PROOFS FROM GEOGRAPHICAL DISTRIBUTION OF ORGANISMS.
It is well known that the kinds of organisms found in widely-separated
countries differ more or less conspicuously. The traveler in Australia
or in Africa finds all, the traveler in Europe nearly all, the animals
and plants wholly different from those he has been accustomed to see
at home. Even the visitor from the Atlantic to the Pacific coast, if
he observes at all, will find nearly all organisms strange to him.
The facts of geographical diversity of organisms are so numerous and
complex that, at first sight, they seem utterly lawless. Only recently
this subject has been redeemed from chaos and reduced to something like
order and law by the light thrown upon it by the theory of evolution.
We will give, in very brief outline, the most important facts, and then
show how they may be explained.
=Geographical Faunas and Floras.=--The group of animals and plants
inhabiting any locality, whether peculiar to that locality or not,
is called, in popular language, its fauna and flora. But, in a true
scientific sense, a fauna and flora is a _natural_ group of animals
and plants in one place, _differing_ more or less conspicuously
from other groups in other places, and _separated from them by
physico-geographical boundaries, or by physical conditions of some
kind._ The members of such a group can only exist in certain harmonic
relations with external conditions, and with one another. These
relations with one another are often complex and nicely adjusted,
so that change in one term is propagated through the whole series
of terms, giving rise often to the most unexpected results, until
finally a new equilibrium is established. Thus, the destruction of
certain insectivorous birds, in mere wanton sport, may give rise to the
multiplication of insect pests, and this to the destruction of certain
kinds of plants, and this to the diminution of certain herbivores, and
this in its turn to the disappearance of certain carnivores. It is well
known that the introduction of rabbits into New Zealand and Australia
has produced the most unexpectedly disastrous effect upon certain
crops, on account of the absence of the fierce and active carnivores
which keep in check their excessive multiplication in Europe.
Now, among the physical conditions which limit faunas and floras, and
separate them from each other, the most important and universal is
temperature.
=Temperature-Regions.=--If we travel from equator to pole, we pass
through mean temperatures varying from 80° to 0°. This gives rise
to a very regular zonal arrangement of plant-forms: 1. We have
first a region in which palms and palm-like forms are abundant and
characteristic, and which therefore may be called the region of palms.
It corresponds with the tropic zone. 2. We next have a region in which
hard-wood foliferous trees are most abundant and characteristic;
first mostly evergreens and then deciduous trees, and therefore may
be called the region of hard-wood forests. This corresponds with the
temperate-zone. 3. Then we find a region characterized predominantly by
pines and pine-like trees and birches, and may be called the region of
pines. This is the sub-Arctic region. 4. Then a region without trees,
but only shrubs and summer plants. This is the Arctic region. 5. And,
finally, an almost wholly plantless region of perpetual ice--the polar
region.
These regions are determined wholly by temperature, and therefore, in
going up a mountain-slope to snowy summits, we pass through similar
regions in smaller space. For example, in going from sea-level to
the summits of the Sierra, 14,000 to 15,000 feet high, we commence
in a region of predominantly hard-wood trees; but at 3,000 feet the
forests become almost wholly coniferous, at 11,000 to 12,000 feet the
vegetation becomes shrubby, and at 13,000 feet we reach perpetual snow.
We have taken plants first, because these, being fixed to the soil and
incapable of voluntary seasonal migrations, are more strictly and
simply limited by temperature--i. e., the arrangement of different
kinds in zones is more simple and conspicuous. But the same rule
holds also for animals. In passing from equator to pole, animal kinds
also change frequently, so that there are many temperature-faunas in
which the animals are all very different. In both animals and plants,
species, genera, families, etc., are limited by temperature. These are
familiar facts; we recall them to the reader in order that we may base
thereon a clearer definition of these limits.
=More Perfect Definition of Regions.=--1. The area over which any form
spreads is called its _range_. Now, the range of a species is more
restricted than that of a genus, because, when a species is limited by
temperature, another species of the same genus may carry on the genus.
For the same reason the range of a family is usually greater than that
of a genus, and so on for higher classification-groups. For example,
pines range on the slopes of the Sierra from about 2,000 feet to
11,000 feet, but not the same species. In ascending, we meet first the
nut-pine (_Pinus Sabiniana_), then the yellow-pine (_P. ponderosa_),
then the sugar-pine (_P. Lambertiana_), then the tamarack-pine (_P.
contorta_), and last, the _Pinus flexilis_, etc.
[Illustration: FIG. 64.]
2. Where two contiguous temperature-regions come in contact, there is
no sharp line between; on the contrary, they _shade gradually_, almost
imperceptibly, into one another, the ranges of species overlapping and
interpenetrating, and the two species coexisting on the borders of
their ranges. This is represented by the diagram (Fig. 64), in which
the horizontal lines represent the north and south ranges of species of
two groups, A and B, separated by the dotted line.
3. Species also pass out gradually on the borders of these ranges and
others come in gradually, so _far as number and vigor of individuals
are concerned_. If _a a′_ and _b b′_ (Fig. 65) represent the north
and south range of two species, and _b a′_ their overlap or area of
coexistence, then the height of the curves A and B will represent the
number and vigor of the individuals in different parts of the range.
[Illustration: FIG. 65.]
4. While, therefore, there is a shading of contiguous groups into each
other by overlap of species-ranges; while there is also a gradual
passing out of species so far as number and vigor of individuals
is concerned, yet, in _specific characters_ we observe usually no
such gradation. Species seem to come in on one border with all their
specific characters perfect, remain substantially unchanged throughout
their range, and pass out on the other border, still the same species.
In other words, one species takes the place of another, usually by
_substitution_, not by _transmutation_. It is _as if_ species had
originated, no matter how, each in its own region, and had spread
in all directions as far as physical conditions and struggle with
other species would allow. This important subject will be more fully
discussed later.
5. We have thus far spoken of species as limited by temperature alone,
but they are limited also by _barriers_. If, then, there be an east and
west barrier, such as a high mountain-range, or a wide sea or desert,
there will be no shading or gradation of any kind, because the barrier
prevents overlapping, interpenetration, and struggle on the margins.
For example: The species north and south of the Himalayas, or north
and south of Sahara, are widely different. It is, again, _as if_ they
originated each where we find them and spread as far as they could, but
the physical barrier prevented mingling and shading.
6. There are temperature-regions south as well as north of the equator.
Now, although the climatic conditions are quite similar, the species of
corresponding temperature-regions north and south are wholly different.
It is, again, as if they originated where we find them, and were kept
separate by the barrier of tropical heat between. If carried over, they
often do perfectly well.
_Continental Faunas and Floras._
If the land-surfaces were continuous all around the globe, there is
little doubt that each temperature region with its characteristic
species would also be substantially continuous. There would, it is
true, be some local variations dependent upon soil and humidity,
etc., but substantially the same species would exist all around.
The distribution would be almost wholly zonal. But the intervening
oceans are complete barriers to continental species. Hence we ought to
expect, and do find, that the faunas and floras of different continents
are almost totally different. Each apparently originated on its own
continent, and did not spread to other continents, only because they
could not get there. It is necessary to explain this in more detail.
[Illustration: FIG. 66.--Polar projection of the earth. 1, tropical; 2,
temperate; 3, sub-arctic; 4, arctic; 5, polar regions.]
Fig. 66 represents a polar view of the earth, showing the eastern and
western continents, and the five temperature zones already described.
Now, if we examine the species in each region, commencing at the pole,
we find that those of Nos. 5 and 4 are almost identical all around.
The reason is obvious. The continents come close together there, with
ice-connection if not land-connection all around. There is but one
circumpolar region. But, as soon as we come down to No. 3 and No. 2,
the species on the two continents are nearly all different, because
there is an impassable barrier between, either in the form of ocean
or of Arctic cold. For example, the animals and plants inhabiting the
United States are almost wholly different from those in Europe, not
only in species, but even largely in genera and to some extent in
families. There are some exceptions to this rule, but these are of the
kind which prove the rule, or rather the principle on which the rule
is founded. These exceptions are mainly of three kinds: 1. _Introduced
species._--All our weeds, many garden-plants, and many animal pests are
of this kind. They were not found here when America was discovered,
only because they could not get here; for, when brought here, they
do so well that they often overrun the country and dispossess the
native species, as we ourselves have done the Indians. 2. _Hardy or
else wide-migrating species._--Hardy species have wide range; they
may belong to No. 4 as well as No. 3. If so, they range down to No. 3
on both continents. Migrating birds, such as ducks and geese, etc.,
breed in summer in No. 4, and migrate southward in winter on both
continents from the common circumpolar ground. 3. _Alpine species._--It
is a curious fact that species on tops of snowy mountains in temperate
regions of the two continents are wonderfully similar, though so
completely isolated. We are not yet prepared to discuss this point.
We shall do so later. Suffice it to say now that it can be completely
explained.
In region No. 1 the continental diversity is still greater. Not only
species and genera, but whole families and even orders, are peculiar
to each continent. The great pachyderms--elephant, rhinoceros,
hippopotamus--are peculiar to the Eastern; the edentates--sloths and
armadillos--to the Western. The humming-birds, those gems of the
forests, of which there are over four hundred species, and the whole
cactus family, are peculiar to America, while the tailless monkeys are
equally characteristic of the Eastern Continent.
The continents do not come together again toward the south, and,
therefore, as might be expected, the great difference between the two
persists to the southern points. The faunas of the southern points of
South America, Africa, and Australia are very different.
=Subdivisions of Continental Faunas and Floras.=--Besides the
subdivisions of continental faunas, north and south, determined
by temperature as already explained, if there be in any continent
an impassable barrier running north and south, there will be a
corresponding difference in the species on the two sides, east and
west. We give but one example: The North American Cordilleras or Rocky
Mountains, with their high ranges and desert plains, constitute a very
great barrier between the eastern and western portions of the United
States. Hence, we find an extraordinary difference between the species
inhabiting California and those found in the eastern portion of the
country. Speaking generally, all the species and many of the genera are
peculiar. The exceptions, too, are significant. Leaving out introduced
species, of which there are many, they are mostly strong-winged or
widely-migrating birds, such as the turtle-dove, the turkey-buzzard,
the bald eagle, and, of course, many water-birds.
=Special Cases.=--If any body of land is widely separated from
all other lands by deep seas, we invariably find a corresponding
peculiarity of its species. Thus, the species inhabiting Australia and
Madagascar are perhaps the most peculiar in the world. We do not dwell
further on these, because we will discuss them hereafter. There is a
little group of very small islands--the Galapagos--about six hundred
miles off the western coast of South America, and surrounded on all
sides by deep sea. These islands are stocked with a collection of
curious animals not found elsewhere on the surface of the earth; but
among them are no mammals at all. We might multiply examples without
limit. Even the rivers emptying in the same sea sometimes have each
its peculiar species of mussels. In the Altamaha River there are
several species of unios--such, for instance, as the wonderful spinous
unio--not found elsewhere. How came they there? Howsoever they may have
come there, they are now kept isolated there by barriers of land and of
salt water.
Many other curious details will come up in our discussion of the origin
of diversity.
=Marine Species.=--Precisely the same principles apply here; but
diversity in the case of marine species is perhaps less marked, and
certainly less general, because of the universal oceanic connection.
Open-sea species are therefore almost universal. But many marine
species are confined to shallow water, and therefore to shore-lines.
The species on the two shores of the same ocean, or the two coasts of
the same continent, are different, being isolated east and west by
barriers of deep sea or of land, and north and south by temperature.
Also about isolated lands, like Australia and Madagascar, the species
are peculiar.
Thus, then, species, genera, etc., are limited in every direction;
north and south by temperature, and in all directions by barriers,
in the form of oceans, deserts, and mountain-chains. Add to these,
peculiar climates and soils, and we see that, from this point of view,
the whole surface of the earth may be divided and sub-divided into
regions, sub-regions, provinces, etc. It would carry us too far to
explain the primary and secondary divisions adopted by Mr. Wallace, and
the somewhat different ones suggested by Mr. Allen. Our main object is
to discuss the _cause_ of this diversity, and especially to show the
light shed upon it by the theory of evolution. We have only given a
sketch of the facts sufficient for this purpose.
_Theory of the Origin of Geographical Diversity._
It will be observed that all along we have assumed a sort of
provisional theory. We have said in every case, it is _as if_ organic
forms originated where we find them, and have gone thence wherever
they could--as far in every direction as physical conditions and
struggle with competing species would allow. This view has been
formulated as the “theory of specific centers of origin.” There would
be less objection to this as a first provisional theory did it not
assume a supernatural _mode_ of origin. But, in the minds of those
who hold it, it has usually assumed expressly or tacitly the form of
“_specific centers of creation_,” thus implying the immutability of
specific types and the supernaturalism of specific origin (page 68). In
this latter or usual form it completely fails to account for the facts
given above. For, if this were the mode of origin, each species ought
in every case to be perfectly adapted to its own environment, and to no
other. But, on the contrary, introduced species often flourish better
than in their own country, and better than the natives of their new
homes. In the less objectionable form of “specific centers of origin,”
without defining the mode of origin, it accounts well for many of the
more obvious facts of geographical diversity, as it _now_ exists, but
not all. According to this view, the amount of diversity ought to be in
strict proportion to the completeness of isolation, or impassableness
of the separating barriers; but this is not _exactly_ true. There is
another element, not yet mentioned, which is just as important as
impassableness, but which until recently has been left entirely out
of account. This is the element of _time_--the amount of time since
the barrier was set up, or during which it has continued to exist.
These two elements, it is true, are closely connected with each other;
for, since all changes in physical geography have taken place very
slowly--since barriers in the form of mountain-ranges and seas have
increased by slow process of growth--it is evident that impassableness
is, to some extent, a measure of time. But they are by no means in
strict proportion. The one or the other may predominate.
Now, this time-element connects geographical distribution with changes
of physical geography and climate in _geological_ times, and especially
with the _latest_ of these changes, viz., those occurring during the
_Glacial epoch_. During that remarkable epoch extraordinary changes
of climate, from extreme Arctic rigor to great mildness, enforced
wide migrations of species southward and northward; while concomitant
changes of physical geography, by elevation of the earth’s crust over
wide areas, opened highways between previously-isolated continents,
permitting migrations in various directions, and by subsequent
depression again isolating the migrated species in their new homes.
It is evident, then, that the recognition of the element of almost
unlimited time at once introduces into the question of geographical
distribution the _idea of evolution_. If the study of geographical
distribution, as _it now exists_, and as a part of science of physical
geography, gave rise naturally to the theory of “specific centers of
origin,” the study of the same, in connection with geological time,
and as a part of geological science, now demands its explanation by the
theory of evolution.
It must be borne in mind, then, that geographical diversity of
organisms is not a question of the present epoch only. There has
been geographical diversity in every previous geological epoch; it
is, therefore, a question of geology as well as of biology. It is
probable, however, that diversity has increased with the course of
geological times, and is greater now than ever before. In other words,
in the evolution of the organic kingdom, the law of differentiation
has prevailed here, as in other departments of biology. A clear
statement of the causes of the _present_ distribution of organisms
must embrace also the causes of geographical diversity _generally_.
We give, therefore, at once a brief statement of what seems to us the
most probable view, and shall then proceed to show how it explains the
present distribution.
=Most Probable View of the General Process.=--Bearing in mind, then,
this time-element, the phenomena of geographical diversity are best
explained by the following suppositions: 1. A gradual progressive
movement (evolution) of the organic kingdom, marching, as it were,
abreast, at equal rate along the whole line--i. e., in all parts of
the earth, and throughout all geological times, under the action of
all the forces or factors, and following all the laws, of evolution
already explained (pages 19 and 73). If this were all, there would be
no _geographical_ diversity, although _organic diversity_ might be
as great as it is now. There would be differentiation of forms and
structure everywhere, but no differentiation of groups in different
localities. 2. Under the influence of different conditions in
different places, more or less isolated from one another by climatic
or physical barriers, the onward movement (evolution) of organic
forms takes different directions and different rates, and gives rise
to local groups, which become more and more differentiated, without
limit as time goes on. This element, acting by itself throughout all
geological times, would ere this have produced an extreme geographical
diversity, such as does not anywhere exist. 3. From time to time, at
long intervals, extensive changes of physical geography and climate,
produced by crust elevations, partly enforce by change of temperature,
and partly permit by opening of gateways, extensive migrations and
dispersals of species, by which mingling and struggle for life and
final readjustment takes place, and extreme diversity is prevented.
Such mingling of different faunas and floras on the same ground,
and the severe struggle for life that thus ensues, and the survival
of the fittest in many directions, are, as already shown, among the
most powerful factors of evolution. They tend to _increase organic_
diversity, but to _diminish geographical_ diversity. 4. At the close
of such great periods of change as indicated in the last, by contrary
movement of the earth-crust--i. e., subsidence--new barriers are set up
and new isolations are produced, and the process of divergence again
commences and increases steadily so long as the barriers continue to
exist.
Now, the last of these periods of great changes and extensive
migrations, and subsequent isolations, was the Glacial epoch. It was
this epoch, therefore, which mainly determined the present geographical
distribution of species. Thus, the present distribution is a key to the
directions of the last great migrations, and therefore to the nature of
the changes in physical geography and climate which then occurred; and,
conversely, the character of these changes, determined in other ways,
_furnishes the only key to the present distribution of species_.
Before applying the foregoing principles in the explanation of special
cases, it may be well to give a very brief outline of the condition of
things during the Glacial epoch.
In America, during this epoch, by increasing cold the southern margin
of the great northern ice-sheet crept slowly southward, until it
reached the latitude of about 38° to 40°. Arctic species were thus
driven southward slowly, from generation to generation, until they
occupied the whole of the United States, as far as the shores of
the Gulf, while temperate species were forced still farther south,
into Central and South America. This period of extreme rigor and
southward migration was followed by a period of great mildness,
during which the ice and its accompanying Arctic conditions retreated
northward, followed by Arctic species. More than one advance and
retreat, apparently, occurred during this time. Again, during the
same time, brought about by northern elevation, there was broader
connection than now exists between North and South America, and free
migrations between, in both directions, enforced by extreme changes
in temperature. Also, during this or previous time, there were broad
connections between North America and Asia, in the region of Behring
Strait, and between America and Europe, in high-latitude regions, and
extensive migrations of faunas and floras between were thus permitted.
The necessary result of all these migrations of species, partly
enforced by changes of climate, partly permitted by opening of gateways
since closed, was exceptionally rapid changes in organic forms. This
was the result of two causes: First, the severer pressure of a changing
physical environment; and, second, a severer struggle for life between
the natives and the invaders.
In Europe, during the same time and from similar causes, there were at
least three or four different faunas struggling together for mastery
on the same soil. First, there were the Pliocene indigenes, who had,
if any, pre-emption right to the soil; second, invaders from Arctic
regions, driven southward by increasing cold; third, invaders from
Asia, permitted by the removal of the old sea-barrier which once
extended from the Black Sea to the Arctic, and of which the Caspian and
Aral are existing remnants, and thus opening a gateway for migration
which has remained open ever since; fourth, invaders from Europe and
Asia into Africa, and sometimes back again into Europe, by opening
of gateways through the Mediterranean, which have been since closed.
One of these highways was through Gibraltar, and one from Italy to
Africa through Sicily. As in America, so here, in even greater degree,
the severe pressure of changing environment and the severe struggle
for life produced rapid changes of organic forms. Many species were
destroyed; others saved themselves by modifications adapted more
perfectly to the changed conditions. There is little doubt that
man came into Europe with the Asiatic invasion, and was one of the
principal agents of change, especially in the way of destruction of
many old forms.
Such is a very brief outline of the last great geological change and
its general results. Being the last, this one has left the strongest
and most universal impress on the _present_ geographical distribution.
But similar changes by crust oscillations, if not also by extreme
changes of climate, have repeatedly occurred in geological times, and
some of the most remarkable geographical faunas and floras are the
result of these earlier geological changes. We will now give a few
examples illustrating these principles:
1. _Australia_ is undoubtedly more peculiar in its fauna and flora than
any other known country. Not only are all its species peculiar, not
found elsewhere on the face of the earth, but its genera, its families,
and even many of its orders of animals and plants, are also peculiar.
These facts are so familiar that it is unnecessary to dwell on them.
I need only mention, among plants, the whole of the simple-leaved
acacias, already mentioned on page 86, of which there are so many
species, and the whole family of the eucalyptids, of which there are
several hundred species. Among animals I need mention only the order
of monotremes, or egg-laying mammals, and nearly the whole order of
marsupials, or pouched animals, of which there are over two hundred
species. On the other hand, the true typical mammals are entirely
absent, with the exception of a few bats and a few rats, which have
evidently been accidentally introduced from abroad.
Another very noteworthy fact, which must be taken in connection with
the last, is that Australian forms are far less advanced in the race of
evolution than those of any other country--i. e., that many old forms
which have long ago become extinct elsewhere are still retained there.
A few examples will suffice. The marsupials just mentioned are an old
form once universally distributed, but now nearly extinct everywhere,
except in Australia; the cestracion, or Port Jackson shark, and the
ceratodus, are Palæozoic and Mesozoic forms retained only in Australia.
What is the explanation of these remarkable facts? We find the
sufficient answer in the fact that Australia has been long isolated
from all other countries. While geographical changes in geological
times have mingled more or less the organic forms of other countries,
and the sharp struggle for life has produced more rapid advance and
the production of many new and higher forms better armed for the
battle of life, Australia has remained isolated from competition, and
therefore comparatively unprogressive.
Can we tell when Australia was finally isolated? Approximately we can.
The class of mammals is divided into two groups, which differ widely
from each other; so widely, that they are called sub-classes. These
are placental mammals, or true typical mammals, and non-placental or
reptilian mammals. The non-placentals include only the marsupials and
the monotremes (ornithorhyncus and echidna). The monotremes actually
lay eggs and incubate them. In the marsupials the embryo has no
placental connection with the mother, and is born in a very imperfect
condition, utterly unfit for independent life, and placed in the pouch
(marsupium), and _permanently_ attached there to the teat until it is
capable of independent life; after which only it voluntarily nurses
like other new-borns. In other words, the gestation commenced in the
womb is completed in the pouch. The uterine gestation in the opossum
is only seventeen days, while the marsupial gestation is about two
and a half months. In a kangaroo seven feet high in sitting position
the embryo at birth is only one inch long--a pink, hairless, almost
amorphous mass. The monotremes are pure oviparous animals, like birds
and reptiles. The marsupials might well be called _semi-oviparous_.
In pure egg-layers the whole embryonic development is outside of the
body; in pure young-bearers the whole is within the body; in marsupials
it is partly within and partly without. Now--1. The monotremes are
found nowhere but in Australia and the neighboring New Guinea. 2. The
marsupials are also all confined to the Australian region, except a few
oppossums in America. 3. There are some two hundred and thirty species
of non-placentals in the Australian region. 4. As already said, there
are no true mammals at all in Australia, except a few bats and rats
which have come accidentally from abroad. 5. But non-placentals existed
abundantly in _Mesozoic times everywhere_, both in Europ-Asia and in
America, while true mammals did not appear at all on the surface of
the earth until the _Tertiary_, when they almost immediately became
very abundant everywhere, except in Australia. _Evidently, therefore,
Australia was isolated before the Tertiary._ The enormous difference
between its fauna and flora and those of other countries is due to
at least three things: 1. So long an isolation necessarily produced
great divergence of forms. This alone, however, would not affect the
_grade of organization_. 2. Saved from wide migrations, and especially
invasions from Eurasia, the great field of competitive struggle, it was
left far behind in the race of evolution. Hence many of its forms are
archaic; its mammalian fauna, for instance, is still in the Mesozoic
stage. 3. Its distance from other large continents is so great that
accidental colonization has been very slight, only extending to a few
bats and a few rats.
I stop a moment to insist on the effect of competitive struggle in
developing organic forms strong for the battle of life. Of all the
continents, Eurasia has been the scene of most frequent geological
changes, and therefore the arena of fiercest competitive struggle
through wide and frequent migrations. Eurasian species, therefore, are
the strongest of all. They have conquered wherever they have gone.
Species in isolated regions are usually the weakest. The great moas
and the dodo could not have continued to exist unless protected in a
sort of bomb-proof. Kangaroos would now be quickly exterminated by the
introduction of fierce Eurasian carnivores.
2. _Africa._--The fauna of that part of Africa north of Sahara is
essentially Mediterranean--i. e., a sub-group of the Eurasian. Sahara,
rather than the Mediterranean Sea, is the true intercontinental
barrier. The true African region, therefore, is south of Sahara. Now,
according to Mr. Wallace, whom I mainly follow here, the true African
mammalian fauna consists of two very different groups of animals. The
one is a group of very small, curious animals, mostly low forms of
insectivores and lemurs, very peculiar to this region, though more
resembling those of Madagascar than of any other region; the other
is a group of large and powerful animals which dominate the region.
These latter are similar to, though not identical with, those which
inhabited Eurasia in Pliocene times. The great carnivores, pachyderms,
and ruminants of the region are examples of this group. Now, the
explanation of these facts is as follows: The indigenes of Africa are
the animals of the first group. Africa, in Tertiary times, was isolated
from the great field of combat, Eurasia, and therefore its animals were
small, of low grade, and peculiar. During later Tertiary (Pliocene)
times, then, Africa was inhabited by animals of the first group, while
Eurasia was dominated by animals of the second group. These two groups
were then separated by the Desert of Sahara, or else by a sea in that
region. Some time during the Glacial epoch geographical changes removed
this barrier, and climatic changes drove the Eurasian animals southward
into Africa, where, finding congenial climate, they took possession
of the continent, dominating the feebler natives. Subsequently they
were isolated there by the formation of the desert, and the process
of divergence commenced, and has gone on to the formation of many
new forms. Meanwhile the change, partly by extinction and partly by
modification, has gone on still more rapidly in Eurasia, but in a
different direction. Hence, Africa is regarded as one of the primary
faunal regions.
3. _Madagascar._--This, next to the Australian, is probably the most
peculiar faunal region known. There is probably not a single mammalian
species found there which is known to occur anywhere else. It is
remarkable also as the principal home of that strange, generalized,
ancient form of monkeys--the lemurs. And yet its animals, though
very different, have a distant resemblance to those of Africa; not,
however, to the present dominant type, but to those we have called
the indigenes. Not one of the northern invaders is found there. The
obvious conclusion from these facts is, that Madagascar was formerly
united with Africa, and both were occupied by the same mammalian fauna
(which may be called African indigenes, although they were considerably
different from their descendants of the present day), but became
separated before the northern invasion. The effect of this invasion
was to hasten the steps of change in the indigenous fauna of Africa,
partly by extermination, partly by modification, while the isolated
portion in Madagascar went on at the usual slow rate of change in
isolated regions. The time since the separation (which was certainly
during the Tertiary period) has been sufficiently long to produce very
great divergence in both, but _especially in the African indigenes_. In
the fauna of Madagascar, therefore, we have a nearer approach to the
original fauna of both. On account of this long isolation, we have here
many ancient types which are extinct elsewhere. The lemurs are such an
ancient type. These are a wonderfully-generalized type of monkeys--a
connecting link between monkeys and other mammals, especially
insectivores. As might be supposed, from the law of differentiation,
already explained (page 11), they are the earliest form, the
progenitors, of monkeys. In fact, in early Tertiary times, they were
found not only in Africa and Madagascar, but all over the earth, as the
only representatives of the monkey family. The true monkeys were not
introduced until the mid-Tertiary. In Eurasia and in America (which at
that time was probably connected with Eurasia) wide migrations and
frequent conflicts of faunas produced comparatively rapid evolution
of new and higher forms, while in isolated Africa old types continued
until the invasion. Madagascar was spared this invasion, and therefore
old types are still preserved there. At present, at least three
quarters of all lemurs are confined to Madagascar, although a few
species are still found in Africa and in the great East Indian islands.
4. =Island-Life.=--Mr. Wallace has divided islands into two kinds,
continental and oceanic islands. The division is undoubtedly a good
one, although we may not always be able to refer an example with
certainty to the one or the other class. _Continental_ islands are
those on the borders of continents, and separated from the latter
only by _shallow water_. _Oceanic_ islands are those, usually very
small, found in the midst of the ocean, with abyssal depth all about.
Continental islands may be regarded as appendages to the neighboring
continent--as outliers of continents separated by submergence, and
have, in fact, been thus formed. Oceanic islands have been formed
geologically recently by volcanic action building up from the
sea-bottom. Continental islands have a continental structure--i. e.,
they are composed of stratified as well as of igneous rocks. Their
structure is a record of geological history, like that of the
neighboring continent. Oceanic islands are composed wholly of volcanic
rocks; or, if there be any stratified rocks, these are only of the most
recent date. As examples of continental islands we have New Zealand
as an appendage of Australia, the great East Indian (Borneo, Java,
Sumatra, etc.) and the Japanese Islands, etc., as appendages of Asia;
the British Islands, appendages of Europe; the West Indian Islands,
appendages of America; Madagascar, an appendage of Africa, etc., etc.
As examples of oceanic islands we have the Azores and Bermudas in the
Atlantic, and the Polynesian islands in mid-Pacific.
_a. Continental Islands._--Now, the fauna of continental islands, as
might be expected from the mode of origin of these islands, is similar
to, though not identical with, that of the neighboring continent; the
amount of difference being in proportion to the _length of time since_
they were separated and the _width of the separation_. _Madagascar_,
for example, has been long separated from its parent continent, and by
a wide and deep channel. Its fauna, therefore, differs greatly from
that of Africa, although resembling it more than that of any other
country. The separation of _New Zealand_ from Australia has been not
quite so long, and the divergence, therefore, is not so great. These
two will be sufficient illustrative examples of long separation, and
therefore of great differentiation of forms.
[Illustration: FIG. 67.--Map of outline of coast of Western Europe, if
elevated 600 feet (after Lyell).]
On the other hand, the British Isles are an excellent example of
comparatively recent separation. These isles have probably been several
times united and separated from Europe, but we are here concerned only
with the more recent. They are now separated from the continent and
from one another only by shallow seas. An elevation of less than six
hundred feet--geologically a very small change--would bare the bottoms
of the Irish and English Channels and the North Sea, and connect these
islands with one another and with the continent (Fig. 67). Now, it is
well known that there were during the Glacial epoch, and subsequently,
several oscillations of level sufficient to connect and separate
these islands. In the mid-Glacial epoch the British Islands, by
submergence, were nearly obliterated, being reduced to an archipelago
of small islets representing the high mountains of Wales and Scotland.
The Pliocene fauna and flora were, therefore, largely exterminated.
During the close of that epoch they were elevated above the present
condition and broadly connected with the continent (Fig. 67), and the
newly-exposed land was taken possession of by European species, man
among the number. Still later--i. e., at the beginning of the present
epoch--the islands by subsidence were again separated, but not widely,
from the continent. This is the condition now. What, then, was the
result? 1. The fauna and flora of the British Isles are substantially
the same, but _less rich_ in species than that of Continental Europe,
some of the European species being wanting. This shows that the last
connection was not a long one; the colonization had not been completed
before re-isolation. 2. This poverty of species is more conspicuous in
Ireland, because colonization is progressive in space as well as in
time. Some species had not reached so far when Ireland was re-isolated
from England. The conspicuous absence of snakes, for example, is thus
accounted for. There is, we all know, another theory to account for
this, but we prefer the natural one. 3. The difference between British
and European fauna and flora is very small, it is true, but there is
some difference, varietal if not specific. The reason is, that the time
since separation is too small to produce much divergence, and the width
of the existing barriers not great enough to prevent colonization by
accidental causes.
The continental islands of the southern coast of Asia are good examples
of an intermediate condition as to the length of time since separation,
and of the consequent degree of differentiation of the faunas and
floras.
_Coast-Islands of California._--We give one more example, and dwell
upon it a little, because it occurs on our own coast.
The recent studies of Mr. E. L. Greene on the flora of the islands off
the coast of California have brought to light some facts which are an
admirable illustration of the principles laid down above.
On looking at a good map of California, any one will observe eight
or ten islands, some of them of considerable size, strung along the
coast from Point Conception southward, and separated from the mainland
by a sound twenty to thirty miles wide. They are in structure true
continental islands--outliers of the mainland separated by a subsidence
of a few hundred feet. Moreover, the date of their separation is known.
They were certainly connected with the mainland during the later
Pliocene and early Quaternary, for bones of the mammoth, characteristic
of that time, have been found on one of them.[28] They were therefore
separated during the Glacial epoch.
The main peculiarities of the flora of these islands are the following:
1. Out of nearly three hundred species of plants gathered by Mr.
Greene, about fifty are wholly peculiar to these islands. 2. Of the
remaining two hundred and fifty species, nearly all are distinctively
Californian. In other words, the distinctively Californian forms are
very abundant, while the common American forms are rare--i. e., the
island flora is distinctively Californian, with many peculiar species
added.
I explain these facts as follows: The whole coast-region of California
is geologically very recent, having emerged from the sea as late as
the beginning of the Pliocene epoch. As soon as emerged it was of
course colonized from adjacent parts. Since that time its peculiar
flora has been formed by gradual modification. The environment has been
sufficiently peculiar, the isolation sufficiently complete, and the
time sufficiently long, to make a very distinct group of organisms. It
is one of Mr. Wallace’s primary divisions of the Ne-arctic region.
During late Pliocene and early Quaternary times, as already said, the
islands were still a part of the mainland, and the whole was occupied
by the same species, viz., the distinctively Californian species
now found in both, together, as I suppose, with the peculiar island
species. During the oscillations of the glacial times the islands were
separated by subsidence of the continental margin. Simultaneously with
this subsidence, or subsequently thereto, came the invasion of northern
species, driven southward by glacial cold. Then came the mingling of
invaders with natives, the struggle for mastery, the extermination
of many forms--viz., the peculiar island species--and the slight
modification of others, and the final result is the California flora
of to-day. But the island flora was spared this invasion by isolation.
Therefore the invading species are mostly wanting, the distinctive
island species were saved, and the result is the island flora of
to-day. The island flora, therefore, somewhat nearly represents the
Pliocene indigenes of both.
It will be observed that this case is somewhat like that of Madagascar,
but with a characteristic difference. In the case of Madagascar, the
separation has been long. The extreme peculiarity of its fauna is the
result partly of progressive divergence and partly of many forms saved
by isolation. In the case of the coast-islands of California, the time
has not been long enough for any great divergence by modification. The
peculiarity of its species is due almost wholly to species saved by
isolation.[29]
_b. Oceanic Islands._--We have seen that faunas and floras of
continental islands are somewhat similar to those of the neighboring
continent, though with varying degrees of difference--the amount of
difference, or divergence by evolution, being in proportion to the
amount of time and the impassableness of the separating barriers. But
oceanic islands have never been connected with any continent. They
are new land formed in the midst of the ocean by volcanic action.
When they first appeared they were, of course, without inhabitants
of any kind, animal or vegetal. How were they peopled? We answer by
_waifs_ from here and there--by _castaways from other lands_. The
dominance of particular kinds will depend on the direction of winds
and currents, bringing from some lands more than others, and upon the
kinds of animals or seeds of plants most liable to be successfully
carried across wide seas. Their faunas and floras, therefore, are
characterized by a mixture of species resembling, though not usually
identical with, those of various lands, with a predominance of those
of some one land, and by the singular and complete absence of mammals
and amphibians, these being unlikely to be transported by floating
timber, as are small reptiles and insects, etc. Among mammals, however,
there is a significant exception in favor of bats, the reason being
both their power of flight and their habit of concealment in hollow
trees, etc. To this explanation, however, we must add that divergence
by isolation will meanwhile go on in proportion to time. The Azores,
for example, have been peopled from Europe, Africa, and America, but
mostly from Europe, on account of the prevailing winds and currents
being favorable to colonization from that direction. There are many
curious peculiarities in the species, however, because colonization is
very slow, and divergent variation has been going on _pari passu_. The
Bermudas, on the other hand, have been colonized mainly from America,
because of the current of the Gulf Stream.
These few examples are sufficient for our purpose, which is only to
illustrate the causes of geographical distribution. If any one desires
to pursue this interesting subject, we would refer him to that most
fascinating book, Mr. Wallace’s “Island-Life.”
5. =Alpine Species.=--These afford an admirable illustration of the
fact that in isolated faunas and floras the amount of difference is
proportioned not only to the completeness of isolation, but also and
mainly to the _time of isolation_.
It is well known that Alpine species--i. e., those species inhabiting
the region bordering the perpetual snow of lofty mountains--are very
similar to one another, even in the most distant localities, where
their isolation from one another is as complete as possible; as, for
example, in the high Alps of Europe, the high mountains of Colorado
and California. Why is this? We find the key to this mystery in the
additional fact that _they are similar also to Arctic species_. A
somewhat full explanation is here necessary.
During Miocene times, magnolias and taxodiums (bald cypress),
like those in forests and swamps of Carolina and Louisiana, and
sequoias and libocedrus like those now in California, and many other
temperate-region forms of plants, grew abundantly in Greenland,
and northward certainly to 75° north latitude. At that time there
could not have been any perpetual polar ice, and therefore no Arctic
species, unless on _high mountains in polar regions_. In Pliocene
times perpetual polar ice, and therefore Arctic species, probably
commenced to appear. As the cold of the Glacial epoch came on and
increased in severity, the polar ice extended southward as a general
ice-sheet, until it reached in America 40° and in Europe about 50°
north latitude. In the United States its margin can be traced as
a distinct moraine through Long Island, middle New Jersey, middle
Pennsylvania; thence, less distinctly, following the Ohio River,
crossing the Mississippi; thence following the Missouri, on its south
side, into Montana. By the increasing cold, Arctic species were driven
slowly southward, generation after generation, until they occupied
the whole of the United States to the Gulf, and the whole of Europe
to the Mediterranean. As these species on the two continents came
from a _common home_ in polar regions, they were similar to one
another, except in so far as some slight divergent modification may
have been produced during their southward travel. When the glacial
rigor declined, and the ice-sheet gradually retreated to its present
position, Arctic species, following the snow-edge, went also northward,
on both continents, to their present home in polar regions. But there
was an alternative way of migration left open which was embraced by
certain plants and insects. While on both continents most individuals
went northward, some of them went upward, following the snow-edge into
high mountains, and were left _stranded there_. Thus it has come to
pass that the plants and insects of high mountains in temperate regions
of different continents, though so widely separated and impassably
isolated, are extremely similar to one another. But, though similar,
they are rarely identical. The time has been long enough for some but
not very great divergent modification. It is impossible to conceive a
more beautiful illustration of the principles we have been trying to
enforce.
* * * * *
Thus, then, undoubtedly all the phenomena of geographical distribution
of species are most rationally explained on the principle of slow
evolution--changes, different in different places, and increasing with
the time of isolation and its completeness.
=Objection.=--The only objection which can be raised against this
view is the manner in which contiguous geographical faunas and floras
pass into one another when they are _limited not by barriers but
by temperature_. In passing from equator to poles, over continuous
land, we of course pass through many successive faunas and floras,
limited wholly or mainly by temperature. Now, if species are indeed
indefinitely modifiable, then on the borders of contiguous faunas or
floras, where one species disappears and another closely allied but
adapted to a colder temperature takes its place, the one species (say
the anti-evolutionists) ought to be gradually _transmuted_ into the
other, so that all the gradations may be traced. But this is certainly
not usually the fact. On the contrary, a species may indeed pass out
gradually, and another come in gradually, so far as _number and vigor
of individuals are concerned_; but, in _specific character_, they
may be said, usually at least, to come in suddenly, with all their
characters perfect, remain unchanged throughout their whole range, and
pass out suddenly at its borders. Another species takes its place,
overlapping in range and coexisting on the borders of both; this also
continues unchanged, as far as it goes, and so on. The change from one
fauna to another is apparently not by _transmutation_ of one species
_into_ another by gradations, but by _substitution_ of one perfect
species _for_ another perfect species. As a broad general statement,
the condition of things is precisely such as would be the case if
specific types were substantially immutable by physical conditions, but
were originated in some inscrutable way (created) in the regions where
we now find them, and have spread in every direction as far as physical
conditions and struggle with other species would allow them--their
ranges therefore interpenetrating and overlapping one another on their
borders.
Two characteristic examples will make our meaning clear. There is not
a more characteristic tree known than the sweet-gum, or liquidambar.
This tree grows from the borders of Florida to the shores of the
Great Lakes. It may indeed be most numerous and vigorous somewhere in
the middle region, and may die out gradually in number and vigor of
individuals on the borders of its range, but in specific character it
is substantially the same throughout, easily recognizable by its dense
wood, its winged bark, its five-starred leaf, its spinous burr, and its
fragrant gum. Physical conditions may diminish its number and vigor,
and limit its extension, but seem powerless to essentially modify its
specific character. It seems to give up its life rather than change its
nature.
Another striking example: The sequoias (redwood and big-tree) are
entirely confined to California, and there are only two species now
existing, viz., the redwood (_S. sempervirens_) of the Coast Ranges,
and the big-tree (_S. gigantea_) of the Sierra Nevada. Doubtless they
are most numerous and vigorous somewhere in the middle of their range,
and die out gradually in number and vigor on the borders north and
south, being replaced there by other genera better adapted to the
physical conditions; but in specific character they remain essentially
unchanged throughout. They are everywhere the same--easily recognizable
by wood, bark, leaf, and burr. Both in this case, and in the previous
one of the sweet-gum, it is as if they were created perfect in their
present localities, and have spread in all directions as far as
physical conditions and the struggle with other competing species would
allow; but physical conditions seem powerless to change them into any
other species by adaptive modification.
=Answer.=--We have, we believe, stated the objection fairly. The
answer is, that the elements of _time_ and of _migrations_ have not
been taken into the account. In fact, this objection was conceived and
formulated before the idea of geological time was fully assimilated by
the human mind, and our theories of origin adjusted to it. If these
species did indeed originate where we now find them, and _in the
present geological epoch_, the argument might at least be entertained;
but this is not the fact. We know something of the geological history
of all these species, and the history of the migrations of some of
them. We know that sweet-gums were abundant and of _many species_ in
the United States in Tertiary times, and all have become extinct except
this remnant. Whatever of modifications there were must be looked
for at or about the time of its origin in Tertiary times, not now.
Species, like individuals, are plastic only when young. This one has
already become rigid, and all the more so as it is a remnant widely
separated from other species. For competition is strongest and most
effective with nearest allies. Present species are mostly isolated
remnants--terminal twiglets of the tree of life. Twiglets are of course
widely separated at their visible ends. Their points of union with
other twiglets must be sought below.
In the case of the sequoias, we know something also of the history of
their migrations. In Miocene times they were abundant, and of many
species in circumpolar regions. Some twenty-four species of fossil
sequoias are known, fourteen of which are Tertiary. By the cold of
the Glacial epoch they were driven slowly southward, both in America
and in Europe--in America as far as Southern California. After the
Glacial epoch, and the return of temperate conditions, they doubtless
attempted to go northward again; but these great changes were too much
for them; they were wholly exterminated in Europe, and nearly so in
America. A few were left stranded high up on the slopes of the Sierra
Nevada, and on the cool, moist slopes of the Coast Ranges. The species
now in California are not identical with those found in the Miocene
strata of Greenland; but the difference is only what we might expect
after such extensive migrations and such long and severe struggle for
life. Further, it is noteworthy that the Miocene species fall into two
groups, viz., the yew-like leaved and the cypress-like leaved. These
are represented to-day in California, the one by the redwood, the other
by the big-tree. They are evidently direct descendants of the Miocene
species, though somewhat modified.
But it will be objected that there ought to be some cases of
transitional forms showing transmutation--in fact, there ought to
be some cases of species now forming under our eyes. There are, we
believe, examples of such cases. But intermediate forms are not likely
to be maintained long, especially if migrations occur to give rise to
severe conflict of forms. In that case the intermediate forms are soon
eliminated, and species become distinct. This important point will be
discussed more fully in the next chapter.