ON THE NUMBER OF INTERMEDIATE FORMS REQUIRED ON THE THEORY OF COMMON
DESCENT; AND ON THEIR ABSENCE IN A FOSSIL STATE
I must here premise that, according to the view ordinarily received, the
myriads of organisms, which have during past and present times peopled
this world, have been created by so many distinct acts of creation. It
is impossible to reason concerning the will of the Creator, and
therefore, according to this view, we can see no cause why or why not
the individual organism should have been created on any fixed scheme.
That all the organisms of this world have been produced on a scheme is
certain from their general affinities; and if this scheme can be shown
to be the same with that which would result from allied organic beings
descending from common stocks, it becomes highly improbable that they
have been separately created by individual acts of the will of a
Creator. For as well might it be said that, although the planets move in
courses conformably to the law of gravity, yet we ought to attribute
the course of each planet to the individual act of the will of the
Creator{306}. It is in every case more conformable with what we know of
the government of this earth, that the Creator should have imposed only
general laws. As long as no method was known by which races could become
exquisitely adapted to various ends, whilst the existence of species was
thought to be proved by the sterility{307} of their offspring, it was
allowable to attribute each organism to an individual act of creation.
But in the two former chapters it has (I think) been shown that the
production, under existing conditions, of exquisitely adapted species,
is at least _possible_. Is there then any direct evidence in favour <of> or
against this view? I believe that the geographical distribution of
organic beings in past and present times, the kind of affinity linking
them together, their so-called "metamorphic" and "abortive" organs,
appear in favour of this view. On the other hand, the imperfect evidence
of the continuousness of the organic series, which, we shall immediately
see, is required on our theory, is against it; and is the most weighty
objection{308}. The evidence, however, even on this point, as far as it
goes, is favourable; and considering the imperfection of our knowledge,
especially with respect to past ages, it would be surprising if evidence
drawn from such sources were not also imperfect.
{306} In the Essay of 1842 the author uses astronomy in the same
manner as an illustration. In the _Origin_ this does not occur; the
reference to the action of secondary causes is more general, _e.g._
Ed. i. p. 488, vi. p. 668.
{307} It is interesting to find the argument from sterility given
so prominent a place. In a corresponding passage in the _Origin_,
Ed. i. p. 480, vi. p. 659, it is more summarily treated. The author
gives, as the chief bar to the acceptance of evolution, the fact
that "we are always slow in admitting any great change of which we
do not see the intermediate steps"; and goes on to quote Lyell on
geological action. It will be remembered that the question of
sterility remained a difficulty for Huxley.
{308} Similar statements occur in the Essay of 1842, p. 24, note 1,
and in the _Origin_, Ed. i. p. 299.
As I suppose that species have been formed in an analogous manner with
the varieties of the domesticated animals and plants, so must there have
existed intermediate forms between all the species of the same group,
not differing more than recognised varieties differ. It must not be
supposed necessary that there should have existed forms exactly
intermediate in character between any two species of a genus, or even
between any two varieties of a species; but it is necessary that there
should have existed every intermediate form between the one species or
variety of the common parent, and likewise between the second species or
variety, and this same common parent. Thus it does not necessarily
follow that there ever has existed <a> series of intermediate sub-varieties
(differing no more than the occasional seedlings from the same
seed-capsule,) between broccoli and common red cabbage; but it is
certain that there has existed, between broccoli and the wild parent
cabbage, a series of such intermediate seedlings, and again between red
cabbage and the wild parent cabbage: so that the broccoli and red
cabbage are linked together, but not _necessarily_ by directly
intermediate forms{309}. It is of course possible that there _may_ have
been directly intermediate forms, for the broccoli may have long since
descended from a common red cabbage, and this from the wild cabbage. So
on my theory, it must have been with species of the same genus. Still
more must the supposition be avoided that there has necessarily ever
existed (though one _may_ have descended from <the> other) directly
intermediate forms between any two genera or families--for instance
between the genus _Sus_ and the Tapir{310}; although it is necessary
that intermediate forms (not differing more than the varieties of our
domestic animals) should have existed between Sus and some unknown
parent form, and Tapir with this same parent form. The latter may have
differed more from Sus and Tapir than these two genera now differ from
each other. In this sense, according to our theory, there has been a
gradual passage (the steps not being wider apart than our domestic
varieties) between the species of the same genus, between genera of the
same family, and between families of the same order, and so on, as far
as facts, hereafter to be given, lead us; and the number of forms which
must have at former periods existed, thus to make good this passage
between different species, genera, and families, must have been almost
infinitely great.
{309} In the _Origin_, Ed. i. p. 280, vi. p. 414 he uses his
newly-acquired knowledge of pigeons to illustrate this point.
{310} Compare the _Origin_, Ed. i. p. 281, vi. p. 414.
What evidence{311} is there of a number of intermediate forms having
existed, making a passage in the above sense, between the species of the
same groups? Some naturalists have supposed that if every fossil which
now lies entombed, together with all existing species, were collected
together, a perfect series in every great class would be formed.
Considering the enormous number of species requisite to effect this,
especially in the above sense of the forms not being _directly_
intermediate between the existing species and genera, but only
intermediate by being linked through a common but often widely different
ancestor, I think this supposition highly improbable. I am however far
from underrating the probable number of fossilised species: no one who
has attended to the wonderful progress of palaeontology during the last
few years will doubt that we as yet have found only an exceedingly small
fraction of the species buried in the crust of the earth. Although the
almost infinitely numerous intermediate forms in no one class may have
been preserved, it does not follow that they have not existed. The
fossils which have been discovered, it is important to remark, do tend,
the little way they go, to make good the series; for as observed by
Buckland they all fall into or between existing groups{312}. Moreover,
those that fall between our existing groups, fall in, according to the
manner required by our theory, for they do not directly connect two
existing species of different groups, but they connect the groups
themselves: thus the Pachydermata and Ruminantia are now separated by
several characters, <for instance> the Pachydermata{313} have both a
tibia and fibula, whilst Ruminantia have only a tibia; now the fossil
Macrauchenia has a leg bone exactly intermediate in this respect, and
likewise has some other intermediate characters. But the Macrauchenia
does not connect any one species of Pachydermata with some one other of
Ruminantia but it shows that these two groups have at one time been less
widely divided. So have fish and reptiles been at one time more closely
connected in some points than they now are. Generally in those groups in
which there has been most change, the more ancient the fossil, if not
identical with recent, the more often it falls between existing groups,
or into small existing groups which now lie between other large existing
groups. Cases like the foregoing, of which there are many, form steps,
though few and far between, in a series of the kind required by my
theory.
{311} _Origin_, Ed. i. p. 301, vi. p. 440.
{312} _Origin_, Ed. i. p. 329, vi. p. 471.
{313} The structure of the Pachyderm leg was a favourite with the
author. It is discussed in the Essay of 1842, p. 48. In the present
Essay the following sentence in the margin appears to refer to
Pachyderms and Ruminants: "There can be no doubt, if we banish all
fossils, existing groups stand more separate." The following occurs
between the lines "The earliest forms would be such as others could
radiate from."
As I have admitted the high improbability, that if every fossil were
disinterred, they would compose in each of the Divisions of Nature a
perfect series of the kind required; consequently I freely admit, that
if those geologists are in the right who consider the lowest known
formation as contemporaneous with the first appearances of life{314}; or
the several formations as at all closely consecutive; or any one
formation as containing a nearly perfect record of the organisms which
existed during the whole period of its deposition in that quarter of the
globe;--if such propositions are to be accepted, my theory must be
abandoned.
{314} _Origin_, Ed. i. p. 307, vi. p. 448.
If the Palaeozoic system is really contemporaneous with the first
appearance of life, my theory must be abandoned, both inasmuch as it
limits _from shortness of time_ the total number of forms which can have
existed on this world, and because the organisms, as fish, mollusca{315}
and star-fish found in its lower beds, cannot be considered as the
parent forms of all the successive species in these classes. But no one
has yet overturned the arguments of Hutton and Lyell, that the lowest
formations known to us are only those which have escaped being
metamorphosed <illegible>; if we argued from some considerable districts,
we might have supposed that even the Cretaceous system was that in which
life first appeared. From the number of distant points, however, in
which the Silurian system has been found to be the lowest, and not
always metamorphosed, there are some objections to Hutton's and Lyell's
view; but we must not forget that the now existing land forms only 1/5
part of the superficies of the globe, and that this fraction is only
imperfectly known. With respect to the fewness of the organisms found in
the Silurian and other Palaeozoic formations, there is less difficulty,
inasmuch as (besides their gradual obliteration) we can expect
formations of this vast antiquity to escape entire denudation, only when
they have been accumulated over a wide area, and have been subsequently
protected by vast superimposed deposits: now this could generally only
hold good with deposits accumulating in a wide and deep ocean, and
therefore unfavourable to the presence of many living things. A mere
narrow and not very thick strip of matter, deposited along a coast where
organisms most abound, would have no chance of escaping denudation and
being preserved to the present time from such immensely distant
ages{316}.
{315} <Pencil insertion by the author.> The parent-forms of Mollusca
would probably differ greatly from all recent,--it is not directly
that any one division of Mollusca would descend from first time
unaltered, whilst others had become metamorphosed from it.
{316} _Origin_, Ed. i. p. 291, vi. p. 426.
If the several known formations are at all nearly consecutive in time,
and preserve a fair record of the organisms which have existed, my
theory must be abandoned. But when we consider the great changes in
mineralogical nature and texture between successive formations, what
vast and entire changes in the geography of the surrounding countries
must generally have been effected, thus wholly to have changed the
nature of the deposits on the same area. What time such changes must
have required! Moreover how often has it not been found, that between
two conformable and apparently immediately successive deposits a vast
pile of water-worn matter is interpolated in an adjoining district. We
have no means of conjecturing in many cases how long a period{317} has
elapsed between successive formations, for the species are often wholly
different: as remarked by Lyell, in some cases probably as long a period
has elapsed between two formations as the whole Tertiary system, itself
broken by wide gaps.
{317} <Note in original.> Reflect on coming in of the Chalk,
extending from Iceland to the Crimea.
Consult the writings of any one who has particularly attended to any one
stage in the Tertiary system (and indeed of every system) and see how
deeply impressed he is with the time required for its accumulation{318}.
Reflect on the years elapsed in many cases, since the latest beds
containing only living species have been formed;--see what Jordan Smith
says of the 20,000 years since the last bed, which is above the boulder
formation in Scotland, has been upraised; or of the far longer period
since the recent beds of Sweden have been upraised 400 feet, what an
enormous period the boulder formation must have required, and yet how
insignificant are the records (although there has been plenty of
elevation to bring up submarine deposits) of the shells, which we know
existed at that time. Think, then, over the entire length of the
Tertiary epoch, and think over the probable length of the intervals,
separating the Secondary deposits. Of these deposits, moreover, those
consisting of sand and pebbles have seldom been favourable, either to
the embedment or to the preservation of fossils{319}.
{318} _Origin_, Ed. i. p. 282, vi. p. 416.
{319} _Origin_, Ed. i. pp. 288, 300, vi. pp. 422, 438.
Nor can it be admitted as probable that any one Secondary formation
contains a fair record even of those organisms which are most easily
preserved, namely hard marine bodies. In how many cases have we not
certain evidence that between the deposition of apparently closely
consecutive beds, the lower one existed for an unknown time as land,
covered with trees. Some of the Secondary formations which contain most
marine remains appear to have been formed in a wide and not deep sea,
and therefore only those marine animals which live in such situations
would be preserved{320}. In all cases, on indented rocky coasts, or any
other coast, where sediment is not accumulating, although often highly
favourable to marine animals, none can be embedded: where pure sand and
pebbles are accumulating few or none will be preserved. I may here
instance the great western line of the S. American coast{321}, tenanted
by many peculiar animals, of which none probably will be preserved to a
distant epoch. From these causes, and especially from such deposits as
are formed along a line of coast, steep above and below water, being
necessarily of little width, and therefore more likely to be
subsequently denuded and worn away, we can see why it is improbable that
our Secondary deposits contain a fair record of the Marine Fauna of any
one period. The East Indian Archipelago offers an area, as large as most
of our Secondary deposits, in which there are wide and shallow seas,
teeming with marine animals, and in which sediment is accumulating; now
supposing that all the hard marine animals, or rather those having hard
parts to preserve, were preserved to a future age, excepting those which
lived on rocky shores where no sediment or only sand and gravel were
accumulating, and excepting those embedded along the steeper coasts,
where only a narrow fringe of sediment was accumulating, supposing all
this, how poor a notion would a person at a future age have of the
Marine Fauna of the present day. Lyell{322} has compared the geological
series to a work of which only the few latter but not consecutive
chapters have been preserved; and out of which, it may be added, very
many leaves have been torn, the remaining ones only illustrating a
scanty portion of the Fauna of each period. On this view, the records
of anteceding ages confirm my theory; on any other they destroy it.
{320} <Note in original.> Neither highest or lowest fish (_i.e._
Myxina <?> or Lepidosiren) could be preserved in intelligible
condition in fossils.
{321} _Origin_, Ed. i. p. 290, vi. p. 425.
{322} See _Origin_, Ed. i. p. 310, vi. p. 452 for Lyell's metaphor.
I am indebted to Prof. Judd for pointing out that Darwin's version
of the metaphor is founded on the first edition of Lyell's
_Principles_, vol. I. and vol. III.; see the Essay of 1842, p. 27.
Finally, if we narrow the question into, why do we not find in some
instances every intermediate form between any two species? the answer
may well be that the average duration of each specific form (as we have
good reason to believe) is immense in years, and that the transition
could, according to my theory, be effected only by numberless small
gradations; and therefore that we should require for this end a most
perfect record, which the foregoing reasoning teaches us not to expect.
It might be thought that in a vertical section of great thickness in the
same formation some of the species ought to be found to vary in the
upper and lower parts{323}, but it may be doubted whether any formation
has gone on accumulating without any break for a period as long as the
duration of a species; and if it had done so, we should require a series
of specimens from every part. How rare must be the chance of sediment
accumulating for some 20 or 30 thousand years on the same spot{324},
with the bottom subsiding, so that a proper depth might be preserved for
any one species to continue living: what an amount of subsidence would
be thus required, and this subsidence must not destroy the source whence
the sediment continued to be derived. In the case of terrestrial
animals, what chance is there when the present time is become a
pleistocene formation (at an earlier period than this, sufficient
elevation to expose marine beds could not be expected), what chance is
there that future geologists will make out the innumerable transitional
sub-varieties, through which the short-horned and long-horned cattle
(so different in shape of body) have been derived from the same parent
stock{325}? Yet this transition has been effected in _the same country_,
and in a far _shorter time_, than would be probable in a wild state,
both contingencies highly favourable for the future hypothetical
geologists being enabled to trace the variation.
{323} See _More Letters_, vol. I. pp. 344-7, for Darwin's interest
in the celebrated observations of Hilgendorf and Hyatt.
{324} This corresponds partly to _Origin_, Ed. i. p. 294, vi. p.
431.
{325} _Origin_, Ed. i. p. 299, vi. p. 437.