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

sea-bottom show, that the whole of the land debris brought down by

rivers to the ocean (with the exception of pumice and other floating

matter), is deposited comparatively near to the shores, and that the

fineness of the material is an indication of the distance to which it

has been carried. Everything in the nature of gravel and sand is laid

down within a very few miles of land, only the finer muddy sediments

being carried out for 20 or 50 miles, and the very finest of all, under

the most favourable conditions, rarely extending beyond 150, or at the

utmost, 300 miles from land into the deep ocean.[164] Beyond these

distances, and covering the entire ocean floor, are various oozes formed

wholly from the debris of marine organisms; while intermingled with

these are found various volcanic products which have been either carried

through the air or floated on the surface, and a small but perfectly

recognisable quantity of meteoric matter. Ice-borne rocks are also found

abundantly scattered over the ocean bottom within a definite distance of

the arctic and antarctic circles, clearly marking out the limit of

floating icebergs in recent geological times.

Palaeozoic to the most recent Tertiary beds, consist of materials

closely corresponding to the land debris now being deposited within a

narrow belt round the shores of all continents; while no rocks have been

found which can be identified with the various oozes now forming in the

deep abysses of the ocean. It follows, therefore, that all the

geological formations have been formed in comparatively shallow water,

and always adjacent to the continental land of the period. The great

thickness of some of the formations is no indication of a deep sea, but

only of slow subsidence during the time that the deposition was in

progress. This view is now adopted by many of the most experienced

geologists, especially by Dr. Archibald Geikie, Director of the

Geological Survey of Great Britain, who, in his lecture on "Geographical

Evolution," says: "From all this evidence we may legitimately conclude

that the present land of the globe, though consisting in great measure

of marine formations, has never lain under the deep sea; but that its

site must always have been near land. Even its thick marine limestones

are the deposits of comparatively shallow water."[165]

mechanical difficulty in the way of repeated change of position of

oceans and continents which has not yet received the attention it

deserves. According to the recent careful estimate by Mr. John Murray,

the land area of the globe is to the water area as ·28 to ·72. The mean

height of the land above sea-level is 2250 feet, while the mean depth of

the ocean is 14,640 feet. Hence the bulk of dry land is 23,450,000 cubic

miles, and that of the waters of the ocean 323,800,000 cubic miles; and

it follows that if the whole of the solid matter of the earth's surface

were reduced to one level, it would be everywhere covered by an ocean

about two miles deep. The accompanying diagram will serve to render

these figures more intelligible. The length of the sections of land and

ocean are in the proportion of their respective areas, while the mean

height of the land and the mean depth of the ocean are exhibited on a

greatly increased vertical scale. If we considered the continents and

their adjacent oceans separately they would differ a little, but not

very materially, from this diagram; in some cases the proportion of land

to ocean would be a little greater, in others a little less.

the sea and land shall completely change places, we shall be met by

insuperable difficulties. We must, in the first place, assume a general

equality between elevation and subsidence during any given period,

because if the elevation over any extensive continental area were not

balanced by some subsidence of approximately equal amount, an

unsupported hollow would be left under the earth's crust. Let us now

suppose a continental area to sink, and an adjacent oceanic area to

rise, it will be seen that the greater part of the land will disappear

long before the new land has approached the surface of the ocean. This

difficulty will not be removed by supposing a portion of a continent to

subside, and the immediately adjacent portion of the ocean on the other

side of the continent to rise, because in almost every case we find that

within a comparatively short distance from the shores of all existing

continents, the ocean floor sinks rapidly to a depth of from 2000 to

3000 fathoms, and maintains a similar depth, generally speaking, over a

large portion of the oceanic areas. In order, therefore, that any area

of continental extent be upraised from the great oceans, there must be a

subsidence of a land area five or six times as great, unless it can be

shown that an extensive elevation of the ocean floor up to and far

above the surface could occur without an equivalent depression

elsewhere. The fact that the waters of the ocean are sufficient to cover

the whole globe to a depth of two miles, is alone sufficient to indicate

that the great ocean basins are permanent features of the earth's

surface, since any process of alternation of these with the land areas

would have been almost certain to result again and again in the total

disappearance of large portions, if not of all, of the dry land of the

globe. But the continuity of terrestrial life since the Devonian and

Carboniferous periods, and the existence of very similar forms in the

corresponding deposits of every continent--as well as the occurrence of

sedimentary rocks, indicating the proximity of land at the time of their

deposit, over a large portion of the surface of all the continents, and

in every geological period--assure us that no such disappearance has

ever occurred.

_Oceanic and Continental Areas._
When we speak of the permanence of oceanic and continental areas as one

of the established facts of modern research, we do not mean that

existing continents and oceans have always maintained the exact areas

and outlines that they now present, but merely, that while all of them

have been undergoing changes in outline and extent from age to age, they

have yet maintained substantially the same positions, and have never

actually changed places with each other. There are, moreover, certain

physical and biological facts which enable us to mark out these areas

with some confidence.
We have seen that there are a large number of islands which may be

classed as oceanic, because they have never formed parts of continents,

but have originated in mid-ocean, and have derived their forms of life

by migration across the sea. Their peculiarities are seen to be very

marked in comparison with those islands which there is good reason to

believe are really fragments of more extensive land areas, and are hence

termed "continental." These continental islands consist in every case of

a variety of stratified rocks of various ages, thus corresponding

closely with the usual structure of continents; although many of the

islands are small like Jersey or the Shetland Islands, or far from

continental land like the Falkland Islands or New Zealand. They all

contain indigenous mammalia or batrachia, and generally a much greater

variety of birds, reptiles, insects, and plants, than do the oceanic

islands. From these various characteristics we conclude that they have

all once formed parts of continents, or at all events of much larger

land areas, and have become isolated, either by subsidence of the

intervening land or by the effects of long-continued marine denudation.
Now, if we trace the thousand-fathom line around all our existing

continents we find that, with only two exceptions, every island which

can be classed as "continental" falls within this line, while all that

lie beyond it have the undoubted characteristics of "oceanic" islands.

We, therefore, conclude that the thousand-fathom line marks out,

approximately, the "continental area,"--that is, the limits within which

continental development and change throughout known geological time have

gone on. There may, of course, have been some extensions of land beyond

this limit, while some areas within it may always have been ocean; but

so far as we have any direct evidence, this line may be taken to mark

out, approximately, the most probable boundary between the "continental

area," which has always consisted of land and shallow sea in varying

proportions, and the great oceanic basins, within the limits of which

volcanic activity has been building up numerous islands, but whose

profound depths have apparently undergone little change.

_Madagascar and New Zealand._

all the evidence goes to show that in these cases the land connection

with the nearest continental area was very remote in time. The

extraordinary isolation of the productions of Madagascar--almost all the

most characteristic forms of mammalia, birds, and reptiles of Africa

being absent from it--renders it certain that it must have been

separated from that continent very early in the Tertiary, if not as far

back as the latter part of the Secondary period; and this extreme

antiquity is indicated by a depth of considerably more than a thousand

fathoms in the Mozambique Channel, though this deep portion is less than

a hundred miles wide between the Comoro Islands and the mainland.[166]

Madagascar is the only island on the globe with a fairly rich mammalian

fauna which is separated from a continent by a depth greater than a

thousand fathoms; and no other island presents so many peculiarities in

these animals, or has preserved so many lowly organised and archaic

forms. The exceptional character of its productions agrees exactly with

its exceptional isolation by means of a very deep arm of the sea.
New Zealand possesses no known mammals and only a single species of

batrachian; but its geological structure is perfectly continental. There

is also much evidence that it does possess one mammal, although no

specimens have been yet obtained.[167] Its reptiles and birds are highly

peculiar and more numerous than in any truly oceanic island. Now the sea

which directly separates New Zealand from Australia is more than 2000

fathoms deep, but in a north-west direction there is an extensive bank

under 1000 fathoms, extending to and including Lord Howe's Island, while

north of this are other banks of the same depth, approaching towards a

submarine extension of Queensland on the one hand, and New Caledonia on

the other, and altogether suggestive of a land union with Australia at

some very remote period. Now the peculiar relations of the New Zealand

fauna and flora with those of Australia and of the tropical Pacific

Islands to the northward indicate such a connection, probably during the

Cretaceous period; and here, again, we have the exceptional depth of the

dividing sea and the form of the ocean bottom according well with the

altogether exceptional isolation of New Zealand, an isolation which has

been held by some naturalists to be great enough to justify its claim to

be one of the primary Zoological Regions.

_The Teachings of the Thousand-Fathom Line._

beyond their present limits our continents may have extended during any

portion of the Tertiary and Secondary periods, we shall obtain a

foundation of inestimable value for our inquiries into those migrations

of animals and plants during past ages which have resulted in their

present peculiarities of distribution. We see, for instance, that the

South American and African continents have always been separated by

nearly as wide an ocean as at present, and that whatever similarities

there may be in their productions must be due to the similar forms

having been derived from a common origin in one of the great northern

continents. The radical difference between the higher forms of life of

the two continents accords perfectly with their permanent separation. If

there had been any direct connection between them during Tertiary times,

we should hardly have found the deep-seated differences between the

Quadrumana of the two regions--no family even being common to both; nor

the peculiar Insectivora of the one continent, and the equally peculiar

Edentata of the other. The very numerous families of birds quite

peculiar to one or other of these continents, many of which, by their

structural isolation and varied development of generic and specific

forms, indicate a high antiquity, equally suggest that there has been no

near approach to a land connection during the same epoch.
Looking to the two great northern continents, we see indications of a

possible connection between them both in the North Atlantic and the

North Pacific oceans; and when we remember that from middle Tertiary

times backward--so far as we know continuously to the earliest

Palaeozoic epoch--a temperate and equable climate, with abundant woody

vegetation, prevailed up to and within the arctic circle, we see what

facilities may have been afforded for migration from one continent to

the other, sometimes between America and Europe, sometimes between

America and Asia. Admitting these highly probable connections, no

bridging of the Atlantic in more southern latitudes (of which there is

not a particle of evidence) will have been necessary to account for all

the intermigration that has occurred between the two continents. If, on

the other hand, we remember how long must have been the route, and how

diverse must always have been the conditions between the more northern

and the more southern portions of the American and Euro-Asiatic

continents, we shall not be surprised that many widespread forms in

either continent have not crossed into the other; and that while the

skunks (Mephitis), the pouched rats (Saccomyidae), and the turkeys

(Meleagris) are confined to America, the pigs and the hedgehogs, the

true flycatchers and the pheasants are found only in the Euro-Asiatic

continent. But, just as there have been periods which facilitated

intermigration between America and the Old World, there have almost

certainly been periods, perhaps of long duration even geologically, when

these continents have been separated by seas as wide as, or even wider

than, those of the present day; and thus may be explained such curious

anomalies as the origination of the camel-tribe in America, and its

entrance into Asia in comparatively recent Tertiary times, while the

introduction of oxen and bears into America from the Euro-Asiatic

continent appears to have been equally recent.[168]
We shall find on examination that this view of the general permanence of

the oceanic and continental areas, with constant minor fluctuations of

land and sea over the whole extent of the latter, enables us to

understand, and offer a rational explanation of, most of the difficult

problems of geographical distribution; and further, that our power of

doing this is in direct proportion to our acquaintance with the

distribution of fossil forms of life during the Tertiary period. We

must, also, take due note of many other facts of almost equal importance

for a due appreciation of the problems presented for solution, the most

essential being, the various powers of dispersal possessed by the

different groups of animals and plants, the geological antiquity of the

species and genera, and the width and depth of the seas which separate

the countries they, inhabit. A few illustrations will now be given of

the way in which these branches of knowledge enable us to deal with the

difficulties and anomalies that present themselves.

_The Distribution of Marsupials._

sole representative of the class in Australia and New Guinea, while it

is entirely unknown in Asia, Africa, or Europe. It reappears in America,

where several species of opossums are found; and it was long thought

necessary to postulate a direct southern connection of these distant

countries, in order to account for this curious fact of distribution.

When, however, we look to what is known of the geological history of the

marsupials the difficulty vanishes. In the Upper Eocene deposits of

Western Europe the remains of several animals closely allied to the

American opossums have been found; and as, at this period, a very mild

climate prevailed far up into the arctic regions, there is no difficulty

in supposing that the ancestors of the group entered America from Europe

or Northern Asia during early Tertiary times.
But we must go much further back for the origin of the Australian

marsupials. All the chief types of the higher mammalia were in existence

in the Eocene, if not in the preceding Cretaceous period, and as we find

none of these in Australia, that country must have been finally

separated from the Asiatic continent during the Secondary or Mesozoic

period. Now during that period, in the Upper and the Lower Oolite and in

the still older Trias, the jaw-bones of numerous small mammalia have

been found, forming eight distinct genera, which are believed to have

been either marsupials or some allied lowly forms. In North America

also, in beds of the Jurassic and Triassic formations, the remains of an

equally great variety of these small mammalia have been discovered; and

from the examination of more than sixty specimens, belonging to at least

six distinct genera, Professor Marsh is of opinion that they represent a

generalised type, from which the more specialised marsupials and

insectivora were developed.
From the fact that very similar mammals occur both in Europe and America

at corresponding periods, and in beds which represent a long succession

of geological time, and that during the whole of this time no fragments

of any higher forms have been discovered, it seems probable that both

the northern continents (or the larger portion of their area) were then

inhabited by no other mammalia than these, with perhaps other equally

low types. It was, probably, not later than the Jurassic age when some

of these primitive marsupials were able to enter Australia, where they

have since remained almost completely isolated; and, being free from

the competition of higher forms, they have developed into the great

variety of types we now behold there. These occupy the place, and have

to some extent acquired the form and structure of distinct orders of the

higher mammals--the rodents, the insectivora, and the carnivora,--while

still preserving the essential characteristics and lowly organisation of

the marsupials. At a much later period--probably in late Tertiary

times--the ancestors of the various species of rats and mice which now

abound in Australia, and which, with the aerial bats, constitute its

only forms of placental mammals, entered the country from some of the

adjacent islands. For this purpose a land connection was not necessary,

as these small creatures might easily be conveyed among the branches or

in the crevices of trees uprooted by floods and carried down to the sea,

and then floated to a shore many miles distant. That no actual land

connection with, or very close approximation to, an Asiatic island has

occurred in recent times, is sufficiently proved by the fact that no

squirrel, pig, civet, or other widespread mammal of the Eastern

hemisphere has been able to reach the Australian continent.

_The Distribution of Tapirs._
These curious animals form one of the puzzles of geographical

distribution, being now confined to two very remote regions of the

globe--the Malay Peninsula and adjacent islands of Sumatra and Borneo,

inhabited by one species, and tropical America, where there are three or

four species, ranging from Brazil to Ecuador and Guatemala. If we

considered these living forms only, we should be obliged to speculate on

enormous changes of land and sea in order that these tropical animals

might have passed from one country to the other. But geological

discoveries have rendered all such hypothetical changes unnecessary.

During Miocene and Pliocene times tapirs abounded over the whole of

Europe and Asia, their remains having been found in the tertiary

deposits of France, India, Burmah, and China. In both North and South

America fossil remains of tapirs occur only in caves and deposits of

Post-Pliocene age, showing that they are comparatively recent immigrants

into that continent. They perhaps entered by the route of Kamchatka and

Alaska, where the climate, even now so much milder and more equable than

on the north-east of America, might have been warm enough in late

Pliocene times to have allowed the migration of these animals. In Asia

they were driven southwards by the competition of numerous higher and

more powerful forms, but have found a last resting-place in the swampy

forests of the Malay region.

_What these Facts Prove._

highest degree instructive, because they show us that, without any

hypothetical bridging of deep oceans, and with only such changes of sea

and land as are indicated by the extent of the comparatively shallow

seas surrounding and connecting the existing continents, we are able to

account for the anomaly of allied forms occurring only in remote and

widely separated areas. These examples really constitute crucial tests,

because, of all classes of animals, mammalia are least able to surmount

physical barriers. They are obviously unable to pass over wide arms of

the sea, while the necessity for constant supplies of food and water

renders sandy deserts or snow-clad plains equally impassable. Then,

again, the peculiar kinds of food on which alone many of them can

subsist, and their liability to the attacks of other animals, put a