our many tertiary and existing species.
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 in the Chalk period. This group includes the large
majority of existing species. Lately, Professor Pictet has carried their
existence one sub-stage further back; and some palaeontologists believe
that certain much older fishes, of which the affinities are as yet
imperfectly known, are really teleostean. Assuming, however, that the
whole of them did appear, as Agassiz believes, at the commencement of the
chalk formation, the fact would certainly be highly remarkable; but I
cannot see that it would be an insuperable difficulty on my theory, unless
it could likewise be shown that the species of this group appeared suddenly
and simultaneously throughout the world at this same period. 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 fish might
formerly have had a similarly confined range, and after having been largely
developed in some one sea, might 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 and similar considerations, but chiefly 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 ideas on many
points, which the discoveries of even the last dozen years have effected,
it seems to me to be about as rash in us to dogmatize on the succession of
organic beings throughout the world, as it would be for a naturalist to
land for five minutes on some one 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 graver. I allude to the manner in which numbers of species of the
same group, 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 have descended from one progenitor, apply with nearly equal
force to the earliest known species. For instance, I cannot doubt that all
the Silurian trilobites have descended from some one crustacean, which must
have lived long before the Silurian age, and which probably differed
greatly from any known animal. Some of the most ancient Silurian animals,
as the Nautilus, Lingula, &c., do not differ much from living species; and
it cannot on my theory be supposed, that these old species were the
progenitors of all the species of the orders to which they belong, for they
do not present characters in any degree intermediate between them. If,
moreover, they had been the progenitors of these orders, they would almost
certainly have been long ago supplanted and exterminated by their numerous
and improved descendants.
Consequently, if my theory be true, it is indisputable that before the
lowest Silurian stratum was deposited, long periods elapsed, as long as, or
probably far longer than, the whole interval from the Silurian age to the
present day; and that during these vast, yet quite unknown, periods of
time, the world swarmed with living creatures.
To the question why we do not find records of these vast primordial
periods, I can give no satisfactory answer. Several of the most eminent
geologists, with Sir R. Murchison at their head, are convinced that we see
in the organic remains of the lowest Silurian stratum the dawn of life on
this planet. Other highly competent judges, as Lyell and the late E.
Forbes, dispute this conclusion. We should not forget that only a small
portion of the world is known with accuracy. M. Barrande has lately added
another and lower stage to the Silurian system, abounding with new and
beneath Barrande's so-called primordial zone. The presence of phosphatic
nodules and bituminous matter in some of the lowest azoic rocks, probably
indicates the former existence of life at these periods. But the
difficulty of understanding the absence of vast piles of fossiliferous
strata, which on my theory no doubt were somewhere accumulated before the
Silurian epoch, is very great. If these most ancient beds had been wholly
worn away by denudation, or obliterated by metamorphic action, we ought to
find only small remnants of the formations next succeeding them in age, and
these ought to be very generally in a metamorphosed condition. But the
descriptions which we now 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 it has suffered the extremity of
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 existing continents of Europe and North
America. But we do not know what was the state of things in the intervals
between the 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 again 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 not one 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 there, 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 we may fairly conclude 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 earliest silurian 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 have we any right to
assume that things have thus remained from eternity? 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 immeasurably antecedent to
the silurian 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 formations older than the silurian 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 bare 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 silurian epoch in a
The several difficulties here discussed, namely our not finding in the
successive formations infinitely numerous transitional links between the
many species which now exist or have existed; the sudden manner in which
whole groups of species appear in our European formations; the almost
entire absence, as at present known, of fossiliferous formations beneath
the Silurian strata, are all undoubtedly of the gravest nature. We see
this in the plainest manner by the fact that all the most eminent
palaeontologists, namely Cuvier, Owen, Agassiz, Barrande, Falconer, E.
Forbes, &c., and all our greatest geologists, as Lyell, Murchison,
Sedgwick, &c., have unanimously, often vehemently, maintained the
immutability of species. But I have reason to believe that one great
authority, Sir Charles Lyell, from further reflexion entertains grave
doubts on this subject. I feel how rash it is to differ from these great
authorities, to whom, with others, we owe all our knowledge. Those who
think the natural geological record in any degree perfect, and who do not
attach much weight to the facts and arguments of other kinds given in this
volume, will undoubtedly at once reject my theory. For my part, following
out Lyell's metaphor, I look at the natural 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, in which the history is supposed to be
written, being more or less different in the interrupted succession of
chapters, may represent the apparently abruptly changed forms of life,
entombed in our consecutive, but widely separated formations. On this
view, the difficulties above discussed are greatly diminished, or even
disappear.
Chapter X
On the Geological Succession of Organic Beings
On the slow and successive appearance of new species -- On their different
rates of change -- Species once lost do not reappear -- Groups of species
follow the same general rules in their appearance and disappearance as do
single species -- On Extinction -- On simultaneous changes in the forms of
life throughout the world -- On the affinities of extinct species to each
other and to living species -- On the state of development of ancient forms
-- On the succession of the same types within the same areas -- Summary of
preceding and present chapters.
Let us now see whether the several facts and rules relating to the
geological succession of organic beings, better accord with the common view
of the immutability of species, or with that of their slow and gradual
modification, through descent and natural selection.
New species have appeared very slowly, one after another, both on the land
and in the waters. Lyell has shown that it is hardly possible to resist
the evidence on this head in the case of the several tertiary stages; and
every year tends to fill up the blanks between them, and to make the
percentage system of lost and new forms more gradual. In some of the most
recent beds, though undoubtedly of high antiquity if measured by years,
only one or two species are lost forms, and only one or two are new forms,
having here appeared for the first time, either locally, or, as far as we
know, on the face of the earth. If we may trust the observations of
Philippi in Sicily, the successive changes in the marine inhabitants of
that island have been many and most gradual. The secondary formations are
more broken; but, as Bronn has remarked, neither the appearance nor
disappearance of their many now extinct species has been simultaneous in
each separate formation.
Species of different genera and classes have not changed at the same rate,
or in the same degree. In the oldest tertiary beds a few living shells may
still be found in the midst of a multitude of extinct forms. Falconer has
given a striking instance of a similar fact, in an existing crocodile
associated with many strange and lost mammals and reptiles in the
sub-Himalayan deposits. The Silurian Lingula differs but little from the
living species of this genus; whereas most of the other Silurian Molluscs
and all the Crustaceans have changed greatly. The productions of the land
seem to change at a quicker rate than those of the sea, of which a striking
instance has lately been observed in Switzerland. There is some reason to
believe that organisms, considered high in the scale of nature, change more
quickly than those that are low: though there are exceptions to this rule.
The amount of organic change, as Pictet has remarked, does not strictly
correspond with the succession of our geological formations; so that
between each two consecutive formations, the forms of life have seldom
changed in exactly the same degree. Yet if we compare any but the most
closely related formations, all the species will be found to have undergone
some change. When a species has once disappeared from the face of the
earth, we have reason to believe that the same identical form never
reappears. The strongest apparent exception to this latter rule, is that
of the so-called 'colonies' of M. Barrande, which intrude for a period in
the midst of an older formation, and then allow the pre-existing fauna to
reappear; but Lyell's explanation, namely, that it is a case of temporary
migration from a distinct geographical province, seems to me satisfactory.
These several facts accord well with my theory. I believe in no fixed law
of development, causing all the inhabitants of a country to change
abruptly, or simultaneously, or to an equal degree. The process of
modification must be extremely slow. The variability of each species is
quite independent of that of all others. Whether such variability be taken
advantage of by natural selection, and whether the variations be
accumulated to a greater or lesser amount, thus causing a greater or lesser
amount of modification in the varying species, depends on many complex
contingencies,--on the variability being of a beneficial nature, on the
power of intercrossing, on the rate of breeding, on the slowly changing
physical conditions of the country, and more especially on the nature of
the other inhabitants with which the varying species comes into
competition. Hence it is by no means surprising that one species should
retain the same identical form much longer than others; or, if changing,
that it should change less. We see the same fact in geographical
distribution; for instance, in the land-shells and coleopterous insects of
Madeira having come to differ considerably from their nearest allies on the
continent of Europe, whereas the marine shells and birds have remained
unaltered. We can perhaps understand the apparently quicker rate of change
in terrestrial and in more highly organised productions compared with
marine and lower productions, by the more complex relations of the higher
beings to their organic and inorganic conditions of life, as explained in a
former chapter. When many of the inhabitants of a country have become
modified and improved, we can understand, on the principle of competition,
and on that of the many all-important relations of organism to organism,
that any form which does not become in some degree modified and improved,
will be liable to be exterminated. Hence we can see why all the species in
the same region do at last, if we look to wide enough intervals of time,
become modified; for those which do not change will become extinct.
In members of the same class the average amount of change, during long and
equal periods of time, may, perhaps, be nearly the same; but as the
accumulation of long-enduring fossiliferous formations depends on great
masses of sediment having been deposited on areas whilst subsiding, our
formations have been almost necessarily accumulated at wide and irregularly
intermittent intervals; consequently the amount of organic change exhibited
by the fossils embedded in consecutive formations is not equal. Each
formation, on this view, does not mark a new and complete act of creation,
but only an occasional scene, taken almost at hazard, in a slowly changing
drama.
We can clearly understand why a species when once lost should never
reappear, even if the very same conditions of life, organic and inorganic,
should recur. For though the offspring of one species might be adapted
(and no doubt this has occurred in innumerable instances) to fill the exact
place of another species in the economy of nature, and thus supplant it;
yet the two forms--the old and the new--would not be identically the same;
for both would almost certainly inherit different characters from their