not extend far enough back to show with unmistakable clearness that within
the known history of the world organisation has largely advanced. Even at
the present day, looking to members of the same class, naturalists are not
unanimous which forms ought to be ranked as highest: thus, some look at
the selaceans or sharks, from their approach in some important points of
structure to reptiles, as the highest fish; others look at the teleosteans
as the highest. The ganoids stand intermediate between the selaceans and
teleosteans; the latter at the present day are largely preponderant in
number; but formerly selaceans and ganoids alone existed; and in this case,
according to the standard of highness chosen, so will it be said that
fishes have advanced or retrograded in organisation. To attempt to compare
members of distinct types in the scale of highness seems hopeless; who will
decide whether a cuttle-fish be higher than a bee--that insect which the
great Von Baer believed to be "in fact more highly organised than a fish,
although upon another type?" In the complex struggle for life it is quite
credible that crustaceans, not very high in their own class, might beat
cephalopods, the highest molluscs; and such crustaceans, though not highly
developed, would stand very high in the scale of invertebrate animals, if
judged by the most decisive of all trials--the law of battle. Beside these
inherent difficulties in deciding which forms are the most advanced in
organisation, we ought not solely to compare the highest members of a class
at any two periods--though undoubtedly this is one and perhaps the most
important element in striking a balance--but we ought to compare all the
members, high and low, at two periods. At an ancient epoch the highest and
lowest molluscoidal animals, namely, cephalopods and brachiopods, swarmed
in numbers; at the present time both groups are greatly reduced, while
others, intermediate in organisation, have largely increased; consequently
some naturalists maintain that molluscs were formerly more highly developed
than at present; but a stronger case can be made out on the opposite side,
by considering the vast reduction of brachiopods, and the fact that our
existing cephalopods, though few in number, are more highly organised than
their ancient representatives. We ought also to compare the relative
proportional numbers, at any two periods, of the high and low classes
throughout the world: if, for instance, at the present day fifty thousand
kinds of vertebrate animals exist, and if we knew that at some former
period only ten thousand kinds existed, we ought to look at this increase
in number in the highest class, which implies a great displacement of lower
forms, as a decided advance in the organisation of the world. We thus see
how hopelessly difficult it is to compare with perfect fairness, under such
extremely complex relations, the standard of organisation of the
imperfectly-known faunas of successive periods.
We shall appreciate this difficulty more clearly by looking to certain
existing faunas and floras. From the extraordinary manner in which
European productions have recently spread over New Zealand, and have seized
on places which must have been previously occupied by the indigenes, we
must believe, that if all the animals and plants of Great Britain were set
free in New Zealand, a multitude of British forms would in the course of
time become thoroughly naturalized there, and would exterminate many of the
natives. On the other hand, from the fact that hardly a single inhabitant
of the southern hemisphere has become wild in any part of Europe, we may
well doubt whether, if all the productions of New Zealand were set free in
Great Britain, any considerable number would be enabled to seize on places
now occupied by our native plants and animals. Under this point of view,
the productions of Great Britain stand much higher in the scale than those
of New Zealand. Yet the most skilful naturalist, from an examination of
the species of the two countries, could not have foreseen this result.
Agassiz and several other highly competent judges insist that ancient
animals resemble to a certain extent the embryos of recent animals
belonging to the same classes; and that the geological succession of
extinct forms is nearly parallel with the embryological development of
existing forms. This view accords admirably well with our theory. In a
future chapter I shall attempt to show that the adult differs from its
embryo, owing to variations having supervened at a not early age, and
having been inherited at a corresponding age. This process, whilst it
leaves the embryo almost unaltered, continually adds, in the course of
successive generations, more and more difference to the adult. Thus the
embryo comes to be left as a sort of picture, preserved by nature, of the
former and less modified condition of the species. This view may be true,
and yet may never be capable of proof. Seeing, for instance, that the
oldest known mammals, reptiles, and fishes strictly belong to their proper
classes, though some of these old forms are in a slight degree less
distinct from each other than are the typical members of the same groups at
the present day, it would be vain to look for animals having the common
embryological character of the Vertebrata, until beds rich in fossils are
discovered far beneath the lowest Cambrian strata--a discovery of which the
chance is small.
ON THE SUCCESSION OF THE SAME TYPES WITHIN THE SAME AREAS, DURING THE LATER
TERTIARY PERIODS.
Mr. Clift many years ago showed that the fossil mammals from the Australian
caves were closely allied to the living marsupials of that continent. In
South America, a similar relationship is manifest, even to an uneducated
eye, in the gigantic pieces of armour, like those of the armadillo, found
in several parts of La Plata; and Professor Owen has shown in the most
striking manner that most of the fossil mammals, buried there in such
numbers, are related to South American types. This relationship is even
more clearly seen in the wonderful collection of fossil bones made by MM.
Lund and Clausen in the caves of Brazil. I was so much impressed with
these facts that I strongly insisted, in 1839 and 1845, on this "law of the
succession of types,"--on "this wonderful relationship in the same
continent between the dead and the living." Professor Owen has
subsequently extended the same generalisation to the mammals of the Old
World. We see the same law in this author's restorations of the extinct
and gigantic birds of New Zealand. We see it also in the birds of the
caves of Brazil. Mr. Woodward has shown that the same law holds good with
sea-shells, but, from the wide distribution of most molluscs, it is not
well displayed by them. Other cases could be added, as the relation
between the extinct and living land-shells of Madeira; and between the
extinct and living brackish water-shells of the Aralo-Caspian Sea.
Now, what does this remarkable law of the succession of the same types
within the same areas mean? He would be a bold man who, after comparing
the present climate of Australia and of parts of South America, under the
same latitude, would attempt to account, on the one hand through dissimilar
physical conditions, for the dissimilarity of the inhabitants of these two
continents; and, on the other hand through similarity of conditions, for
the uniformity of the same types in each continent during the later
tertiary periods. Nor can it be pretended that it is an immutable law that
marsupials should have been chiefly or solely produced in Australia; or
that Edentata and other American types should have been solely produced in
South America. For we know that Europe in ancient times was peopled by
numerous marsupials; and I have shown in the publications above alluded to,
that in America the law of distribution of terrestrial mammals was formerly
different from what it now is. North America formerly partook strongly of
the present character of the southern half of the continent; and the
southern half was formerly more closely allied, than it is at present, to
the northern half. In a similar manner we know, from Falconer and
Cautley's discoveries, that Northern India was formerly more closely
related in its mammals to Africa than it is at the present time. Analogous
facts could be given in relation to the distribution of marine animals.
On the theory of descent with modification, the great law of the long
enduring, but not immutable, succession of the same types within the same
areas, is at once explained; for the inhabitants of each quarter of the
world will obviously tend to leave in that quarter, during the next
succeeding period of time, closely allied though in some degree modified
descendants. If the inhabitants of one continent formerly differed greatly
from those of another continent, so will their modified descendants still
differ in nearly the same manner and degree. But after very long intervals
of time, and after great geographical changes, permitting much
intermigration, the feebler will yield to the more dominant forms, and
there will be nothing immutable in the distribution of organic beings.
It may be asked in ridicule whether I suppose that the megatherium and
other allied huge monsters, which formerly lived in South America, have
left behind them the sloth, armadillo, and anteater, as their degenerate
descendants. This cannot for an instant be admitted. These huge animals
have become wholly extinct, and have left no progeny. But in the caves of
Brazil there are many extinct species which are closely allied in size and
in all other characters to the species still living in South America; and
some of these fossils may have been the actual progenitors of the living
species. It must not be forgotten that, on our theory, all the species of
the same genus are the descendants of some one species; so that, if six
genera, each having eight species, be found in one geological formation,
and in a succeeding formation there be six other allied or representative
genera, each with the same number of species, then we may conclude that
generally only one species of each of the older genera has left modified
descendants, which constitute the new genera containing the several
species; the other seven species of each old genus having died out and left
no progeny. Or, and this will be a far commoner case, two or three species
in two or three alone of the six older genera will be the parents of the
new genera: the other species and the other old genera having become
utterly extinct. In failing orders, with the genera and species decreasing
in numbers as is the case with the Edentata of South America, still fewer
genera and species will leave modified blood-descendants.
SUMMARY OF THE PRECEDING AND PRESENT CHAPTERS.
I have attempted to show that the geological record is extremely imperfect;
that only a small portion of the globe has been geologically explored with
care; that only certain classes of organic beings have been largely
preserved in a fossil state; that the number both of specimens and of
species, preserved in our museums, is absolutely as nothing compared with
the number of generations which must have passed away even during a single
formation; that, owing to subsidence being almost necessary for the
accumulation of deposits rich in fossil species of many kinds, and thick
enough to outlast future degradation, great intervals of time must have
elapsed between most of our successive formations; that there has probably
been more extinction during the periods of subsidence, and more variation
during the periods of elevation, and during the latter the record will have
been least perfectly kept; that each single formation has not been
continuously deposited; that the duration of each formation is probably
short compared with the average duration of specific forms; that migration
has played an important part in the first appearance of new forms in any
one area and formation; that widely ranging species are those which have
varied most frequently, and have oftenest given rise to new species; that
varieties have at first been local; and lastly, although each species must
have passed through numerous transitional stages, it is probable that the
periods, during which each underwent modification, though many and long as
measured by years, have been short in comparison with the periods during
which each remained in an unchanged condition. These causes, taken
conjointly, will to a large extent explain why--though we do find many
links--we do not find interminable varieties, connecting together all
extinct and existing forms by the finest graduated steps. It should also
be constantly borne in mind that any linking variety between two forms,
which might be found, would be ranked, unless the whole chain could be
perfectly restored, as a new and distinct species; for it is not pretended
that we have any sure criterion by which species and varieties can be
discriminated.
He who rejects this view of the imperfection of the geological record, will
rightly reject the whole theory. For he may ask in vain where are the
numberless transitional links which must formerly have connected the
closely allied or representative species, found in the successive stages of
the same great formation? He may disbelieve in the immense intervals of
time which must have elapsed between our consecutive formations; he may
overlook how important a part migration has played, when the formations of
any one great region, as those of Europe, are considered; he may urge the
apparent, but often falsely apparent, sudden coming in of whole groups of
species. He may ask where are the remains of those infinitely numerous
organisms which must have existed long before the Cambrian system was
deposited? We now know that at least one animal did then exist; but I can
answer this last question only by supposing that where our oceans now
extend they have extended for an enormous period, and where our oscillating
continents now stand they have stood since the commencement of the Cambrian
system; but that, long before that epoch, the world presented a widely
different aspect; and that the older continents, formed of formations older
than any known to us, exist now only as remnants in a metamorphosed
condition, or lie still buried under the ocean.
Passing from these difficulties, the other great leading facts in
palaeontology agree admirably with the theory of descent with modification
through variation and natural selection. We can thus understand how it is
that new species come in slowly and successively; how species of different
classes do not necessarily change together, or at the same rate, or in the
same degree; yet in the long run that all undergo modification to some
extent. The extinction of old forms is the almost inevitable consequence
of the production of new forms. We can understand why, when a species has
once disappeared, it never reappears. Groups of species increase in
numbers slowly, and endure for unequal periods of time; for the process of
modification is necessarily slow, and depends on many complex
contingencies. The dominant species belonging to large and dominant groups
tend to leave many modified descendants, which form new sub-groups and
groups. As these are formed, the species of the less vigorous groups, from
their inferiority inherited from a common progenitor, tend to become
extinct together, and to leave no modified offspring on the face of the
earth. But the utter extinction of a whole group of species has sometimes
been a slow process, from the survival of a few descendants, lingering in
protected and isolated situations. When a group has once wholly
disappeared, it does not reappear; for the link of generation has been
broken.
We can understand how it is that dominant forms which spread widely and
yield the greatest number of varieties tend to people the world with
allied, but modified, descendants; and these will generally succeed in
displacing the groups which are their inferiors in the struggle for
existence. Hence, after long intervals of time, the productions of the
world appear to have changed simultaneously.
We can understand how it is that all the forms of life, ancient and recent,
make together a few grand classes. We can understand, from the continued
tendency to divergence of character, why the more ancient a form is, the
more it generally differs from those now living. Why ancient and extinct
forms often tend to fill up gaps between existing forms, sometimes blending
two groups, previously classed as distinct into one; but more commonly
bringing them only a little closer together. The more ancient a form is,
the more often it stands in some degree intermediate between groups now
distinct; for the more ancient a form is, the more nearly it will be
related to, and consequently resemble, the common progenitor of groups,
since become widely divergent. Extinct forms are seldom directly
intermediate between existing forms; but are intermediate only by a long
and circuitous course through other extinct and different forms. We can
clearly see why the organic remains of closely consecutive formations are
closely allied; for they are closely linked together by generation. We can
clearly see why the remains of an intermediate formation are intermediate
in character.
The inhabitants of the world at each successive period in its history have
beaten their predecessors in the race for life, and are, in so far, higher
in the scale, and their structure has generally become more specialised;
and this may account for the common belief held by so many
palaeontologists, that organisation on the whole has progressed. Extinct
and ancient animals resemble to a certain extent the embryos of the more
recent animals belonging to the same classes, and this wonderful fact
receives a simple explanation according to our views. The succession of
the same types of structure within the same areas during the later
geological periods ceases to be mysterious, and is intelligible on the
principle of inheritance.
If, then, the geological record be as imperfect as many believe, and it may
at least be asserted that the record cannot be proved to be much more
perfect, the main objections to the theory of natural selection are greatly
diminished or disappear. On the other hand, all the chief laws of
palaeontology plainly proclaim, as it seems to me, that species have been
produced by ordinary generation: old forms having been supplanted by new
and improved forms of life, the products of variation and the survival of
the fittest.
CHAPTER XII.
GEOGRAPHICAL DISTRIBUTION.
Present distribution cannot be accounted for by differences in physical
conditions -- Importance of barriers -- Affinity of the productions of the
same continent -- Centres of creation -- Means of dispersal by changes of
climate and of the level of the land, and by occasional means -- Dispersal
during the Glacial period -- Alternate Glacial periods in the North and
South.
In considering the distribution of organic beings over the face of the
globe, the first great fact which strikes us is, that neither the
similarity nor the dissimilarity of the inhabitants of various regions can
be wholly accounted for by climatal and other physical conditions. Of
late, almost every author who has studied the subject has come to this
conclusion. The case of America alone would almost suffice to prove its
truth; for if we exclude the arctic and northern temperate parts, all
authors agree that one of the most fundamental divisions in geographical
distribution is that between the New and Old Worlds; yet if we travel over
the vast American continent, from the central parts of the United States to
its extreme southern point, we meet with the most diversified conditions;
humid districts, arid deserts, lofty mountains, grassy plains, forests,
marshes, lakes and great rivers, under almost every temperature. There is
hardly a climate or condition in the Old World which cannot be paralleled
in the New--at least so closely as the same species generally require. No
doubt small areas can be pointed out in the Old World hotter than any in
the New World; but these are not inhabited by a fauna different from that
of the surrounding districts; for it is rare to find a group of organisms
confined to a small area, of which the conditions are peculiar in only a
slight degree. Notwithstanding this general parallelism in the conditions
of Old and New Worlds, how widely different are their living productions!
In the southern hemisphere, if we compare large tracts of land in
Australia, South Africa, and western South America, between latitudes 25
and 35 degrees, we shall find parts extremely similar in all their
conditions, yet it would not be possible to point out three faunas and
floras more utterly dissimilar. Or, again, we may compare the productions
of South America south of latitude 35 degrees with those north of 25
degrees, which consequently are separated by a space of ten degrees of
latitude, and are exposed to considerably different conditions; yet they
are incomparably more closely related to each other than they are to the
productions of Australia or Africa under nearly the same climate.
Analogous facts could be given with respect to the inhabitants of the sea.
A second great fact which strikes us in our general review is, that
barriers of any kind, or obstacles to free migration, are related in a
close and important manner to the differences between the productions of
various regions. We see this in the great difference in nearly all the
terrestrial productions of the New and Old Worlds, excepting in the
northern parts, where the land almost joins, and where, under a slightly
different climate, there might have been free migration for the northern
temperate forms, as there now is for the strictly arctic productions. We
see the same fact in the great difference between the inhabitants of
Australia, Africa, and South America under the same latitude; for these
countries are almost as much isolated from each other as is possible. On
each continent, also, we see the same fact; for on the opposite sides of
lofty and continuous mountain-ranges, and of great deserts and even of
large rivers, we find different productions; though as mountain chains,
deserts, etc., are not as impassable, or likely to have endured so long, as
the oceans separating continents, the differences are very inferior in
degree to those characteristic of distinct continents.
Turning to the sea, we find the same law. The marine inhabitants of the
eastern and western shores of South America are very distinct, with
extremely few shells, crustacea, or echinodermata in common; but Dr.
Gunther has recently shown that about thirty per cent of the fishes are the
same on the opposite sides of the isthmus of Panama; and this fact has led
naturalists to believe that the isthmus was formerly open. Westward of the
shores of America, a wide space of open ocean extends, with not an island
as a halting-place for emigrants; here we have a barrier of another kind,
and as soon as this is passed we meet in the eastern islands of the Pacific
with another and totally distinct fauna. So that three marine faunas range
northward and southward in parallel lines not far from each other, under
corresponding climate; but from being separated from each other by
impassable barriers, either of land or open sea, they are almost wholly
distinct. On the other hand, proceeding still further westward from the
eastern islands of the tropical parts of the Pacific, we encounter no
impassable barriers, and we have innumerable islands as halting-places, or
continuous coasts, until, after travelling over a hemisphere, we come to
the shores of Africa; and over this vast space we meet with no well-defined
and distinct marine faunas. Although so few marine animals are common to
the above-named three approximate faunas of Eastern and Western America and
the eastern Pacific islands, yet many fishes range from the Pacific into
the Indian Ocean, and many shells are common to the eastern islands of the
Pacific and the eastern shores of Africa on almost exactly opposite
meridians of longitude.
A third great fact, partly included in the foregoing statement, is the
affinity of the productions of the same continent or of the same sea,
though the species themselves are distinct at different points and
stations. It is a law of the widest generality, and every continent offers
innumerable instances. Nevertheless, the naturalist, in travelling, for
instance, from north to south, never fails to be struck by the manner in
which successive groups of beings, specifically distinct, though nearly
related, replace each other. He hears from closely allied, yet distinct
kinds of birds, notes nearly similar, and sees their nests similarly
constructed, but not quite alike, with eggs coloured in nearly the same
manner. The plains near the Straits of Magellan are inhabited by one
species of Rhea (American ostrich), and northward the plains of La Plata by
another species of the same genus; and not by a true ostrich or emu, like
those inhabiting Africa and Australia under the same latitude. On these
same plains of La Plata we see the agouti and bizcacha, animals having
nearly the same habits as our hares and rabbits, and belonging to the same
order of Rodents, but they plainly display an American type of structure.
We ascend the lofty peaks of the Cordillera, and we find an alpine species
of bizcacha; we look to the waters, and we do not find the beaver or
muskrat, but the coypu and capybara, rodents of the South American type.
Innumerable other instances could be given. If we look to the islands off
the American shore, however much they may differ in geological structure,
the inhabitants are essentially American, though they may be all peculiar
species. We may look back to past ages, as shown in the last chapter, and
we find American types then prevailing on the American continent and in the
American seas. We see in these facts some deep organic bond, throughout
space and time, over the same areas of land and water, independently of
physical conditions. The naturalist must be dull who is not led to inquire
what this bond is.
The bond is simply inheritance, that cause which alone, as far as we
positively know, produces organisms quite like each other, or, as we see in
the case of varieties, nearly alike. The dissimilarity of the inhabitants
of different regions may be attributed to modification through variation
and natural selection, and probably in a subordinate degree to the definite
influence of different physical conditions. The degrees of dissimilarity
will depend on the migration of the more dominant forms of life from one
region into another having been more or less effectually prevented, at
periods more or less remote--on the nature and number of the former
immigrants--and on the action of the inhabitants on each other in leading
to the preservation of different modifications; the relation of organism to
organism in the struggle for life being, as I have already often remarked,
the most important of all relations. Thus the high importance of barriers
comes into play by checking migration; as does time for the slow process of
modification through natural selection. Widely-ranging species, abounding
in individuals, which have already triumphed over many competitors in their
own widely-extended homes, will have the best chance of seizing on new
places, when they spread out into new countries. In their new homes they
will be exposed to new conditions, and will frequently undergo further
modification and improvement; and thus they will become still further
victorious, and will produce groups of modified descendants. On this
principle of inheritance with modification we can understand how it is that
sections of genera, whole genera, and even families, are confined to the
same areas, as is so commonly and notoriously the case.
There is no evidence, as was remarked in the last chapter, of the existence
of any law of necessary development. As the variability of each species is
an independent property, and will be taken advantage of by natural
selection, only so far as it profits each individual in its complex
struggle for life, so the amount of modification in different species will
be no uniform quantity. If a number of species, after having long competed
with each other in their old home, were to migrate in a body into a new and
afterwards isolated country, they would be little liable to modification;
for neither migration nor isolation in themselves effect anything. These
principles come into play only by bringing organisms into new relations
with each other and in a lesser degree with the surrounding physical
conditions. As we have seen in the last chapter that some forms have
retained nearly the same character from an enormously remote geological
period, so certain species have migrated over vast spaces, and have not
become greatly or at all modified.
According to these views, it is obvious that the several species of the
same genus, though inhabiting the most distant quarters of the world, must
originally have proceeded from the same source, as they are descended from
the same progenitor. In the case of those species which have undergone,
during whole geological periods, little modification, there is not much
difficulty in believing that they have migrated from the same region; for
during the vast geographical and climatical changes which have supervened
since ancient times, almost any amount of migration is possible. But in
many other cases, in which we have reason to believe that the species of a
genus have been produced within comparatively recent times, there is great
difficulty on this head. It is also obvious that the individuals of the
same species, though now inhabiting distant and isolated regions, must have
proceeded from one spot, where their parents were first produced: for, as
has been explained, it is incredible that individuals identically the same
should have been produced from parents specifically distinct.
SINGLE CENTRES OF SUPPOSED CREATION.
We are thus brought to the question which has been largely discussed by
naturalists, namely, whether species have been created at one or more
points of the earth's surface. Undoubtedly there are many cases of extreme
difficulty in understanding how the same species could possibly have
migrated from some one point to the several distant and isolated points,
where now found. Nevertheless the simplicity of the view that each species
was first produced within a single region captivates the mind. He who
rejects it, rejects the vera causa of ordinary generation with subsequent
migration, and calls in the agency of a miracle. It is universally
admitted, that in most cases the area inhabited by a species is continuous;
and that when a plant or animal inhabits two points so distant from each
other, or with an interval of such a nature, that the space could not have
been easily passed over by migration, the fact is given as something
remarkable and exceptional. The incapacity of migrating across a wide sea
is more clear in the case of terrestrial mammals than perhaps with any
other organic beings; and, accordingly, we find no inexplicable instances
of the same mammals inhabiting distant points of the world. No geologist
feels any difficulty in Great Britain possessing the same quadrupeds with
the rest of Europe, for they were no doubt once united. But if the same
species can be produced at two separate points, why do we not find a single
mammal common to Europe and Australia or South America? The conditions of
life are nearly the same, so that a multitude of European animals and
plants have become naturalised in America and Australia; and some of the
aboriginal plants are identically the same at these distant points of the
northern and southern hemispheres? The answer, as I believe, is, that
mammals have not been able to migrate, whereas some plants, from their
varied means of dispersal, have migrated across the wide and broken
interspaces. The great and striking influence of barriers of all kinds, is
intelligible only on the view that the great majority of species have been
produced on one side, and have not been able to migrate to the opposite
side. Some few families, many subfamilies, very many genera, a still
greater number of sections of genera, are confined to a single region; and
it has been observed by several naturalists that the most natural genera,
or those genera in which the species are most closely related to each
other, are generally confined to the same country, or if they have a wide
range that their range is continuous. What a strange anomaly it would be
if a directly opposite rule were to prevail when we go down one step lower
in the series, namely to the individuals of the same species, and these had
not been, at least at first, confined to some one region!
Hence, it seems to me, as it has to many other naturalists, that the view
of each species having been produced in one area alone, and having
subsequently migrated from that area as far as its powers of migration and
subsistence under past and present conditions permitted, is the most
probable. Undoubtedly many cases occur in which we cannot explain how the
same species could have passed from one point to the other. But the
geographical and climatical changes which have certainly occurred within
recent geological times, must have rendered discontinuous the formerly
continuous range of many species. So that we are reduced to consider
whether the exceptions to continuity of range are so numerous, and of so
grave a nature, that we ought to give up the belief, rendered probable by
general considerations, that each species has been produced within one
area, and has migrated thence as far as it could. It would be hopelessly
tedious to discuss all the exceptional cases of the same species, now
living at distant and separated points; nor do I for a moment pretend that
any explanation could be offered of many instances. But, after some
preliminary remarks, I will discuss a few of the most striking classes of
facts, namely, the existence of the same species on the summits of distant
mountain ranges, and at distant points in the Arctic and Antarctic regions;
and secondly (in the following chapter), the wide distribution of fresh
water productions; and thirdly, the occurrence of the same terrestrial
species on islands and on the nearest mainland, though separated by
hundreds of miles of open sea. If the existence of the same species at
distant and isolated points of the earth's surface can in many instances be
explained on the view of each species having migrated from a single
birthplace; then, considering our ignorance with respect to former
climatical and geographical changes, and to the various occasional means of
transport, the belief that a single birthplace is the law seems to me
incomparably the safest.
In discussing this subject we shall be enabled at the same time to consider
a point equally important for us, namely, whether the several species of a
genus which must on our theory all be descended from a common progenitor,
can have migrated, undergoing modification during their migration from some
one area. If, when most of the species inhabiting one region are different
from those of another region, though closely allied to them, it can be
shown that migration from the one region to the other has probably occurred
at some former period, our general view will be much strengthened; for the
explanation is obvious on the principle of descent with modification. A
volcanic island, for instance, upheaved and formed at the distance of a few
hundreds of miles from a continent, would probably receive from it in the
course of time a few colonists, and their descendants, though modified,
would still be related by inheritance to the inhabitants of that continent.
Cases of this nature are common, and are, as we shall hereafter see,
inexplicable on the theory of independent creation. This view of the
relation of the species of one region to those of another, does not differ
much from that advanced by Mr. Wallace, who concludes that "every species
has come into existence coincident both in space and time with a
pre-existing closely allied species." And it is now well known that he
attributes this coincidence to descent with modification.
The question of single or multiple centres of creation differs from another
though allied question, namely, whether all the individuals of the same
species are descended from a single pair, or single hermaphrodite, or
whether, as some authors suppose, from many individuals simultaneously
created. With organic beings which never intercross, if such exist, each
species, must be descended from a succession of modified varieties, that
have supplanted each other, but have never blended with other individuals
or varieties of the same species, so that, at each successive stage of
modification, all the individuals of the same form will be descended from a
single parent. But in the great majority of cases, namely, with all
organisms which habitually unite for each birth, or which occasionally
intercross, the individuals of the same species inhabiting the same area
will be kept nearly uniform by intercrossing; so that many individuals will
go on simultaneously changing, and the whole amount of modification at each
stage will not be due to descent from a single parent. To illustrate what
I mean: our English race-horses differ from the horses of every other
breed; but they do not owe their difference and superiority to descent from
any single pair, but to continued care in the selecting and training of
many individuals during each generation.
Before discussing the three classes of facts, which I have selected as
presenting the greatest amount of difficulty on the theory of "single
centres of creation," I must say a few words on the means of dispersal.
MEANS OF DISPERSAL.
Sir C. Lyell and other authors have ably treated this subject. I can give
here only the briefest abstract of the more important facts. Change of
climate must have had a powerful influence on migration. A region now
impassable to certain organisms from the nature of its climate, might have
been a high road for migration, when the climate was different. I shall,
however, presently have to discuss this branch of the subject in some
detail. Changes of level in the land must also have been highly
influential: a narrow isthmus now separates two marine faunas; submerge
it, or let it formerly have been submerged, and the two faunas will now
blend together, or may formerly have blended. Where the sea now extends,
land may at a former period have connected islands or possibly even
continents together, and thus have allowed terrestrial productions to pass
from one to the other. No geologist disputes that great mutations of level
have occurred within the period of existing organisms. Edward Forbes
insisted that all the islands in the Atlantic must have been recently
connected with Europe or Africa, and Europe likewise with America. Other
authors have thus hypothetically bridged over every ocean, and united
almost every island with some mainland. If, indeed, the arguments used by
Forbes are to be trusted, it must be admitted that scarcely a single island
exists which has not recently been united to some continent. This view
cuts the Gordian knot of the dispersal of the same species to the most
distant points, and removes many a difficulty; but to the best of my
judgment we are not authorized in admitting such enormous geographical
changes within the period of existing species. It seems to me that we have
abundant evidence of great oscillations in the level of the land or sea;
but not of such vast changes in the position and extension of our
continents, as to have united them within the recent period to each other
and to the several intervening oceanic islands. I freely admit the former
existence of many islands, now buried beneath the sea, which may have
served as halting places for plants and for many animals during their
migration. In the coral-producing oceans such sunken islands are now
marked by rings of coral or atolls standing over them. Whenever it is
fully admitted, as it will some day be, that each species has proceeded
from a single birthplace, and when in the course of time we know something
definite about the means of distribution, we shall be enabled to speculate
with security on the former extension of the land. But I do not believe
that it will ever be proved that within the recent period most of our
continents which now stand quite separate, have been continuously, or
almost continuously united with each other, and with the many existing
oceanic islands. Several facts in distribution--such as the great
difference in the marine faunas on the opposite sides of almost every
continent--the close relation of the tertiary inhabitants of several lands
and even seas to their present inhabitants--the degree of affinity between
the mammals inhabiting islands with those of the nearest continent, being
in part determined (as we shall hereafter see) by the depth of the
intervening ocean--these and other such facts are opposed to the admission
of such prodigious geographical revolutions within the recent period, as
are necessary on the view advanced by Forbes and admitted by his followers.
The nature and relative proportions of the inhabitants of oceanic islands
are likewise opposed to the belief of their former continuity of
continents. Nor does the almost universally volcanic composition of such
islands favour the admission that they are the wrecks of sunken continents;
if they had originally existed as continental mountain ranges, some at
least of the islands would have been formed, like other mountain summits,
of granite, metamorphic schists, old fossiliferous and other rocks, instead
of consisting of mere piles of volcanic matter.
I must now say a few words on what are called accidental means, but which
more properly should be called occasional means of distribution. I shall
here confine myself to plants. In botanical works, this or that plant is
often stated to be ill adapted for wide dissemination; but the greater or
less facilities for transport across the sea may be said to be almost
wholly unknown. Until I tried, with Mr. Berkeley's aid, a few experiments,
it was not even known how far seeds could resist the injurious action of
sea-water. To my surprise I found that out of eighty-seven kinds, sixty-
four germinated after an immersion of twenty-eight days, and a few survived
an immersion of 137 days. It deserves notice that certain orders were far
more injured than others: nine Leguminosae were tried, and, with one
exception, they resisted the salt-water badly; seven species of the allied
orders, Hydrophyllaceae and Polemoniaceae, were all killed by a month's
immersion. For convenience sake I chiefly tried small seeds without the
capsules or fruit; and as all of these sank in a few days, they could not
have been floated across wide spaces of the sea, whether or not they were
injured by salt water. Afterwards I tried some larger fruits, capsules,
etc., and some of these floated for a long time. It is well known what a
difference there is in the buoyancy of green and seasoned timber; and it
occurred to me that floods would often wash into the sea dried plants or
branches with seed-capsules or fruit attached to them. Hence I was led to
dry the stems and branches of ninety-four plants with ripe fruit, and to
place them on sea-water. The majority sank quickly, but some which, whilst
green, floated for a very short time, when dried floated much longer; for
instance, ripe hazel-nuts sank immediately, but when dried they floated for
ninety days, and afterwards when planted germinated; an asparagus plant
with ripe berries floated for twenty-three days, when dried it floated for
eighty-five days, and the seeds afterwards germinated: the ripe seeds of
Helosciadium sank in two days, when dried they floated for above ninety
days, and afterwards germinated. Altogether, out of the ninety-four dried
plants, eighteen floated for above twenty-eight days; and some of the
eighteen floated for a very much longer period. So that as 64/87 kinds of
seeds germinated after an immersion of twenty-eight days; and as 18/94
distinct species with ripe fruit (but not all the same species as in the
foregoing experiment) floated, after being dried, for above twenty-eight
days, we may conclude, as far as anything can be inferred from these scanty
facts, that the seeds of 14/100 kinds of plants of any country might be
floated by sea-currents during twenty-eight days, and would retain their
power of germination. In Johnston's Physical Atlas, the average rate of
the several Atlantic currents is thirty-three miles per diem (some currents
running at the rate of sixty miles per diem); on this average, the seeds of
14/100 plants belonging to one country might be floated across 924 miles of
sea to another country; and when stranded, if blown by an inland gale to a
favourable spot, would germinate.
Subsequently to my experiments, M. Martens tried similar ones, but in a
much better manner, for he placed the seeds in a box in the actual sea, so
that they were alternately wet and exposed to the air like really floating
plants. He tried ninety-eight seeds, mostly different from mine, but he
chose many large fruits, and likewise seeds, from plants which live near
the sea; and this would have favoured both the average length of their
flotation and their resistance to the injurious action of the salt-water.
On the other hand, he did not previously dry the plants or branches with
the fruit; and this, as we have seen, would have caused some of them to
have floated much longer. The result was that 18/98 of his seeds of
different kinds floated for forty-two days, and were then capable of
germination. But I do not doubt that plants exposed to the waves would
float for a less time than those protected from violent movement as in our
experiments. Therefore, it would perhaps be safer to assume that the seeds
of about 10/100 plants of a flora, after having been dried, could be
floated across a space of sea 900 miles in width, and would then germinate.
The fact of the larger fruits often floating longer than the small, is
interesting; as plants with large seeds or fruit which, as Alph. de
Candolle has shown, generally have restricted ranges, could hardly be
transported by any other means.
Seeds may be occasionally transported in another manner. Drift timber is
thrown up on most islands, even on those in the midst of the widest oceans;
and the natives of the coral islands in the Pacific procure stones for
their tools, solely from the roots of drifted trees, these stones being a
valuable royal tax. I find that when irregularly shaped stones are
embedded in the roots of trees, small parcels of earth are very frequently
enclosed in their interstices and behind them, so perfectly that not a
particle could be washed away during the longest transport: out of one
small portion of earth thus COMPLETELY enclosed by the roots of an oak
about fifty years old, three dicotyledonous plants germinated: I am
certain of the accuracy of this observation. Again, I can show that the
carcasses of birds, when floating on the sea, sometimes escape being
immediately devoured; and many kinds of seeds in the crops of floating
birds long retain their vitality: peas and vetches, for instance, are
killed by even a few days' immersion in sea-water; but some taken out of
the crop of a pigeon, which had floated on artificial sea-water for thirty
days, to my surprise nearly all germinated.
Living birds can hardly fail to be highly effective agents in the
transportation of seeds. I could give many facts showing how frequently
birds of many kinds are blown by gales to vast distances across the ocean.
We may safely assume that under such circumstances their rate of flight
would often be thirty-five miles an hour; and some authors have given a far
higher estimate. I have never seen an instance of nutritious seeds passing
through the intestines of a bird; but hard seeds of fruit pass uninjured
through even the digestive organs of a turkey. In the course of two
months, I picked up in my garden twelve kinds of seeds, out of the
excrement of small birds, and these seemed perfect, and some of them, which
were tried, germinated. But the following fact is more important: the
crops of birds do not secrete gastric juice, and do not, as I know by
trial, injure in the least the germination of seeds; now, after a bird has
found and devoured a large supply of food, it is positively asserted that
all the grains do not pass into the gizzard for twelve or even eighteen
hours. A bird in this interval might easily be blown to the distance of
five hundred miles, and hawks are known to look out for tired birds, and
the contents of their torn crops might thus readily get scattered. Some
hawks and owls bolt their prey whole, and after an interval of from twelve
to twenty hours, disgorge pellets, which, as I know from experiments made
in the Zoological Gardens, include seeds capable of germination. Some
seeds of the oat, wheat, millet, canary, hemp, clover, and beet germinated
after having been from twelve to twenty-one hours in the stomachs of
different birds of prey; and two seeds of beet grew after having been thus
retained for two days and fourteen hours. Fresh-water fish, I find, eat
seeds of many land and water plants; fish are frequently devoured by birds,
and thus the seeds might be transported from place to place. I forced many
kinds of seeds into the stomachs of dead fish, and then gave their bodies
to fishing-eagles, storks, and pelicans; these birds, after an interval of
many hours, either rejected the seeds in pellets or passed them in their
excrement; and several of these seeds retained the power of germination.
Certain seeds, however, were always killed by this process.
Locusts are sometimes blown to great distances from the land. I myself
caught one 370 miles from the coast of Africa, and have heard of others
caught at greater distances. The Rev. R.T. Lowe informed Sir C. Lyell that
in November, 1844, swarms of locusts visited the island of Madeira. They
were in countless numbers, as thick as the flakes of snow in the heaviest
snowstorm, and extended upward as far as could be seen with a telescope.
During two or three days they slowly careered round and round in an immense
ellipse, at least five or six miles in diameter, and at night alighted on
the taller trees, which were completely coated with them. They then
disappeared over the sea, as suddenly as they had appeared, and have not
since visited the island. Now, in parts of Natal it is believed by some
farmers, though on insufficient evidence, that injurious seeds are
introduced into their grass-land in the dung left by the great flights of
locusts which often visit that country. In consequence of this belief Mr.
Weale sent me in a letter a small packet of the dried pellets, out of which
I extracted under the microscope several seeds, and raised from them seven
grass plants, belonging to two species, of two genera. Hence a swarm of
locusts, such as that which visited Madeira, might readily be the means of
introducing several kinds of plants into an island lying far from the
mainland.
Although the beaks and feet of birds are generally clean, earth sometimes
adheres to them: in one case I removed sixty-one grains, and in another
case twenty-two grains of dry argillaceous earth from the foot of a
partridge, and in the earth there was a pebble as large as the seed of a
vetch. Here is a better case: the leg of a woodcock was sent to me by a
friend, with a little cake of dry earth attached to the shank, weighing
only nine grains; and this contained a seed of the toad-rush (Juncus
bufonius) which germinated and flowered. Mr. Swaysland, of Brighton, who
during the last forty years has paid close attention to our migratory
birds, informs me that he has often shot wagtails (Motacillae), wheatears,
and whinchats (Saxicolae), on their first arrival on our shores, before
they had alighted; and he has several times noticed little cakes of earth
attached to their feet. Many facts could be given showing how generally
soil is charged with seeds. For instance, Professor Newton sent me the leg
of a red-legged partridge (Caccabis rufa) which had been wounded and could
not fly, with a ball of hard earth adhering to it, and weighing six and a
half ounces. The earth had been kept for three years, but when broken,
watered and placed under a bell glass, no less than eighty-two plants
sprung from it: these consisted of twelve monocotyledons, including the
common oat, and at least one kind of grass, and of seventy dicotyledons,
which consisted, judging from the young leaves, of at least three distinct
species. With such facts before us, can we doubt that the many birds which
are annually blown by gales across great spaces of ocean, and which
annually migrate--for instance, the millions of quails across the
Mediterranean--must occasionally transport a few seeds embedded in dirt
adhering to their feet or beaks? But I shall have to recur to this
subject.
As icebergs are known to be sometimes loaded with earth and stones, and
have even carried brushwood, bones, and the nest of a land-bird, it can
hardly be doubted that they must occasionally, as suggested by Lyell, have
transported seeds from one part to another of the arctic and antarctic
regions; and during the Glacial period from one part of the now temperate
regions to another. In the Azores, from the large number of plants common
to Europe, in comparison with the species on the other islands of the
Atlantic, which stand nearer to the mainland, and (as remarked by Mr. H.C.
Watson) from their somewhat northern character, in comparison with the
latitude, I suspected that these islands had been partly stocked by
ice-borne seeds during the Glacial epoch. At my request Sir C. Lyell wrote
to M. Hartung to inquire whether he had observed erratic boulders on these
islands, and he answered that he had found large fragments of granite and
other rocks, which do not occur in the archipelago. Hence we may safely
infer that icebergs formerly landed their rocky burdens on the shores of
these mid-ocean islands, and it is at least possible that they may have
brought thither the seeds of northern plants.
Considering that these several means of transport, and that other means,
which without doubt remain to be discovered, have been in action year after
year for tens of thousands of years, it would, I think, be a marvellous
fact if many plants had not thus become widely transported. These means of
transport are sometimes called accidental, but this is not strictly
correct: the currents of the sea are not accidental, nor is the direction
of prevalent gales of wind. It should be observed that scarcely any means
of transport would carry seeds for very great distances; for seeds do not
retain their vitality when exposed for a great length of time to the action
of sea water; nor could they be long carried in the crops or intestines of
birds. These means, however, would suffice for occasional transport across
tracts of sea some hundred miles in breadth, or from island to island, or
from a continent to a neighbouring island, but not from one distant
continent to another. The floras of distant continents would not by such
means become mingled; but would remain as distinct as they now are. The
currents, from their course, would never bring seeds from North America to
Britain, though they might and do bring seeds from the West Indies to our
western shores, where, if not killed by their very long immersion in salt
water, they could not endure our climate. Almost every year, one or two
land-birds are blown across the whole Atlantic Ocean, from North America to
the western shores of Ireland and England; but seeds could be transported
by these rare wanderers only by one means, namely, by dirt adhering to
their feet or beaks, which is in itself a rare accident. Even in this
case, how small would be the chance of a seed falling on favourable soil,
and coming to maturity! But it would be a great error to argue that
because a well-stocked island, like Great Britain, has not, as far as is
known (and it would be very difficult to prove this), received within the
last few centuries, through occasional means of transport, immigrants from
Europe or any other continent, that a poorly-stocked island, though
standing more remote from the mainland, would not receive colonists by
similar means. Out of a hundred kinds of seeds or animals transported to
an island, even if far less well-stocked than Britain, perhaps not more
than one would be so well fitted to its new home, as to become naturalised.
But this is no valid argument against what would be effected by occasional
means of transport, during the long lapse of geological time, whilst the
island was being upheaved, and before it had become fully stocked with
inhabitants. On almost bare land, with few or no destructive insects or
birds living there, nearly every seed which chanced to arrive, if fitted
for the climate, would germinate and survive.
DISPERSAL DURING THE GLACIAL PERIOD.
The identity of many plants and animals, on mountain-summits, separated
from each other by hundreds of miles of lowlands, where Alpine species
could not possibly exist, is one of the most striking cases known of the
same species living at distant points, without the apparent possibility of
their having migrated from one point to the other. It is indeed a
remarkable fact to see so many plants of the same species living on the
snowy regions of the Alps or Pyrenees, and in the extreme northern parts of
Europe; but it is far more remarkable, that the plants on the White
Mountains, in the United States of America, are all the same with those of
Labrador, and nearly all the same, as we hear from Asa Gray, with those on
the loftiest mountains of Europe. Even as long ago as 1747, such facts led
Gmelin to conclude that the same species must have been independently
created at many distinct points; and we might have remained in this same
belief, had not Agassiz and others called vivid attention to the Glacial
period, which, as we shall immediately see, affords a simple explanation of
these facts. We have evidence of almost every conceivable kind, organic
and inorganic, that, within a very recent geological period, central Europe
and North America suffered under an Arctic climate. The ruins of a house
burnt by fire do not tell their tale more plainly than do the mountains of
Scotland and Wales, with their scored flanks, polished surfaces, and
perched boulders, of the icy streams with which their valleys were lately
filled. So greatly has the climate of Europe changed, that in Northern
Italy, gigantic moraines, left by old glaciers, are now clothed by the vine
and maize. Throughout a large part of the United States, erratic boulders
and scored rocks plainly reveal a former cold period.
The former influence of the glacial climate on the distribution of the
inhabitants of Europe, as explained by Edward Forbes, is substantially as
follows. But we shall follow the changes more readily, by supposing a new
glacial period slowly to come on, and then pass away, as formerly occurred.
As the cold came on, and as each more southern zone became fitted for the
inhabitants of the north, these would take the places of the former
inhabitants of the temperate regions. The latter, at the same time would
travel further and further southward, unless they were stopped by barriers,
in which case they would perish. The mountains would become covered with
snow and ice, and their former Alpine inhabitants would descend to the
plains. By the time that the cold had reached its maximum, we should have
an arctic fauna and flora, covering the central parts of Europe, as far
south as the Alps and Pyrenees, and even stretching into Spain. The now
temperate regions of the United States would likewise be covered by arctic
plants and animals and these would be nearly the same with those of Europe;
for the present circumpolar inhabitants, which we suppose to have
everywhere travelled southward, are remarkably uniform round the world.
As the warmth returned, the arctic forms would retreat northward, closely
followed up in their retreat by the productions of the more temperate
regions. And as the snow melted from the bases of the mountains, the
arctic forms would seize on the cleared and thawed ground, always
ascending, as the warmth increased and the snow still further disappeared,
higher and higher, whilst their brethren were pursuing their northern
journey. Hence, when the warmth had fully returned, the same species,
which had lately lived together on the European and North American
lowlands, would again be found in the arctic regions of the Old and New
Worlds, and on many isolated mountain-summits far distant from each other.
Thus we can understand the identity of many plants at points so immensely
remote as the mountains of the United States and those of Europe. We can
thus also understand the fact that the Alpine plants of each mountain-range
are more especially related to the arctic forms living due north or nearly
due north of them: for the first migration when the cold came on, and the
re-migration on the returning warmth, would generally have been due south
and north. The Alpine plants, for example, of Scotland, as remarked by Mr.
H.C. Watson, and those of the Pyrenees, as remarked by Ramond, are more
especially allied to the plants of northern Scandinavia; those of the
United States to Labrador; those of the mountains of Siberia to the arctic
regions of that country. These views, grounded as they are on the
perfectly well-ascertained occurrence of a former Glacial period, seem to
me to explain in so satisfactory a manner the present distribution of the
Alpine and Arctic productions of Europe and America, that when in other
regions we find the same species on distant mountain-summits, we may almost
conclude, without other evidence, that a colder climate formerly permitted
their migration across the intervening lowlands, now become too warm for
their existence.
As the arctic forms moved first southward and afterwards backward to the
north, in unison with the changing climate, they will not have been exposed
during their long migrations to any great diversity of temperature; and as
they all migrated in a body together, their mutual relations will not have
been much disturbed. Hence, in accordance with the principles inculcated
in this volume, these forms will not have been liable to much modification.
But with the Alpine productions, left isolated from the moment of the
returning warmth, first at the bases and ultimately on the summits of the
mountains, the case will have been somewhat different; for it is not likely
that all the same arctic species will have been left on mountain ranges far
distant from each other, and have survived there ever since; they will
also, in all probability, have become mingled with ancient Alpine species,
which must have existed on the mountains before the commencement of the
Glacial epoch, and which during the coldest period will have been
temporarily driven down to the plains; they will, also, have been
subsequently exposed to somewhat different climatical influences. Their
mutual relations will thus have been in some degree disturbed; consequently
they will have been liable to modification; and they have been modified;
for if we compare the present Alpine plants and animals of the several
great European mountain ranges, one with another, though many of the
species remain identically the same, some exist as varieties, some as
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