THE GEOGRAPHICAL DISTRIBUTION OF ORGANISMS
The facts to be explained--The conditions which have determined
distribution--The permanence of oceans--Oceanic and continental
areas--Madagascar and New Zealand--The teachings of the
thousand-fathom line--The distribution of marsupials--The
distribution of tapirs--Powers of dispersal as illustrated by
insular organisms--Birds and insects at sea--Insects at great
altitudes--The dispersal of plants--Dispersal of seeds by the
wind--Mineral matter carried by the wind--Objections to the
theory of wind-dispersal answered--Explanation of north
temperate plants in the southern hemisphere--No proof of
glaciation in the tropics--Lower temperature not needed to
explain the facts--Concluding remarks.
The theory which we may now take as established--that all the existing
forms of life have been derived from other forms by a natural process of
descent with modification, and that this same process has been in action
during past geological time--should enable us to give a rational account
not only of the peculiarities of form and structure presented by animals
and plants, but also of their grouping together in certain areas, and
their general distribution over the earth's surface.
In the absence of any exact knowledge of the facts of distribution, a
student of the theory of evolution might naturally anticipate that all
groups of allied organisms would be found in the same region, and that,
as he travelled farther and farther from any given centre, the forms of
life would differ more and more from those which prevailed at the
starting-point, till, in the remotest regions to which he could
penetrate, he would find an entirely new assemblage of animals and
plants, altogether unlike those with which he was familiar. He would
also anticipate that diversities of climate would always be associated
with a corresponding diversity in the forms of life.
Now these anticipations are to a considerable extent justified.
Remoteness on the earth's surface is usually an indication of diversity
in the fauna and flora, while strongly contrasted climates are always
accompanied by a considerable contrast in the forms of life. But this
correspondence is by no means exact or proportionate, and the converse
propositions are often quite untrue. Countries which are near to each
other often differ radically in their animal and vegetable productions;
while similarity of climate, together with moderate geographical
proximity, are often accompanied by marked diversities in the prevailing
forms of life. Again, while many groups of animals--genera, families,
and sometimes even orders--are confined to limited regions, most of the
families, many genera, and even some species are found in every part of
the earth. An enumeration of a few of these anomalies will better
illustrate the nature of the problem we have to solve.
As examples of extreme diversity, notwithstanding geographical
proximity, we may adduce Madagascar and Africa, whose animal and
vegetable productions are far less alike than are those of Great Britain
and Japan at the remotest extremities of the great northern continent;
while an equal, or perhaps even a still greater, diversity exists
between Australia and New Zealand. On the other hand, Northern Africa
and South Europe, though separated by the Mediterranean Sea, have faunas
and floras which do not differ from each other more than do the various
countries of Europe. As a proof that similarity of climate and general
adaptability have had but a small part in determining the forms of life
in each country, we have the fact of the enormous increase of rabbits
and pigs in Australia and New Zealand, of horses and cattle in South
America, and of the common sparrow in North America, though in none of
these cases are the animals natives of the countries in which they
thrive so well. And lastly, in illustration of the fact that allied
forms are not always found in adjacent regions, we have the tapirs,
which are found only on opposite sides of the globe, in tropical America
and the Malayan Islands; the camels of the Asiatic deserts, whose
nearest allies are the llamas and alpacas of the Andes; and the
marsupials, only found in Australia and on the opposite side of the
globe, in America. Yet, again, although mammalia may be said to be
universally distributed over the globe, being found abundantly on all
the continents and on a great many of the larger islands, yet they are
entirely wanting in New Zealand, and in a considerable number of other
islands which are, nevertheless, perfectly able to support them when
introduced.
Now most of these difficulties can be solved by means of well-known
geographical and geological facts. When the productions of remote
countries resemble each other, there is almost always continuity of land
with similarity of climate between them. When adjacent countries differ
greatly in their productions, we find them separated by a sea or strait
whose great depth is an indication of its antiquity or permanence. When
a group of animals inhabits two countries or regions separated by wide
oceans, it is found that in past geological times the same group was
much more widely distributed, and may have reached the countries it
inhabits from an intermediate region in which it is now extinct. We
know, also, that countries now united by land were divided by arms of
the sea at a not very remote epoch; while there is good reason to
believe that others now entirely isolated by a broad expanse of sea were
formerly united and formed a single land area. There is also another
important factor to be taken account of in considering how animals and
plants have acquired their present peculiarities of
distribution,--changes of climate. We know that quite recently a glacial
epoch extended over much of what are now the temperate regions of the
northern hemisphere, and that consequently the organisms which inhabit
those parts must be, comparatively speaking, recent immigrants from more
southern lands. But it is a yet more important fact that, down to middle
Tertiary times at all events, an equable temperate climate, with a
luxuriant vegetation, extended to far within the arctic circle, over
what are now barren wastes, covered for ten months of the year with snow
and ice. The arctic zone has, therefore, been in past times capable of
supporting almost all the forms of life of our temperate regions; and we
must take account of this condition of things whenever we have to
speculate on the possible migrations of organisms between the old and
new continents.
_The Conditions which have determined Distribution._
When we endeavour to explain in detail the facts of the existing
distribution of organic beings, we are confronted by several preliminary
questions, upon the solution of which will depend our treatment of the
phenomena presented to us. Upon the theory of descent which we have
adopted, all the different species of a genus, as well as all the genera
which compose a family or higher group, have descended from some common
ancestor, and must therefore, at some remote epoch, have occupied the
same area, from which their descendants have spread to the regions they
now inhabit. In the numerous cases in which the same group now occupies
countries separated by oceans or seas, by lofty mountain-chains, by wide
deserts, or by inhospitable climates, we have to consider how the
migration which must certainly have taken place has been effected. It is
possible that during some portion of the time which has elapsed since
the origin of the group the interposing barriers have not been in
existence; or, on the other hand, the particular organisms we are
dealing with may have the power of overpassing the barriers, and thus
reaching their present remote dwelling-places. As this is really the
fundamental question of distribution on which the solution of all its
more difficult problems depends, we have to inquire, in the first place,
what is the nature of, and what are the limits to, the changes of the
earth's surface, especially during the Tertiary and latter part of the
Secondary periods, as it was during those periods that most of the
existing types of the higher animals and plants came into existence;
and, in the next place, what are the extreme limits of the powers of
dispersal possessed by the chief groups of animals and plants. We will
first consider the question of barriers, more especially those formed by
seas and oceans.
_The Permanence of Oceans._
It was formerly a very general belief, even amongst geologists, that the
great features of the earth's surface, no less than the smaller ones,
were subject to continual mutations, and that during the course of
known geological time the continents and great oceans had again and
again changed places with each other. Sir Charles Lyell, in the last
edition of his _Principles of Geology_ (1872), said: "Continents,
therefore, although permanent for whole geological epochs, shift their
positions entirely in the course of ages;" and this may be said to have
been the orthodox opinion down to the very recent period when, by means
of deep-sea soundings, the nature of the ocean bottom was made known.
The first person to throw doubt on this view appears to have been the
veteran American geologist, Professor Dana. In 1849, in the Report of
Wilke's Exploring Expedition, he adduced the argument against a former
continent in the Pacific during the Tertiary period, from the absence of
all native quadrupeds. In 1856, in articles in the _American Journal_,
he discussed the development of the American continent, and argued for
its general permanence; and in his _Manual of Geology_ in 1863 and later
editions, the same views were more fully enforced and were latterly
applied to all continents. Darwin, in his _Journal of Researches_,
published in 1845, called attention to the fact that all the small
islands far from land in the Pacific, Indian, and Atlantic Oceans are
either of coralline or volcanic formation. He excepted, however, the
Seychelles and St. Paul's rocks; but the former have since been shown to
be no exception, as they consist entirely of coral rock; and although
Darwin himself spent a few hours on St. Paul's rocks on his outward
voyage in the _Beagle_, and believed he had found some portions of them
to be of a "cherty," and others of a "felspathic" nature, this also has
been shown to be erroneous, and the careful examination of the rocks by
the Abbé Renard clearly proves them to be wholly of volcanic
origin.[163] We have, therefore, at the present time, absolutely no
exception whatever to the remarkable fact that all the oceanic islands
of the globe are either of volcanic or coral formation; and there is,
further, good reason to believe that those of the latter class in every
case rest upon a volcanic foundation.
In his _Origin of Species_, Darwin further showed that no true oceanic
island had any native mammals or batrachia when first discovered, this
fact constituting the test of the class to which an island belongs;
whence he argued that none of them had ever been connected with
continents, but all had originated in mid-ocean. These considerations
alone render it almost certain that the areas now occupied by the great
oceans have never, during known geological time, been occupied by
continents, since it is in the highest degree improbable that every
fragment of those continents should have completely disappeared, and
have been replaced by volcanic islands rising out of profound oceanic
abysses; but recent research into the depth of the oceans and the nature
of the deposits now forming on their floors, adds greatly to the
evidence in this direction, and renders it almost a certainty that they
represent very ancient if not primaeval features of the earth's surface.
A very brief outline of the nature of this evidence will be now given.
The researches of the _Challenger_ expedition into the nature of the
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.
Now the whole series of marine stratified rocks, from the earliest
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]
But besides these geological and physical considerations, there is a
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.
[Illustration: FIG. 32.]
Now, if we try to imagine a process of elevation and depression by which
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._
The two exceptions just referred to are Madagascar and New Zealand, and
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._
If now we accept the annexed map as showing us approximately how far
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._
This singular and lowly organised type of mammals constitutes almost the
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._
Now these two cases, of the marsupials and the tapirs, are in the
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
further check upon their migrations. In these respects almost all other
organisms have great advantages over mammals. Birds can often fly long
distances, and can thus cross arms of the sea, deserts, or mountain
ranges; insects not only fly, but are frequently carried great distances
by gales of wind, as shown by the numerous cases of their visits to
ships hundreds of miles from land. Reptiles, though slow of movement,
have advantages in their greater capacity for enduring hunger or thirst,
their power of resisting cold or drought in a state of torpidity, and
they have also some facilities for migration across the sea by means of
their eggs, which may be conveyed in crevices of timber or among masses
of floating vegetable matter. And when we come to the vegetable kingdom,
the means of transport are at their maximum, numbers of seeds having
special adaptations for being carried by mammalia or birds, and for
floating in the water, or through the air, while many are so small and
so light that there is practically no limit to the distances they may be
carried by gales and hurricanes.
We may, therefore, feel quite certain that the means of distribution
that have enabled the larger mammalia to reach the most remote regions
from a common starting-point, will be at least as efficacious, and
usually far more efficacious, with all other land animals and plants;
and if in every case the existing distribution of this class can be
explained on the theory of oceanic and continental permanence, with the
limited changes of sea and land already referred to, no valid objections
can be taken against this theory founded on anomalies of distribution in
other orders. Yet nothing is more common than for students of this or
that group to assort that the theory of oceanic permanence is quite
inconsistent with the distribution of its various species and genera.
Because a few Indian genera and closely allied species of birds are
found in Madagascar, a land termed "Lemuria" has been supposed to have
united the two countries during a comparatively recent geological epoch;
while the similarity of fossil plants and reptiles, from the Permian and
Miocene formations of India and South Africa, has been adduced as
further evidence of this connection. But there are also genera of
snakes, of insects, and of plants, common to Madagascar and South
America only, which have been held to necessitate a direct land
connection between these countries. These views evidently refute
themselves, because any such land connections must have led to a far
greater similarity in the productions of the several countries than
actually exists, and would besides render altogether inexplicable the
absence of all the chief types of African and Indian mammalia from
Madagascar, and its marvellous individuality in every department of the
organic world.[169]
_Powers of Dispersal as illustrated by Insular Organisms._
Having arrived at the conclusion that our existing oceans have remained
practically unaltered throughout the Tertiary and Secondary periods of
geology, and that the distribution of the mammalia is such as might
have been brought about by their known powers of dispersal, and by such
changes of land and sea as have probably or certainly occurred, we are,
of course, restricted to similar causes to explain the much wider and
sometimes more eccentric distribution of other classes of animals and of
plants. In doing so, we have to rely partly on direct evidence of
dispersal, afforded by the land organisms that have been observed far
out at sea, or which have taken refuge on ships, as well as by the
periodical visitants to remote islands; but very largely on indirect
evidence, afforded by the frequent presence of certain groups on remote
oceanic islands, which some ancestral forms must, therefore, have
reached by transmission across the ocean from distant lands.
_Birds._
These vary much in their powers of flight, and their capability of
traversing wide seas and oceans. Many swimming and wading birds can
continue long on the wing, fly swiftly, and have, besides, the power of
resting safely on the surface of the water. These would hardly be
limited by any width of ocean, except for the need of food; and many of
them, as the gulls, petrels, and divers, find abundance of food on the
surface of the sea itself. These groups have a wide distribution
_across_ the oceans; while waders--especially plovers, sandpipers,
snipes, and herons--are equally cosmopolitan, travelling _along_ the
coasts of all the continents, and across the narrow seas which separate
them. Many of these birds seem unaffected by climate, and as the
organisms on which they feed are equally abundant on arctic, temperate,
and tropical shores, there is hardly any limit to the range even of some
of the species.
Land-birds are much more restricted in their range, owing to their
usually limited powers of flight, their inability to rest on the surface
of the sea or to obtain food from it, and their greater specialisation,
which renders them less able to maintain themselves in the new countries
they may occasionally reach. Many of them are adapted to live only in
woods, or in marshes, or in deserts; they need particular kinds of food
or a limited range of temperature; and they are adapted to cope only
with the special enemies or the particular group of competitors among
which they have been developed. Such birds as these may pass again and
again to a new country, but are never able to establish themselves in
it; and it is this organic barrier, as it is termed, rather than any
physical barrier, which, in many cases, determines the presence of a
species in one area and its absence from another. We must always
remember, therefore, that, although the presence of a species in a
remote oceanic island clearly proves that its ancestors must at one time
have found their way there, the absence of a species does not prove the
contrary, since it also may have reached the island, but have been
unable to maintain itself, owing to the inorganic or organic conditions
not being suitable to it. This general principle applies to all classes
of organisms, and there are many striking illustrations of it. In the
Azores there are eighteen species of land-birds which are permanent
residents, but there are also several others which reach the islands
almost every year after great storms, but have never been able to
establish themselves. In Bermuda the facts are still more striking,
since there are only ten species of resident birds, while no less than
twenty other species of land-birds and more than a hundred species of
waders and aquatics are frequent visitors, often in great numbers, but
are never able to establish themselves. On the same principle we account
for the fact that, of the many continental insects and birds that have
been let loose, or have escaped from confinement, in this country,
hardly one has been able to maintain itself, and the same phenomenon is
still more striking in the case of plants. Of the thousands of hardy
plants which grow easily in our gardens, very few have ever run wild,
and when the experiment is purposely tried it invariably fails. Thus A.
de Candolle informs us that several botanists of Paris, Geneva, and
especially of Montpellier, have sown the seeds of many hundreds of
species of exotic hardy plants, in what appeared to be the most
favourable situations, but that in hardly a single case has any one of
them become naturalised.[170] Still more, then, in plants than in
animals the absence of a species does not prove that it has never
reached the locality, but merely that it has not been able to maintain
itself in competition with the native productions. In other cases, as
we have seen, facts of an exactly opposite nature occur. The rat, the
pig, and the rabbit, the water-cress, the clover, and many other plants,
when introduced into New Zealand, nourish exceedingly, and even
exterminate their native competitors; so that in these cases we may feel
sure that the species in question did not exist in New Zealand simply
because they had been unable to reach that country by their natural
means of dispersal. I will now give a few cases, in addition to those
recorded in my previous works, of birds and insects which have been
observed far from any land.
_Birds and Insects at Sea._
Captain D. Fullarton of the ship _Timaru_ recorded in his log the
occurrence of a great number of small land-birds about the ship on 15th
March 1886, when in Lat. 48° 31' N., Long. 8° 16' W. He says: "A great
many small land-birds about us; put about sixty into a coop, evidently
tired out." And two days later, 17th March, "Over fifty of the birds
cooped on 15th died, though fed. Sparrows, finches, water-wagtails, two
small birds, name unknown, one kind like a linnet, and a large bird like
a starling. In all there have been on board over seventy birds, besides
some that hovered about us for some time and then fell into the sea
exhausted." Easterly winds and severe weather were experienced at the
time.[171] The spot where this remarkable flight of birds was met with
is about 160 miles due west of Brest, and this is the least distance the
birds must have been carried. It is interesting to note that the
position of the ship is nearly in the line from the English and French
coasts to the Azores, where, after great storms, so many bird stragglers
arrive annually. These birds were probably blown out to sea during their
spring migration along the south coast of England to Wales and Ireland.
During the autumnal migration, however, great flocks of
birds--especially starlings, thrushes, and fieldfares--have been
observed every year flying out to sea from the west coast of Ireland,
almost the whole of which must perish. At the Nash Lighthouse, in the
Bristol Channel on the coast of Glamorganshire, an enormous number of
small birds were observed on 3d September, including nightjars,
buntings, white-throats, willow-wrens, cuckoos, house-sparrows, robins,
wheatears, and blackbirds. These had probably crossed from
Somersetshire, and had they been caught by a storm the larger portion of
them must have been blown out to sea.[172]
These facts enable us to account sufficiently well for the birds of
oceanic islands, the number and variety of which are seen to be
proportionate to their facilities for reaching the island and
maintaining themselves in it. Thus, though more birds yearly reach
Bermuda than the Azores, the number of residents in the latter islands
is much larger, due to the greater extent of the islands, their number,
and their more varied surface. In the Galapagos the land-birds are still
more numerous, due in part to their larger area and greater proximity to
the continent, but chiefly to the absence of storms, so that the birds
which originally reached the islands have remained long isolated and
have developed into many closely allied species adapted to the special
conditions. All the species of the Galapagos but one are peculiar to the
islands, while the Azores possess only one peculiar species, and Bermuda
none--a fact which is clearly due to the continual immigration of fresh
individuals keeping up the purity of the breed by intercrossing. In the
Sandwich Islands, which are extremely isolated, being more than 2000
miles from any continent or large island, we have a condition of things
similar to what prevails in the Galapagos, the land-birds, eighteen in
number, being all peculiar, and belonging, except one, to peculiar
genera. These birds have probably all descended from three or four
original types which reached the islands at some remote period, probably
by means of intervening islets that have since disappeared. In St.
Helena we have a degree of permanent isolation which has prevented any
land-birds from reaching the island; for although its distance from the
continent, 1100 miles, is not so great as in the case of the Sandwich
Islands, it is situated in an ocean almost entirely destitute of small
islands, while its position within the tropics renders it free from
violent storms. Neither is there, on the nearest part of the coast of
Africa, a perpetual stream of migrating birds like that which supplies
the innumerable stragglers which every year reach Bermuda and the
Azores.
_Insects._
Winged insects have been mainly dispersed in the same way as birds, by
their power of flight, aided by violent or long-continued winds. Being
so small, and of such low specific gravity, they are occasionally
carried to still greater distances; and thus no islands, however remote,
are altogether without them. The eggs of insects, being often deposited
in borings or in crevices of timber, may have been conveyed long
distances by floating trees, as may the larvae of those species which
feed on wood. Several cases have been published of insects coming on
board ships at great distances from land; and Darwin records having
caught a large grasshopper when the ship was 370 miles from the coast of
Africa, whence the insect had probably come.
In the _Entomologists' Monthly Magazine_ for June 1885, Mr. MacLachlan
has recorded the occurrence of a swarm of moths in the Atlantic ocean,
from the log of the ship _Pleione_. The vessel was homeward bound from
New Zealand, and in Lat. 6° 47' N., Long. 32° 50' W., hundreds of moths
appeared about the ship, settling in numbers on the spars and rigging.
The wind for four days previously had been very light from north,
north-west, or north-east, and sometimes calm. The north-east trade wind
occasionally extends to the ship's position at that time of year. The
captain adds that "frequently, in that part of the ocean, he has had
moths and butterflies come on board." The position is 960 miles
south-west of the Cape Verde Islands, and about 440 north-east of the
South American coast. The specimen preserved is Deiopeia pulchella, a
very common species in dry localities in the Eastern tropics, and rarely
found in Britain, but, Mr. MacLachlan thinks, not found in South
America. They must have come, therefore, from the Cape Verde Islands, or
from some parts of the African coast, and must have traversed about a
thousand miles of ocean with the assistance, no doubt, of a strong
north-east trade wind for a great part of the distance. In the British
Museum collection there is a specimen of the same moth caught at sea
during the voyage of the _Rattlesnake_, in Lat. 6° N., Long. 22-1/2°
W., being between the former position and Sierra Leone, thus rendering
it probable that the moths came from that part of the African coast, in
which case the swarm encountered by the _Pleione_ must have travelled
more than 1200 miles.
A similar case was recorded by Mr. F.A. Lucas in the American periodical
_Science_ of 8th April 1887. He states that in 1870 he met with numerous
moths of many species while at sea in the South Atlantic (Lat. 25° S.,
Long. 24° W.), about 1000 miles from the coast of Brazil. As this
position is just beyond the south-east trades, the insects may have been
brought from the land by a westerly gale. In the _Zoologist_ (1864, p.
8920) is the record of a small longicorn beetle which flew on board a
ship 500 miles off the west coast of Africa. Numerous other cases are
recorded of insects at less distances from land, and, taken in
connection with those already given, they are sufficient to show that
great numbers must be continually carried out to sea, and that
occasionally they are able to reach enormous distances. But the
reproductive powers of insects are so great that all we require, in
order to stock a remote island, is that some few specimens shall reach
it even once in a century, or once in a thousand years.
_Insects at great Altitudes._
Equally important is the proof we possess that insects are often carried
to great altitudes by upward currents of air. Humboldt noticed them up
to heights of 15,000 and 18,000 feet in South America, and Mr. Albert
Müller has collected many interesting cases of the same character in
Europe.[173] A moth (Plusia gamma) has been found on the summit of Mont
Blanc; small hymenoptera and moths have been seen on the Pyrenees at a
height of 11,000 feet, while numerous flies and beetles, some of
considerable size, have been caught on the glaciers and snow-fields of
various parts of the Alps. Upward currents of air, whirlwinds and
tornadoes, occur in all parts of the world, and large numbers of insects
are thus carried up into the higher regions of the atmosphere, where
they are liable to be caught by strong winds, and thus conveyed enormous
distances over seas or continents. With such powerful means of
dispersal the distribution of insects over the entire globe, and their
presence in the most remote oceanic islands, offer no difficulties.
_The Dispersal of Plants._
The dispersal of seeds is effected in a greater variety of ways than are
available in the case of any animals. Some fruits or seed-vessels, and
some seeds, will float for many weeks, and after immersion in salt water
for that period the seeds will often germinate. Extreme cases are the
double cocoa-nut of the Seychelles, which has been found on the coast of
Sumatra, about 3000 miles distant; the fruits of the Sapindus saponaria
(soap-berry), which has been brought to Bermuda by the Gulf Stream from
the West Indies, and has grown after a journey in the sea of about 1500
miles; and the West Indian bean, Entada scandens, which reached the
Azores from the West Indies, a distance of full 3000 miles, and
afterwards germinated at Kew. By these means we can account for the
similarity in the shore flora of the Malay Archipelago and most of the
islands of the Pacific; and from an examination of the fruits and seeds,
collected among drift during the voyage of the _Challenger_, Mr. Hemsley
has compiled a list of 121 species which are probably widely dispersed
by this means.
A still larger number of species owe their dispersal to birds in several
distinct ways. An immense number of fruits in all parts of the world are
devoured by birds, and have been attractively coloured (as we have
seen), in order to be so devoured, because the seeds pass through the
birds' bodies and germinate where they fall. We have seen how frequently
birds are forced by gales of wind across a wide expanse of ocean, and
thus seeds must be occasionally carried. It is a very suggestive fact,
that all the trees and shrubs in the Azores bear berries or small fruits
which are eaten by birds; while all those which bear larger fruits, or
are eaten chiefly by mammals--such as oaks, beeches, hazels, crabs,
etc.--are entirely wanting. Game-birds and waders often have portions of
mud attached to their feet, and Mr. Darwin has proved by experiment that
such mud frequently contains seeds. One partridge had such a quantity of
mud attached to its foot as to contain seeds from which eighty-two
plants germinated; this proves that a very small portion of mud may
serve to convey seeds, and such an occurrence repeated even at long
intervals may greatly aid in stocking remote islands with vegetation.
Many seeds also adhere to the feathers of birds, and thus, again, may be
conveyed as far as birds are ever carried. Dr. Guppy found a small hard
seed in the gizzard of a Cape Petrel, taken about 550 miles east of
Tristan da Cunha.
_Dispersal of Seeds by the Wind._
In the preceding cases we have been able to obtain direct evidence of
transportal; but although we know that many seeds are specially adapted
to be dispersed by the wind, we cannot obtain direct proof that they are
so carried for hundreds or thousands of miles across the sea, owing to
the difficulty of detecting single objects which are so small and
inconspicuous. It is probable, however, that the wind as an agent of
dispersal is really more effective than any of those we have hitherto
considered, because a very large number of plants have seeds which are
very small and light, and are often of such a form as to facilitate
aerial carriage for enormous distances. It is evident that such seeds
are especially liable to be transported by violent winds, because they
become ripe in autumn at the time when storms are most prevalent, while
they either lie upon the surface of the ground, or are disposed in dry
capsules on the plant ready to be blown away. If inorganic particles
comparable in weight, size, or form with such seeds are carried for
great distances, we may be sure that seeds will also be occasionally
carried in the same way. It will, therefore, be necessary to give a few
examples of wind-carriage of small objects.
On 27th July 1875 a remarkable shower of small pieces of hay occurred at
Monkstown, near Dublin. They appeared floating slowly down from a great
height, as if falling from a dark cloud which hung overhead. The pieces
picked up were wet, and varied from single blades of grass to tufts
weighing one or two ounces. A similar shower occurred a few days earlier
in Denbighshire, and was observed to travel in a direction contrary to
that of the wind in the lower atmosphere.[174] There is no evidence of
the distance from which the hay was brought, but as it had been carried
to a great height, it was in a position to be conveyed to almost any
distance by a violent wind, had such occurred at the time.
_Mineral Matter carried by the Wind._
The numerous cases of sand and volcanic dust being carried enormous
distances through the atmosphere sufficiently prove the importance of
wind as a carrier of solid matter, but unfortunately the matter
collected has not been hitherto examined with a view to determine the
maximum size and weight of the particles. A few facts, however, have
been kindly furnished me by Professor Judd, F.R.S. Some dust which fell
at Genoa on 15th October 1885, and was believed to have been brought
from the African desert, consisted of quartz, hornblende, and other
minerals, and contained particles having a diameter of 1/500 inch, each
weighing 1/200,000 grain. This dust had probably travelled over 600
miles. In the dust from Krakatoa, which fell at Batavia, about 100 miles
distant, during the great eruption, there are many solid particles even
larger than those mentioned above. Some of this dust was given me by
Professor Judd, and I found in it several ovoid particles of a much
larger size, being 1/50 inch long, and 1/70 wide and deep. The dust from
the same eruption, which fell on board the ship _Arabella_, 970 miles
from the volcano, also contained solid particles 1/500 inch diameter.
Mr. John Murray of the _Challenger_ Expedition writes to me that he
finds in the deep sea deposits 500 and even 700 miles west of the coast
of Africa, rounded particles of quartz, having a diameter of 1/250 inch,
and similar particles are found at equally great distances from the
south-west coasts of Australia; and he considers these to be atmospheric
dust carried to that distance by the wind. Taking the sp. gr. of quartz
at 2.6, these particles would weigh about 1/25,000 grain each. These
interesting facts can, however, by no means be taken as indicating the
extreme limits of the power of wind in carrying solid particles. During
the Krakatoa eruption no gale of special violence occurred, and the
region is one of comparative calms. The grains of quartz found by Mr.
Murray more nearly indicate the limit, but the very small portions of
matter brought up by the dredge, as compared with the enormous areas of
sea-bottom, over which the atmospheric dust must have been scattered,
render it in the highest degree improbable that the maximum limit either
of size of particles, or of distance from land has been reached.
Let us, however, assume that the quartz grains, found by Mr. Murray in
the deep-sea ooze 700 miles from land, give us the extreme limit of the
power of the atmosphere as a carrier of solid particles, and let us
compare with these the weights of some seeds. From a small collection of
the seeds of thirty species of herbaceous plants sent me from Kew, those
in the above table were selected, and small portions of eight of them
carefully weighed in a chemical balance.[175] By counting these portions
I was able to estimate the number of seeds weighing one grain. The three
very minute species, whose numbers are marked with an asterisk (*), were
estimated by the comparison of their sizes with those of the smaller
weighed seeds.
No| Species. |Approximate | Approximate | Remarks.
| |No. of Seeds| Dimensions. |
| |In one Grain| |
| | | in. in. in. |
1|Draba verna | 1,800 |1/60 x 1/90 x 1/160|Oval, flat.
2|Hypericum perforatum | 520 | 1/30 x 1/80 |Cylindrical.
3|Astilbe rivularis | 4,500 | 1/50 x 1/100 |Elongate, flat, tailed,
| | | | wavy.
4|Saxifraga coriophylla| 750 | 1/40 x 1/75 |Surface rough, adhere
| | | | to the dry capsules.
5|Oenothera rosea | 640 | 1/40 x 1/80 |Ovate.
6|Hypericum hirsutum | 700 | 1/30 x 1/100 |Cylindrical, rough.
7|Mimulus luteus | 2,900 | 1/60 x 1/100 |Oval, minute.
8|Penthorum sedoides | 8,000* | 1/70 x 1/150 |Flattened, very minute.
9|Sagina procumbens | 12,000* | 1/120 |Sub-triangular, flat.
10|Orchis maculata | 15,000* | --- |Margined, flat,
| | | | very minute.
11|Gentiana purpurea | 35 | 1/25 |Wavy, rough, with this
| | | | coriaceous margins.
12|Silene alpina | --- | 1/30 |Flat, with fringed
| | | | margins.
13|Adenophora communis | --- | 1/20 x 1/40 |Very thin, wavy, light.
|Quartz grains | 25,000 | 1/250 |Deep sea ... 700 miles.
|Do. |200,000 | 1/500 |Genoa ... 600 miles.
If now we compare the seeds with the quartz grains, we find that
several are from twice to three times the weight of the grains found by
Mr. Murray, and others five times, eight times, and fifteen times as
heavy; but they are proportionately very much larger, and, being usually
irregular in shape or compressed, they expose a very much larger surface
to the air. The surface is often rough, and several have dilated margins
or tailed appendages, increasing friction and rendering the uniform rate
of falling through still air immensely less than in the case of the
smooth, rounded, solid quartz grains. With these advantages it is a
moderate estimate that seeds ten times the weight of the quartz grains
could be carried quite as far through the air by a violent gale and
under the most favourable conditions. These limits will include five of
the seeds here given, as well as hundreds of others which do not exceed
them in weight; and to these we may add some larger seeds which have
other favourable characteristics, as is the case with numbers 11-13,
which, though very much larger than the rest, are so formed as in all
probability to be still more easily carried great distances by a gale of
wind. It appears, therefore, to be absolutely certain that every
autumnal gale capable of conveying solid mineral particles to great
distances, must also carry numbers of small seeds at least as far; and
if this is so, the wind alone will form one of the most effective agents
in the dispersal of plants.
Hitherto this mode of conveyance, as applying to the transmission of
seeds for great distances across the ocean, has been rejected by
botanists, for two reasons. In the first place, there is said to be no
direct evidence of such conveyance; and, secondly, the peculiar plants
of remote oceanic islands do not appear to have seeds specially adapted
for aerial transmission. I will consider briefly each of these
objections.
_Objection to the Theory of Wind-Dispersal._
To obtain direct evidence of the transmission of such minute and
perishable objects, which do not exist in great quantities, and are
probably carried to the greatest distances but rarely and as single
specimens, is extremely difficult. A bird or insect can be seen if it
comes on board ship, but who would ever detect the seeds of Mimulus or
Orchis even if a score of them fell on a ship's deck? Yet if but one
such seed per century were carried to an oceanic island, that island
might become rapidly overrun by the plant, if the conditions were
favourable to its growth and reproduction. It is further objected that
search has been made for such seeds, and they have not been found.
Professor Kerner of Innsbruck examined the snow on the surface of
glaciers, and assiduously collected all the seeds he could find, and
these were all of plants which grew in the adjacent mountains or in the
same district. In like manner, the plants growing on moraines were found
to be those of the adjacent mountains, plateaux, or lowlands. Hence he
concluded that the prevalent opinion that seeds may be carried through
the air for very great distances "is not supported by fact."[176] The
opinion is certainly not supported by Kerner's facts, but neither is it
opposed by them. It is obvious that the seeds that would be carried by
the wind to moraines or to the surface of glaciers would be, first and
in the greatest abundance, those of the immediately surrounding
district; then, very much more rarely, those from more remote mountains;
and lastly, in extreme rarity, those from distant countries or
altogether distinct mountain ranges. Let us suppose the first to be so
abundant that a single seed could be found by industrious search on each
square yard of the surface of the glacier; the second so scarce that
only one could possibly be found in a hundred yards square; while to
find one of the third class it would be necessary exhaustively to
examine a square mile of surface. Should we expect that _one_ ever to be
found, and should the fact that it could not be found be taken as a
proof that it was not there? Besides, a glacier is altogether in a bad
position to receive such remote wanderers, since it is generally
surrounded by lofty mountains, often range behind range, which would
intercept the few air-borne seeds that might have been carried from a
distant land. The conditions in an oceanic island, on the other hand,
are the most favourable, since the land, especially if high, will
intercept objects carried by the wind, and will thus cause more of the
solid matter to fall on it than on an equal area of ocean. We know that
winds at sea often blow violently for days together, and the rate of
motion is indicated by the fact that 72 miles an hour was the average
velocity of the wind observed during twelve hours at the Ben Nevis
observatory, while the velocity sometimes rises to 120 miles an hour. A
twelve hours' gale might, therefore, carry light seeds a thousand miles
as easily and certainly as it could carry quartz-grains of much greater
specific gravity, rotundity, and smoothness, 500 or even 100 miles; and
it is difficult even to imagine a sufficient reason why they should not
be so carried--perhaps very rarely and under exceptionally favourable
conditions,--but this is all that is required.
As regards the second objection, it has been observed that orchideae,
which have often exceedingly small and light seeds, are remarkably
absent from oceanic islands. This, however, may be very largely due to
their extreme specialisation and dependence on insect agency for their
fertilisation; while the fact that they do occur in such very remote
islands as the Azores, Tahiti, and the Sandwich Islands, proves that
they must have once reached these localities either by the agency of
birds or by transmission through the air; and the facts I have given
above render the latter mode at least as probable as the former. Sir
Joseph Hooker remarks on the composite plant of Kerguelen Island (Cotula
plumosa) being found also on Lord Auckland and MacQuarrie Islands, and
yet having no pappus, while other species of the genus possess it. This
is certainly remarkable, and proves that the plant must have, or once
have had, some other means of dispersal across wide oceans.[177] One of
the most widely dispersed species in the whole world (Sonchus oleraceus)
possesses pappus, as do four out of five of the species which are common
to Europe and New Zealand, all of which have a very wide distribution.
The same author remarks on the limited area occupied by most species of
Compositae, notwithstanding their facilities for dispersal by means of
their feathered seeds; but it has been already shown that limitations
of area are almost always due to the competition of allied forms,
facilities for dispersal being only one of many factors in determining
the wide range of species. It is, however, a specially important factor
in the case of the inhabitants of remote oceanic islands, since, whether
they are peculiar species or not, they or their remote ancestors must at
some time or other have reached their present position by natural means.
I have already shown elsewhere, that the flora of the Azores strikingly
supports the view of the species having been introduced by aerial
transmission only, that is, by the agency of birds and the wind, because
all plants that could not possibly have been carried by these means are
absent.[178] In the same way we may account for the extreme rarity of
Leguminosae in all oceanic islands. Mr. Hemsley, in his Report on
Insular Floras, says that they "are wanting in a large number of oceanic
islands where there is no true littoral flora," as St. Helena, Juan
Fernandez, and all the islands of the South Atlantic and South Indian
Oceans. Even in the tropical islands, such as Mauritius and Bourbon,
there are no endemic species, and very few in the Galapagos and the
remoter Pacific Islands. All these facts are quite in accordance with
the absence of facilities for transmission through the air, either by
birds or the wind, owing to the comparatively large size and weight of
the seeds; and an additional proof is thus afforded of the extreme
rarity of the successful floating of seeds for great distances across
the ocean.[179]
_Explanation of North Temperate Plants in the Southern Hemisphere._
If we now admit that many seeds which are either minute in size, of thin
texture or wavy form, or so fringed or margined as to afford a good hold
to the air, are capable of being carried for many hundreds of miles by
exceptionally violent and long-continued gales of wind, we shall not
only be better able to account for the floras of some of the remotest
oceanic islands, but shall also find in the fact a sufficient
explanation of the wide diffusion of many genera, and even species, of
arctic and north temperate plants in the southern hemisphere or on the
summits of tropical mountains. Nearly fifty of the flowering plants of
Tierra-del-Fuego are found also in North America or Europe, but in no
intermediate country; while fifty-eight species are common to New
Zealand and Northern Europe; thirty-eight to Australia, Northern Europe,
and Asia; and no less than seventy-seven common to New Zealand,
Australia, and South America.[180] On lofty mountains far removed from
each other, identical or closely allied plants often occur. Thus the
fine Primula imperialis of a single mountain peak in Java has been found
(or a closely allied species) in the Himalayas; and many other plants of
the high mountains of Java, Ceylon, and North India are either identical
or closely allied forms. So, in Africa, some species, found on the
summits of the Cameroons and Fernando Po in West Africa, are closely
allied to species in the Abyssinian highlands and in Temperate Europe;
while other Abyssinian and Cameroons species have recently been found on
the mountains of Madagascar. Some peculiar Australian forms have been
found represented on the summit of Kini Balu in Borneo. Again, on the
summit of the Organ mountains in Brazil there are species allied to
those of the Andes, but not found in the intervening lowlands.
_No Proof of Recent Lower Temperature in the Tropics._
Now all these facts, and numerous others of like character, were
supposed by Mr. Darwin to be due to a lowering of temperature during
glacial epochs, which allowed these temperate forms to migrate across
the intervening tropical lowlands. But any such change within the epoch
of existing species is almost inconceivable. In the first place, it
would necessitate the extinction of much of the tropical flora (and with
it of the insect life), because without such extinction alpine
herbaceous plants could certainly never spread over tropical forest
lowlands; and, in the next place, there is not a particle of direct
evidence that any such lowering of temperature in inter-tropical
lowlands ever took place. The only alleged evidence of the kind is that
adduced by the late Professor Agassiz and Mr. Hartt; but I am informed
by my friend, Mr. J.C. Branner (now State Geologist of Arkansas, U.S.),
who succeeded Mr. Hartt, and spent several years completing the
geological survey of Brazil, that the supposed moraines and glaciated
granite rocks near Rio Janeiro and elsewhere, as well as the so-called
boulder-clay of the same region, are entirely explicable as the results
of sub-aerial denudation and weathering, and that there is no proof
whatever of glaciation in any part of Brazil.
_Lower Temperature not needed to Explain the Facts._
But any such vast physical change as that suggested by Darwin, involving
as it does such tremendous issues as regards its effects on the tropical
fauna and flora of the whole world, is really quite uncalled for,
because the facts to be explained are of the same essential nature as
those presented by remote oceanic islands, between which and the nearest
continents no temperate land connection is postulated. In proportion to
their limited area and extreme isolation, the Azores, St. Helena, the
Galapagos, and the Sandwich Islands, each possess a fairly rich--the
last a very rich--indigenous flora; and the means which sufficed to
stock them with a great variety of plants would probably suffice to
transmit others from mountain-top to mountain-top in various parts of
the globe. In the case of the Azores, we have large numbers of species
identical with those of Europe, and others closely allied, forming an
exactly parallel case to the species found on the various mountain
summits which have been referred to. The distances from Madagascar to
the South African mountains and to Kilimandjaro, and from the latter to
Abyssinia, are no greater than from Spain to the Azores, while there are
other equatorial mountains forming stepping-stones at about an equal
distance to the Cameroons. Between Java and the Himalayas we have the
lofty mountains of Sumatra and of North-western Burma, forming steps at
about the same distance apart; while between Kini Balu and the
Australian Alps we have the unexplored snow mountains of New Guinea,
the Bellenden Ker mountains in Queensland, and the New England and Blue
Mountains of New South Wales. Between Brazil and Bolivia the distances
are no greater; while the unbroken range of mountains from Arctic
America to Tierra-del-Fuego offers the greatest facilities for
transmission, the partial gap between the lofty peak of Chiriqui and the
high Andes of New Grenada being far less than from Spain to the Azores.
Thus, whatever means have sufficed for stocking oceanic islands must
have been to some extent effective in transmitting northern forms from
mountain to mountain, across the equator, to the southern hemisphere;
while for this latter form of dispersal there are special facilities, in
the abundance of fresh and unoccupied surfaces always occurring in
mountain regions, owing to avalanches, torrents, mountain-slides, and
rock-falls, thus affording stations on which air-borne seeds may
germinate and find a temporary home till driven out by the inroads of
the indigenous vegetation. These temporary stations may be at much lower
altitudes than the original habitat of the species, if other conditions
are favourable. Alpine plants often descend into the valleys on glacial
moraines, while some arctic species grow equally well on mountain
summits and on the seashore. The distances above referred to between the
loftier mountains may thus be greatly reduced by the occurrence of
suitable conditions at lower altitudes, and the facilities for
transmission by means of aerial currents proportionally increased.[181]
_Facts Explained by the Wind-Carriage of Seeds._
But if we altogether reject aerial transmission of seeds for great
distances, except by the agency of birds, it will be difficult, if not
impossible, to account for the presence of so many identical species of
plants on remote mountain summits, or for that "continuous current of
vegetation" described by Sir Joseph Hooker as having apparently long
existed from the northern to the southern hemisphere. It may be admitted
that we can, possibly, account for the greater portion of the floras of
remote oceanic islands by the agency of birds alone; because, when blown
out to sea land-birds must reach some island or perish, and all which
come within sight of an island will struggle to reach it as their only
refuge. But, with mountain summits the case is altogether different,
because, being surrounded by land instead of by sea, no bird would need
to fly, or to be carried by the wind, for several hundred miles at a
stretch to another mountain summit, but would find a refuge in the
surrounding uplands, ridges, valleys, or plains. As a rule the birds
that frequent lofty mountain tops are peculiar species, allied to those
of the surrounding district; and there is no indication whatever of the
passage of birds from one remote mountain to another in any way
comparable with the flights of birds which are known to reach the Azores
annually, or even with the few regular migrants from Australia to New
Zealand. It is almost impossible to conceive that the seeds of the
Himalayan primula should have been thus carried to Java; but, by means
of gales of wind, and intermediate stations from fifty to a few hundred
miles apart, where the seeds might vegetate for a year or two and
produce fresh seed to be again carried on in the same manner, the
transmission might, after many failures, be at last effected.
A very important consideration is the vastly larger scale on which
wind-carriage of seeds must act, as compared with bird-carriage. It can
only be a few birds which carry seeds attached to their feathers or
feet. A very small proportion of these would carry the seeds of Alpine
plants; while an almost infinitesimal fraction of these latter would
convey the few seeds attached to them safely to an oceanic island or
remote mountain. But winds, in the form of whirlwinds or tornadoes,
gales or hurricanes, are perpetually at work over large areas of land
and sea. Insects and light particles of matter are often carried up to
the tops of high mountains; and, from the very nature and origin of
winds, they usually consist of ascending or descending currents, the
former capable of suspending such small and light objects as are many
seeds long enough for them to be carried enormous distances. For each
single seed carried away by external attachment to the feet or feathers
of a bird, countless millions are probably carried away by violent
winds; and the chance of conveyance to a great distance and in a
definite direction must be many times greater by the latter mode than
by the former.[182] We have seen that inorganic particles of much
greater specific gravity than seeds, and nearly as heavy as the smallest
kinds, are carried to great distances through the air, and we can
therefore hardly doubt that some seeds are carried as far. The direct
agency of the wind, as a supplement to bird-transport, will help to
explain the presence in oceanic islands of plants growing in dry or
rocky places whose small seeds are not likely to become attached to
birds; while it seems to be the only effective agency possible in the
dispersal of those species of alpine or sub-alpine plants found on the
summits of distant mountains, or still more widely separated in the
temperate zones of the northern and southern hemispheres.
_Concluding Remarks._
On the general principles that have been now laid down, it will be found
that all the chief facts of the geographical distribution of animals and
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.]
[Footnote 164: Even the extremely fine Mississippi mud is nowhere found
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.]
[Footnote 167: See _Island Life_, p. 446, and the whole of chaps. xxi.
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.]
[Footnote 170: _Géographie Botanique_, p. 798.]
[Footnote 171: _Nature_, 1st April 1886.]
[Footnote 172: Report of the Brit. Assoc. Committee on Migration of
Birds during 1886.]
[Footnote 173: _Trans. Ent. Soc._, 1871, p. 184.]
[Footnote 174: _Nature_ (1875), vol. xii. pp. 279, 298.]
[Footnote 175: I am indebted to Professor R. Meldola of the Finsbury
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.]
[Footnote 179: Mr. Hemsley suggests that it is not so much the
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.]
[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.]