Darwinism (1889) by Alfred Russel Wallace

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[Footnote 180: For fuller particulars, see Sir J. Hooker's _Introduction
to Floras of New Zealand and Australia_, and a summary in my _Island
Life_, chaps. xxii. xxiii.]

[Footnote 181: For a fuller discussion of this subject, see my _Island
Life_, chap. xxiii.]

[Footnote 182: A very remarkable case of wind conveyance of seeds on a
large scale is described in a letter from Mr. Thomas Hanbury to his
brother, the late Daniel Hanbury, which has been kindly communicated to
me by Mr. Hemsley of Kew. The letter is dated "Shanghai, 1st May 1856,"
and the passage referred to is as follows:--

"For the past three days we have had very warm weather for this time of
year, in fact almost as warm as the middle of summer. Last evening the
wind suddenly changed round to the north and blew all night with
considerable violence, making a great change in the atmosphere.

"This morning, myriads of small white particles are floating about in
the air; there is not a single cloud and no mist, yet the sun is quite
obscured by this substance, and it looks like a white fog in England. I
enclose thee a sample, thinking it may interest. It is evidently a
vegetable production; I think, apparently, some kind of seed."

Mr. Hemsley adds, that this substance proves to be the plumose seeds of
a poplar or willow. In order to produce the effects described--_quite
obscuring the sun like a white fog_,--the seeds must have filled the air
to a very great height; and they must have been brought from some
district where there were extensive tracts covered with the tree which
produced them.]

CHAPTER XIII

THE GEOLOGICAL EVIDENCES OF EVOLUTION

What we may expect--The number of known species of extinct
animals--Causes of the imperfection of the geological
record--Geological evidences of
evolution--Shells--Crocodiles--The rhinoceros tribe--The
pedigree of the horse tribe--Development of deer's horns--Brain
development--Local relations of fossil and living animals--Cause
of extinction of large animals--Indications of general progress
in plants and animals--The progressive development of
plants--Possible cause of sudden late appearance of
exogens--Geological distribution of insects--Geological
succession of vertebrata--Concluding remarks.

The theory of evolution in the organic world necessarily implies that
the forms of animals and plants have, broadly speaking, progressed from
a more generalised to a more specialised structure, and from simpler to
more complex forms. We know, however, that this progression has been by
no means regular, but has been accompanied by repeated degradation and
degeneration; while extinction on an enormous scale has again and again
stopped all progress in certain directions, and has often compelled a
fresh start in development from some comparatively low and imperfect
type.

The enormous extension of geological research in recent times has made
us acquainted with a vast number of extinct organisms, so vast that in
some important groups--such as the mollusca--the fossil are more
numerous than the living species; while in the mammalia they are not
much less numerous, the preponderance of living species being chiefly in
the smaller and in the arboreal forms which have not been so well
preserved as the members of the larger groups. With such a wealth of
material to illustrate the successive stages through which animals have
passed, it will naturally be expected that we should find important
evidence of evolution. We should hope to learn the steps by which some
isolated forms have been connected with their nearest allies, and in
many cases to have the gaps filled up which now separate genus from
genus, or species from species. In some cases these expectations are
fulfilled, but in many other cases we seek in vain for evidence of the
kind we desire; and this absence of evidence with such an apparent
wealth of material is held by many persons to throw doubt on the theory
of evolution itself. They urge, with much appearance of reason, that all
the arguments we have hitherto adduced fall short of demonstration, and
that the crucial test consists in being able to show, in a great number
of cases, those connecting links which we say must have existed. Many of
the gaps that still remain are so vast that it seems incredible to these
writers that they could ever have been filled up by a close succession
of species, since these must have spread over so many ages, and have
existed in such numbers, that it seems impossible to account for their
total absence from deposits in which great numbers of species belonging
to other groups are preserved and have been discovered. In order to
appreciate the force, or weakness, of these objections, we must inquire
into the character and completeness of that record of the past life of
the earth which geology has unfolded, and ascertain the nature and
amount of the evidence which, under actual conditions, we may expect to
find.

_The Number of known Species of Extinct Animals._

When we state that the known fossil mollusca are considerably more
numerous than those which now live on the earth, it appears at first
sight that our knowledge is very complete, but this is far from being
the case. The species have been continually changing throughout
geological time, and at each period have probably been as numerous as
they are now. If we divide the fossiliferous strata into twelve great
divisions--the Pliocene, Miocene, Eocene, Cretaceous, Oolite, Lias,
Trias, Permian, Carboniferous, Devonian, Silurian, and Cambrian,--we
find not only that each has a very distinct and characteristic molluscan
fauna, but that the different subdivisions often present a widely
different series of species; so that although a certain number of
species are common to two or more of the great divisions, the totality
of the species that have lived upon the earth must be very much more
than twelve times--perhaps even thirty or forty times--the number now
living. In like manner, although the species of fossil mammals now
recognised by more or less fragmentary fossil remains may not be much
less numerous than the living species, yet the duration of existence of
these was comparatively so short that they were almost completely
changed, perhaps six or seven times, during the Tertiary period; and
this is certainly only a fragment of the geological time during which
mammalia existed on the globe.

There is also reason to believe that the higher animals were much more
abundant in species during past geological epochs than now, owing to the
greater equability of the climate which rendered even the arctic regions
as habitable as the temperate zones are in our time.

The same equable climate would probably cause a more uniform
distribution of moisture, and render what are now desert regions capable
of supporting abundance of animal life. This is indicated by the number
and variety of the species of large animals that have been found fossil
in very limited areas which they evidently inhabited at one period. M.
Albert Gaudry found, in the deposits of a mountain stream at Pikermi in
Greece, an abundance of large mammalia such as are nowhere to be found
living together at the present time. Among them were two species of
Mastodon, two different rhinoceroses, a gigantic wild boar, a camel and
a giraffe larger than those now living, several monkeys, carnivora
ranging from martens and civets to lions and hyaenas of the largest
size, numerous antelopes of at least five distinct genera, and besides
these many forms altogether extinct. Such were the great herds of
Hipparion, an ancestral form of horse; the Helladotherium, a huge animal
bigger than the giraffe; the Ancylotherium, one of the Edentata; the
huge Dinotherium; the Aceratherium, allied to the rhinoceros; and the
monstrous Chalicotherium, allied to the swine and ruminants, but as
large as a rhinoceros; and to prey upon these, the great Machairodus or
sabre-toothed tiger. And all these remains were found in a space 300
paces long by 60 paces broad, many of the species existing in enormous
quantities.

The Pikermi fossils belong to the Upper Miocene formation, but an
equally rich deposit of Upper Eocene age has been discovered in
South-Western France at Quercy, where M. Filhol has determined the
presence of no less than forty-two species of beasts of prey alone.
Equally remarkable are the various discoveries of mammalian fossils in
North America, especially in the old lake bottoms now forming what are
called the "bad lands" of Dakota and Nebraska, belonging to the Miocene
period. Here are found an enormous assemblage of remains, often perfect
skeletons, of herbivora and carnivora, as varied and interesting as
those from the localities already referred to in Europe; but altogether
distinct, and far exceeding, in number and variety of species of the
larger animals, the whole existing fauna of North America. Very similar
phenomena occur in South America and in Australia, leading us to the
conclusion that the earth at the present time is impoverished as regards
the larger animals, and that at each successive period of Tertiary time,
at all events, it contained a far greater number of species than now
inhabit it. The very richness and abundance of the remains which we find
in limited areas, serve to convince us how imperfect and fragmentary
must be our knowledge of the earth's fauna at any one past epoch; since
we cannot believe that all, or nearly all, of the animals which
inhabited any district were entombed in a single lake, or overwhelmed by
the floods of a single river.

But the spots where such rich deposits occur are exceedingly few and far
between when compared with the vast areas of continental land, and we
have every reason to believe that in past ages, as now, numbers of
curious species were rare or local, the commoner and more abundant
species giving a very imperfect idea of the existing series of animal
forms. Yet more important, as showing the imperfection of our knowledge,
is the enormous lapse of time between the several formations in which we
find organic remains in any abundance, so vast that in many cases we
find ourselves almost in a new world, all the species and most of the
genera of the higher animals having undergone a complete change.

_Causes of the Imperfection of the Geological Record._

These facts are quite in accordance with the conclusions of geologists
as to the necessary imperfection of the geological record, since it
requires the concurrence of a number of favourable conditions to
preserve any adequate representation of the life of a given epoch. In
the first place, the animals to be preserved must not die a natural
death by disease, or old age, or by being the prey of other animals, but
must be destroyed by some accident which shall lead to their being
embedded in the soil. They must be either carried away by floods, sink
into bogs or quicksands, or be enveloped in the mud or ashes of a
volcanic eruption; and when thus embedded they must remain undisturbed
amid all the future changes of the earth's surface.

But the chances against this are enormous, because denudation is always
going on, and the rocks we now find at the earth's surface are only a
small fragment of those which were originally laid down. The
alternations of marine and freshwater deposits, and the frequent
unconformability of strata with those which overlie them, tell us
plainly of repeated elevations and depressions of the surface, and of
denudation on an enormous scale. Almost every mountain range, with its
peaks, ridges, and valleys, is but the remnant of some vast plateau
eaten away by sub-aerial agencies; every range of sea-cliffs tell us of
long slopes of land destroyed by the waves; while almost all the older
rocks which now form the surface of the earth have been once covered
with newer deposits which have long since disappeared. Nowhere are the
evidences of this denudation more apparent than in North and South
America, where granitic or metamorphic rocks cover an area hardly less
than that of all Europe. The same rocks are largely developed in Central
Africa and Eastern Asia; while, besides those portions that appear
exposed on the surface, areas of unknown extent are buried under strata
which rest on them uncomformably, and could not, therefore, constitute
the original capping under which the whole of these rocks must once have
been deeply buried; because granite can only be formed, and metamorphism
can only go on, deep down in the crust of the earth. What an
overwhelming idea does this give us of the destruction of whole piles
of rock, miles in thickness and covering areas comparable with those of
continents; and how great must have been the loss of the innumerable
fossil forms which those rocks contained! In view of such destruction we
are forced to conclude that our palaeontological collections, rich
though they may appear, are really but small and random samples, giving
no adequate idea of the mighty series of organism which have lived upon
the earth.[183]

Admitting, however, the extreme imperfection of the geological record as
a whole, it may be urged that certain limited portions of it are fairly
complete--as, for example, the various Miocene deposits of India,
Europe, and North America,--and that in these we ought to find many
examples of species and genera linked together by intermediate forms. It
may be replied that in several cases this really occurs; and the reason
why it does not occur more often is, that the theory of evolution
requires that distinct genera should be linked together, not by a direct
passage, but by the descent of both from a common ancestor, which may
have lived in some much earlier age the record of which is either
wanting or very incomplete. An illustration given by Mr. Darwin will
make this more clear to those who have not studied the subject. The
fantail and pouter pigeons are two very distinct and unlike breeds,
which we yet know to have been both derived from the common wild
rock-pigeon. Now, if we had every variety of living pigeon before us, or
even all those which have lived during the present century, we should
find no intermediate types between these two--none combining in any
degree the characters of the pouter with that of the fantail. Neither
should we ever find such an intermediate form, even had there been
preserved a specimen of every breed of pigeon since the ancestral
rock-pigeon was first tamed by man--a period of probably several
thousand years. We thus see that a complete passage from one very
distinct species to another could not be expected even had we a complete
record of the life of any one period. What we require is a complete
record of all the species that have existed since the two forms began
to diverge from their common ancestor, and this the known imperfection
of the record renders it almost impossible that we should ever attain.
All that we have a right to expect is, that, as we multiply the fossil
forms in any group, the gaps that at first existed in that group shall
become less wide and less numerous; and also that, in some cases, a
tolerably direct series shall be found, by which the more specialised
forms of the present day shall be connected with more generalised
ancestral types. We might also expect that when a country is now
characterised by special groups of animals, the fossil forms that
immediately preceded them shall, for the most part, belong to the same
groups; and further, that, comparing the more ancient with the more
modern types, we should find indications of progression, the earlier
forms being, on the whole, lower in organisation, and less specialised
in structure than the later. Now evidence of evolution of these varied
kinds is what we do find, and almost every fresh discovery adds to their
number and cogency. In order, therefore, to show that the testimony
given by geology is entirely in favour of the theory of descent with
modification, some of the more striking of the facts will now be given.

_Geological Evidences of Evolution._

In an article in _Nature_ (vol. xiv. p. 275), Professor Judd calls
attention to some recent discoveries in the Hungarian plains, of fossil
lacustrine shells, and their careful study by Dr. Neumayr and M. Paul of
the Austrian Geological Survey. The beds in which they occur have
accumulated to the thickness of 2000 feet, containing throughout
abundance of fossils, and divisible into eight zones, each of which
exhibits a well-marked and characteristic fauna. Professor Judd then
describes the bearing of these discoveries as follows--

"The group of shells which affords the most interesting evidence
of the origin of new forms through descent with modification is
that of the genus Vivipara or Paludina, which occurs in
prodigious abundance throughout the whole series of freshwater
strata. We shall not, of course, attempt in this place to enter
into any details concerning the forty distinct _forms_ of this
genus (Dr. Neumayr very properly hesitates to call them all
_species_), which are named and described in this monograph,
and between which, as the authors show, so many connecting
links, clearly illustrating the derivation of the newer from the
older types, have been detected. On the minds of those who
carefully examine the admirably engraved figures given in the
plates accompanying this valuable memoir, or still better, the
very large series of specimens from among which the subjects of
these figures are selected, and which are now in the museum of
the Reichsanstalt of Vienna, but little doubt will, we suspect,
remain that the authors have fully made out their case, and have
demonstrated that, beyond all controversy, the series with
highly complicated ornamentation were variously derived by
descent--the lines of which are in most cases perfectly clear
and obvious--from the simple and unornamented Vivipara
achatinoides of the Congerien-Schichten (the lower division of
the series of strata). It is interesting to notice that a large
portion of these unquestionably derived forms depart so widely
from the type of the genus Vivipara, that they have been
separated on so high an authority as that of Sandberger, as a
new genus, under the name of Tulotoma. And hence we are led to
the conclusion that a vast number of forms, certainly exhibiting
specific distinctions, and according to some naturalists,
differences even entitled to be regarded of generic value, have
all a common ancestry."

It is, as Professor Judd remarks, owing to the exceptionally favourable
circumstances of a long-continued and unbroken series of deposits being
formed under physical conditions either identical or very slowly
changing, that we owe so complete a record of the process of organic
change. Usually, some disturbing elements, such as a sudden change of
physical conditions, or the immigration of new sets of forms from other
areas and the consequent retreat or partial extinction of the older
fauna, interferes with the continuity of organic development, and
produces those puzzling discordances so generally met with in geological
formations of marine origin. While a case of the kind now described
affords evidence of the origin of species complete and conclusive,
though on a necessarily very limited scale, the very rarity of the
conditions which are essential to such completeness serves to explain
why it is that in most cases the direct evidence of evolution is not to
be obtained.

Another illustration of the filling up of gaps between existing groups
is afforded by Professor Huxley's researches on fossil crocodiles. The
gap between the existing crocodiles and the lizards is very wide, but as
we go back in geological time we meet with fossil forms which are to
some extent intermediate and form a connected series. The three living
genera--Crocodilus, Alligator, and Gavialis--are found in the Eocene
formation, and allied forms of another genus, Holops, in the Chalk. From
the Chalk backward to the Lias another group of genera occurs, having
anatomical characteristics intermediate between the living crocodiles
and the most ancient forms. These, forming two genera Belodon and
Stagonolepis, are found in a still older formation, the Trias. They have
characters resembling some lizards, especially the remarkable Hatteria
of New Zealand, and have also some resemblances to the
Dinosaurians--reptiles which in some respects approach birds.
Considering how comparatively few are the remains of this group of
animals, the evidence which it affords of progressive development is
remarkably clear.[184]

Among the higher animals the rhinoceros, the horse, and the deer afford
good evidence of advance in organisation and of the filling up of the
gaps which separate the living forms from their nearest allies. The
earliest ancestral forms of the rhinoceroses occur in the Middle Eocene
of the United States, and were to some extent intermediate between the
rhinoceros and tapir families, having like the latter four toes to the
front feet, and three to those behind. These are followed in the Upper
Eocene by the genus Amynodon, in which the skull assumes more distinctly
the rhinocerotic type. Following this in the Lower Miocene we have the
Aceratherium, like the last in its feet, but still more decidedly a
rhinoceros in its general structure. From this there are two diverging
lines--one in the Old World, the other in the New. In the former, to
which the Aceratherium is supposed to have migrated in early Miocene
times, when a mild climate and luxuriant vegetation prevailed far within
the arctic circle, it gave rise to the Ceratorhinus and the various
horned rhinoceroses of late Tertiary times and of those now living. In
America a number of large hornless rhinoceroses were developed--they
are found in the Upper Miocene, Pliocene, and Post-Pliocene
formations--and then became extinct. The true rhinoceroses have three
toes on all the feet.[185]

_The Pedigree of the Horse Tribe._

Yet more remarkable is the evidence afforded by the ancestral forms of
the horse tribe which have been discovered in the American tertiaries.
The family Equidae, comprising the living horse, asses, and zebras,
differ widely from all other mammals in the peculiar structure of the
feet, all of which terminate in a single large toe forming the hoof.
They have forty teeth, the molars being formed of hard and soft material
in crescentic folds, so as to be a powerful agent in grinding up hard
grasses and other vegetable food. The former peculiarities depend upon
modifications of the skeleton, which have been thus described by
Professor Huxley:--

"Let us turn in the first place to the fore-limb. In most
quadrupeds, as in ourselves, the fore-arm contains distinct
bones, called the radius and the ulna. The corresponding region
in the horse seems at first to possess but one bone. Careful
observation, however, enables us to distinguish in this bone a
part which clearly answers to the upper end of the ulna. This is
closely united with the chief mass of the bone which represents
the radius, and runs out into a slender shaft, which may be
traced for some distance downwards upon the back of the radius,
and then in most cases thins out and vanishes. It takes still
more trouble to make sure of what is nevertheless the fact, that
a small part of the lower end of the bone of a horse's fore-arm,
which is only distinct in a very young foal, is really the lower
extremity of the ulna.

"What is commonly called the knee of a horse is its wrist. The
'cannon bone' answers to the middle bone of the five metacarpal
bones which support the palm of the hand in ourselves. The
pastern, coronary, and coffin bones of veterinarians answer to
the joints of our middle fingers, while the hoof is simply a
greatly enlarged and thickened nail. But if what lies below the
horse's 'knee' thus corresponds to the middle finger in
ourselves, what has become of the four other fingers or digits?
We find in the places of the second and fourth digits only two
slender splintlike bones, about two-thirds as long as the cannon
bone, which gradually taper to their lower ends and bear no
finger joints, or, as they are termed, phalanges. Sometimes,
small bony or gristly nodules are to be found at the bases of
these two metacarpal splints, and it is probable that these
represent rudiments of the first and fifth toes. Thus, the part
of the horse's skeleton which corresponds with that of the human
hand, contains one overgrown middle digit, and at least two
imperfect lateral digits; and these answer, respectively, to the
third, the second, and the fourth fingers in man.

"Corresponding modifications are found in the hind limb. In
ourselves, and in most quadrupeds, the leg contains two distinct
bones, a large bone, the tibia, and a smaller and more slender
bone, the fibula. But, in the horse, the fibula seems, at first,
to be reduced to its upper end; a short slender bone united with
the tibia, and ending in a point below, occupying its place.
Examination of the lower end of a young foal's shin-bone,
however, shows a distinct portion of osseous matter which is the
lower end of the fibula; so that the, apparently single, lower
end of the shin-bone is really made up of the coalesced ends of
the tibia and fibula, just as the, apparently single, lower end
of the fore-arm bone is composed of the coalesced radius and
ulna.