The Project Gutenberg ebook of Darwinism (1889), by Alfred Russel Wallace

plants can be sufficiently understood. There will, of course, be many

cases of difficulty and some seeming anomalies, but these can usually be

seen to depend on our ignorance of some of the essential factors of the

problem. Either we do not know the distribution of the group in recent

geological times, or we are still ignorant of the special methods by

which the organisms are able to cross the sea. The latter difficulty

applies especially to the lizard tribe, which are found in almost all

the tropical oceanic islands; but the particular mode in which they are

able to traverse a wide expanse of ocean, which is a perfect barrier to

batrachia and almost so to snakes, has not yet been discovered. Lizards

are found in all the larger Pacific Islands as far as Tahiti, while

snakes do not extend beyond the Fiji Islands; and the latter are also

absent from Mauritius and Bourbon, where lizards of seven or eight

species abound. Naturalists resident in the Pacific Islands would make a

valuable contribution to our science by studying the life-history of the

native lizards, and endeavouring to ascertain the special facilities

they possess for crossing over wide spaces of ocean.
FOOTNOTES:
[Footnote 163: See A. Agassiz, _Three Cruises of the Blake_ (Cambridge,

Mass., 1888), vol. i. p. 127, footnote.]

beyond a hundred miles from the mouths of the river in the Gulf of

Mexico (A. Agassiz, _Three Cruises of the Blake_, vol. i. p. 128).]
[Footnote 165: I have given a full summary of the evidence for the

permanence of oceanic and continental areas in my _Island Life_, chap.

vi.]
[Footnote 166: For a full account of the peculiarities of the Madagascar

fauna, see my _Island Life_, chap. xix.]

xxii. More recent soundings have shown that the Map at p. 443, as well

as that of the Madagascar group at p. 387, are erroneous, the ocean

around Norfolk Island and in the Straits of Mozambique being more than

1000 fathoms deep. The general argument is, however, unaffected.]
[Footnote 168: For some details of these migrations, see the author's

_Geographical Distribution of Animals_, vol. i. p. 140; also Heilprin's

_Geographical and Geological Distribution of Animals_.]
[Footnote 169: For a full discussion of this question, see _Island

Life_, pp. 390-420.]

Birds during 1886.]

Technical Institute, and Rev. T.D. Titmas of Charterhouse for furnishing

me with the weights required.]
[Footnote 176: See _Nature_, vol. vi. p. 164, for a summary of Kerner's

paper.]
[Footnote 177: It seems quite possible that the absence of pappus in

this case is a recent adaptation, and that it has been brought about by

causes similar to those which have reduced or aborted the wings of

insects in oceanic islands. For when a plant has once reached one of the

storm-swept islands of the southern ocean, the pappus will be injurious

for the same reason that the wings of insects are injurious, since it

will lead to the seeds being blown out to sea and destroyed. The seeds

which are heaviest and have least pappus will have the best chance of

falling on the ground and remaining there to germinate, and this process

of selection might rapidly lead to the entire disappearance of the

pappus.]
[Footnote 178: See _Island Life_, p. 251.]

difficulty of transmission by floating, as the bad conditions the seeds

are usually exposed to when they reach land. Many, even if they

germinate, are destroyed by the waves, as Burchell noticed at St.

Helena; while even a flat and sheltered shore would be an unsuitable

position for many inland plants. Air-borne seeds, on the other hand, may

be carried far inland, and so scattered that some of them are likely to

reach suitable stations.]

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.]

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

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.

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.

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.

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._

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