for the slow process of variation; hence the deposit will generally have to
be a very thick one; and the species undergoing modification will have had
to live on the same area throughout this whole time. But we have seen that
a thick fossiliferous formation can only be accumulated during a period of
subsidence; and to keep the depth approximately the same, which is
necessary in order to enable the same species to live on the same space,
the supply of sediment must nearly have counterbalanced the amount of
subsidence. But this same movement of subsidence will often tend to sink
the area whence the sediment is derived, and thus diminish the supply
whilst the downward movement continues. In fact, this nearly exact
balancing between the supply of sediment and the amount of subsidence is
probably a rare contingency; for it has been observed by more than one
palaeontologist, that very thick deposits are usually barren of organic
remains, except near their upper or lower limits.
It would seem that each separate formation, like the whole pile of
formations in any country, has generally been intermittent in its
accumulation. When we see, as is so often the case, a formation composed
of beds of different mineralogical composition, we may reasonably suspect
that the process of deposition has been much interrupted, as a change in
the currents of the sea and a supply of sediment of a different nature will
generally have been due to geographical changes requiring much time. Nor
will the closest inspection of a formation give any idea of the time which
its deposition has consumed. Many instances could be given of beds only a
few feet in thickness, representing formations, elsewhere thousands of feet
in thickness, and which must have required an enormous period for their
accumulation; yet no one ignorant of this fact would have suspected the
vast lapse of time represented by the thinner formation. Many cases could
be given of the lower beds of a formation having been upraised, denuded,
submerged, and then re-covered by the upper beds of the same
formation,--facts, showing what wide, yet easily overlooked, intervals have
occurred in its accumulation. In other cases we have the plainest evidence
in great fossilised trees, still standing upright as they grew, of many
long intervals of time and changes of level during the process of
deposition, which would never even have been suspected, had not the trees
chanced to have been preserved: thus, Messrs. Lyell and Dawson found
carboniferous beds 1400 feet thick in Nova Scotia, with ancient
root-bearing strata, one above the other, at no less than sixty-eight
different levels. Hence, when the same species occur at the bottom,
middle, and top of a formation, the probability is that they have not lived
on the same spot during the whole period of deposition, but have
disappeared and reappeared, perhaps many times, during the same geological
period. So that if such species were to undergo a considerable amount of
modification during any one geological period, a section would not probably
include all the fine intermediate gradations which must on my theory have
existed between them, but abrupt, though perhaps very slight, changes of
form.
It is all-important to remember that naturalists have no golden rule by
which to distinguish species and varieties; they grant some little
variability to each species, but when they meet with a somewhat greater
amount of difference between any two forms, they rank both as species,
unless they are enabled to connect them together by close intermediate
gradations. And this from the reasons just assigned we can seldom hope to
effect in any one geological section. Supposing B and C to be two species,
and a third, A, to be found in an underlying bed; even if A were strictly
intermediate between B and C, it would simply be ranked as a third and
distinct species, unless at the same time it could be most closely
connected with either one or both forms by intermediate varieties. Nor
should it be forgotten, as before explained, that A might be the actual
progenitor of B and C, and yet might not at all necessarily be strictly
intermediate between them in all points of structure. So that we might
obtain the parent-species and its several modified descendants from the
lower and upper beds of a formation, and unless we obtained numerous
transitional gradations, we should not recognise their relationship, and
should consequently be compelled to rank them all as distinct species.
It is notorious on what excessively slight differences many
palaeontologists have founded their species; and they do this the more
readily if the specimens come from different sub-stages of the same
formation. Some experienced conchologists are now sinking many of the very
fine species of D'Orbigny and others into the rank of varieties; and on
this view we do find the kind of evidence of change which on my theory we
ought to find. Moreover, if we look to rather wider intervals, namely, to
distinct but consecutive stages of the same great formation, we find that
the embedded fossils, though almost universally ranked as specifically
different, yet are far more closely allied to each other than are the
species found in more widely separated formations; but to this subject I
shall have to return in the following chapter.
One other consideration is worth notice: with animals and plants that can
propagate rapidly and are not highly locomotive, there is reason to
suspect, as we have formerly seen, that their varieties are generally at
first local; and that such local varieties do not spread widely and
supplant their parent-forms until they have been modified and perfected in
some considerable degree. According to this view, the chance of
discovering in a formation in any one country all the early stages of
transition between any two forms, is small, for the successive changes are
supposed to have been local or confined to some one spot. Most marine
animals have a wide range; and we have seen that with plants it is those
which have the widest range, that oftenest present varieties; so that with
shells and other marine animals, it is probably those which have had the
widest range, far exceeding the limits of the known geological formations
of Europe, which have oftenest given rise, first to local varieties and
ultimately to new species; and this again would greatly lessen the chance
of our being able to trace the stages of transition in any one geological
formation.
It should not be forgotten, that at the present day, with perfect specimens
for examination, two forms can seldom be connected by intermediate
varieties and thus proved to be the same species, until many specimens have
been collected from many places; and in the case of fossil species this
could rarely be effected by palaeontologists. We shall, perhaps, best
perceive the improbability of our being enabled to connect species by
numerous, fine, intermediate, fossil links, by asking ourselves whether,
for instance, geologists at some future period will be able to prove, that
our different breeds of cattle, sheep, horses, and dogs have descended from
a single stock or from several aboriginal stocks; or, again, whether
certain sea-shells inhabiting the shores of North America, which are ranked
by some conchologists as distinct species from their European
representatives, and by other conchologists as only varieties, are really
varieties or are, as it is called, specifically distinct. This could be
effected only by the future geologist discovering in a fossil state
numerous intermediate gradations; and such success seems to me improbable
in the highest degree.
Geological research, though it has added numerous species to existing and
extinct genera, and has made the intervals between some few groups less
wide than they otherwise would have been, yet has done scarcely anything in
breaking down the distinction between species, by connecting them together
by numerous, fine, intermediate varieties; and this not having been
effected, is probably the gravest and most obvious of all the many
objections which may be urged against my views. Hence it will be worth
while to sum up the foregoing remarks, under an imaginary illustration.
The Malay Archipelago is of about the size of Europe from the North Cape to
the Mediterranean, and from Britain to Russia; and therefore equals all the
geological formations which have been examined with any accuracy, excepting
those of the United States of America. I fully agree with Mr.
Godwin-Austen, that the present condition of the Malay Archipelago, with
its numerous large islands separated by wide and shallow seas, probably
represents the former state of Europe, when most of our formations were
accumulating. The Malay Archipelago is one of the richest regions of the
whole world in organic beings; yet if all the species were to be collected
which have ever lived there, how imperfectly would they represent the
natural history of the world!
But we have every reason to believe that the terrestrial productions of the
archipelago would be preserved in an excessively imperfect manner in the
formations which we suppose to be there accumulating. I suspect that not
many of the strictly littoral animals, or of those which lived on naked
submarine rocks, would be embedded; and those embedded in gravel or sand,
would not endure to a distant epoch. Wherever sediment did not accumulate
on the bed of the sea, or where it did not accumulate at a sufficient rate
to protect organic bodies from decay, no remains could be preserved.
In our archipelago, I believe that fossiliferous formations could be formed
of sufficient thickness to last to an age, as distant in futurity as the
secondary formations lie in the past, only during periods of subsidence.
These periods of subsidence would be separated from each other by enormous
intervals, during which the area would be either stationary or rising;
whilst rising, each fossiliferous formation would be destroyed, almost as
soon as accumulated, by the incessant coast-action, as we now see on the
shores of South America. During the periods of subsidence there would
probably be much extinction of life; during the periods of elevation, there
would be much variation, but the geological record would then be least
perfect.
It may be doubted whether the duration of any one great period of
subsidence over the whole or part of the archipelago, together with a
contemporaneous accumulation of sediment, would exceed the average duration
of the same specific forms; and these contingencies are indispensable for
the preservation of all the transitional gradations between any two or more
species. If such gradations were not fully preserved, transitional
varieties would merely appear as so many distinct species. It is, also,
probable that each great period of subsidence would be interrupted by
oscillations of level, and that slight climatal changes would intervene
during such lengthy periods; and in these cases the inhabitants of the
archipelago would have to migrate, and no closely consecutive record of
their modifications could be preserved in any one formation.
Very many of the marine inhabitants of the archipelago now range thousands
of miles beyond its confines; and analogy leads me to believe that it would
be chiefly these far-ranging species which would oftenest produce new
varieties; and the varieties would at first generally be local or confined
to one place, but if possessed of any decided advantage, or when further
modified and improved, they would slowly spread and supplant their
parent-forms. When such varieties returned to their ancient homes, as they
would differ from their former state, in a nearly uniform, though perhaps
extremely slight degree, they would, according to the principles followed
by many palaeontologists, be ranked as new and distinct species.
If then, there be some degree of truth in these remarks, we have no right
to expect to find in our geological formations, an infinite number of those
fine transitional forms, which on my theory assuredly have connected all
the past and present species of the same group into one long and branching
chain of life. We ought only to look for a few links, some more closely,
some more distantly related to each other; and these links, let them be
ever so close, if found in different stages of the same formation, would,
by most palaeontologists, be ranked as distinct species. But I do not
pretend that I should ever have suspected how poor a record of the
mutations of life, the best preserved geological section presented, had not
the difficulty of our not discovering innumerable transitional links
between the species which appeared at the commencement and close of each
formation, pressed so hardly on my theory.
On the sudden appearance of whole groups of Allied Species. -- The abrupt
manner in which whole groups of species suddenly appear in certain
formations, has been urged by several palaeontologists, for instance, by
Agassiz, Pictet, and by none more forcibly than by Professor Sedgwick, as a
fatal objection to the belief in the transmutation of species. If numerous
species, belonging to the same genera or families, have really started into
life all at once, the fact would be fatal to the theory of descent with
slow modification through natural selection. For the development of a
group of forms, all of which have descended from some one progenitor, must
have been an extremely slow process; and the progenitors must have lived
long ages before their modified descendants. But we continually over-rate
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. 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 of the United States. We do not make due
allowance for
the enormous intervals of time, which have probably elapsed
between our consecutive formations,--longer perhaps in some 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 or some few
parent-forms; and in the succeeding formation such 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; but that when this had 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 be able to spread rapidly and widely
throughout the world.
I will now give a few examples to illustrate these remarks; and to show how
liable we are to error in supposing that whole groups of species have
suddenly been produced. I may recall the well-known fact that in
geological treatises, published not many years ago, the great class of
mammals was 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 one true
mammal has 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 even as far back as the
eocene stage. The most striking case, however, is that of the Whale
family; as these animals have huge bones, are marine, and range over the
world, the fact of not a single bone of a whale having been discovered in
any secondary formation, seemed fully to justify the belief that this great
and distinct order had been suddenly produced in the interval between the
latest secondary and earliest tertiary formation. But now we may read in
the Supplement to Lyell's 'Manual,' published in 1858, clear evidence of
the existence of whales in the upper greensand, some time before the close
of the secondary period.
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 have stated
that, from the 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 50 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 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 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 unmistakeable
sessile cirripede, which he had himself extracted from the chalk of
Belgium. And, as if to make the case as striking as possible, this sessile
cirripede was a Chthamalus, a very common, large, and ubiquitous genus, of
which not one specimen has as yet been found even in any tertiary stratum.
Hence we now positively know that sessile cirripedes existed during the
secondary period; and these cirripedes might have been the progenitors of