MIGRATION AND DISTRIBUTION OF ORGANISMS. CHOROLOGY AND THE ICE-PERIOD OF
THE EARTH.
Chorological Facts and Causes.—Origin of most Species in
one Single Locality: “Centres of Creation.”—Distribution
by Migration.—Active and Passive Migrations of Animals
and Plants.—Means of Transport.—Transport of Germs by
Water and by Wind.—Continual Change of the Area of
Distribution by Elevations and Depressions of the
Ground.—Chorological Importance of Geological
Processes.—Influence of the Change of Climate.—Ice or
Glacial Period.—Its Importance to Chorology.—Importance
of Migrations for the Origin of New Species.—Isolation
of Colonists.—Wagner’s Law of Migration.—Connection
between the Theory of Migration and the Theory of
Selection.—Agreement of its Results with the Theory of
Descent.
As I have repeatedly said, but cannot too much emphasize, the actual
value and invincible strength of the Theory of Descent does not lie in
its explaining this or that single phenomenon, but in the fact that it
explains _all_ biological phenomena, that it makes _all_ botanical and
zoological series of phenomena intelligible in their relations to one
another. Hence every thoughtful investigator is the more firmly and
deeply convinced of its truth the more he advances from single
biological observations to a general view of the whole domain of animal
and vegetable life. Let us now, starting from this comprehensive point
of view, survey a biological domain, the varied and complicated
phenomena of which may be explained with remarkable simplicity and
clearness by the theory of selection. I mean _Chorology_, or the theory
of the _local distribution of organisms over the surface of the earth_.
By this I do not only mean the _geographical_ distribution of animal and
vegetable species over the different parts and provinces of the earth,
over continents and islands, seas, and rivers; but also their
_topographical_ distribution in a _vertical_ direction, their ascending
to the heights of mountains, and their descending into the depths of the
ocean. (Gen. Morph. ii. 286.)
The strange chorological series of phenomena which show the horizontal
distribution of organisms over parts of the earth, and their vertical
distribution in heights and depths, have long since excited general
interest. In recent times Alexander Humboldt(39) and Frederick Schouw
have especially discussed the geography of plants, and Berghaus and
Schmarda the geography of animals, on a large scale. But although these
and several other naturalists have in many ways increased our knowledge
of the distribution of animal and vegetable forms, and laid open to us a
new domain of science, full of wonderful and interesting phenomena, yet
Chorology as a whole remained, as far as their labours were concerned,
only a desultory knowledge of a mass of individual _facts_. It could not
be called a science as long as the _causes_ for the explanation of these
facts were wanting. These causes were first disclosed by the theory of
selection and its doctrine of the _migrations_ of animal and vegetable
species, and it is only since the works of Darwin and Wallace that we
have been able to speak of an independent _science of Chorology_.
If all the phenomena of the geographical and topographical distribution
of organisms are examined by themselves, without considering the gradual
development of species, and if at the same time, following the customary
superstition, the individual species of animals and plants are
considered as forms independently created and independent of one
another, then there remains nothing for us to do but to gaze at those
phenomena as a confused collection of incomprehensible and inexplicable
miracles. But as soon as we leave this low stand-point, and rise to the
height of the theory of development, by means of the supposition of a
blood-relationship between the different species, then all at once a
clear light falls upon this strange series of miracles, and we see that
all chorological facts can be understood quite simply and clearly by the
supposition of a common descent of the species, and their passive and
active migrations.
The most important principle from which we must start in chorology, and
of the truth of which we are convinced by due examination of the theory
of selection, is that, as a rule, every animal and vegetable species has
arisen only _once_ in the course of time and only in _one_ place on the
earth—its so-called “centre of creation”—by natural selection. I share
this opinion of Darwin’s unconditionally, in respect to the great
majority of higher and perfect organisms, and in respect to most animals
and plants in which the division of labour, or differentiation of the
cells and organs of which they are composed, has attained a certain
stage. For it is quite incredible, or could at best only be an
exceedingly rare accident, that all the manifold and complicated
circumstances—all the different conditions of the struggle for life,
which influence the origin of a new species by natural selection—should
have worked together in exactly the same agreement and combination more
than once in the earth’s history, or should have been active at the same
time at several different points of the earth’s surface.
On the other hand, I consider it to be very probable that certain
exceedingly imperfect organisms of the simplest structure, forms of
species of an exceedingly indifferent nature, as, for example, many
single-celled Protista, but especially the Monera, the simplest of them
all, should have several times or simultaneously arisen in their
specific form in several parts of the earth. For the few and very simple
conditions by which their specific form was changed in the struggle for
life may surely have often been repeated, in the course of time,
independently in different parts of the earth. Further, those higher
specific forms also, which have not arisen by natural selection, but by
_hybridism_ (the previously-mentioned hybrid species, pp. 147 and 275),
may have repeatedly arisen anew in different localities. As, however,
this proportionately small number of organisms does not especially
interest us here, we may, in respect of chorology, leave them alone, and
need only take into consideration the distribution of the great majority
of animal and vegetable species in regard to which the _single origin of
every species in a single locality_, in its so-called “central point of
creation,” can be considered as tolerably certain.
Every animal and vegetable species from the beginning of its existence
has possessed the tendency to spread beyond the limited locality of its
origin, beyond the boundary of its “centre of creation,” or, in other
words, beyond its _primæval home_, or its natal place. This is a
necessary consequence of the relations of population and over-population
(pp. 161 and 256). The more an animal or vegetable species increases,
the less is its limited natal place sufficient for its sustenance, and
the fiercer the struggle for life; the more rapid the _over-population_
of the natal spot, the more it leads to _emigration_. These _migrations_
are common to all organisms, and are the real cause of the wide
distribution of the different species of organisms over the earth’s
surface. Just as men leave over-crowded states, so all animals and
plants migrate from their over-crowded primæval homes.
Many distinguished naturalists, especially Lyell(11) and Schleiden, have
before this repeatedly drawn attention to the great importance of these
very interesting migrations of organisms. The means of transport by
which they are effected are extremely varied. Darwin has discussed these
most excellently in the eleventh and twelfth chapters of his work, which
are exclusively devoted to “geographical distribution.” The means of
transport are partly active, partly passive; that is to say, the
organism effects its migration partly by free locomotion due to its own
activity, and partly by the movements of other natural bodies in which
it has no active share.
It is self-evident that _active migrations_ play the chief part in
animals able to move freely. The more freely an animal’s organization
permits it to all move in directions, the more easily the animal species
can migrate, and the more rapidly it will spread over the earth.
_Flying_ animals are of course most favoured in this respect, among
vertebrate animals especially birds, and among articulated animals,
insects. These two classes, as soon as they came into existence, can
have more easily spread over the whole earth than any other animal, and
this fact partly explains the extraordinary uniformity of structure
which characterizes these two great classes of animals. For, although
they contain an exceedingly large number of different species, and
although the insect class alone is said to possess more different
species than all other classes of animals together, yet all the
innumerable species of insects, and in like manner, also, the different
species of birds, agree most strikingly in all essential peculiarities
of their organization. Hence, in the class of insects, as well as in
that of birds, we can distinguish only a very small number of large
natural groups or orders, and these few orders differ but very little
from one another in their internal structure. The orders of birds with
their numerous species are not nearly as distinct from one another as
the orders of the mammalian class, containing much fewer species; and
the orders of insects, which are extremely rich in genera and species,
resemble one another much more closely in their internal structure than
do the much smaller orders of the crab class. The general parallelism
between birds and insects is also very interesting in relation to
systematic zoology; and the great importance of their richness in forms,
for scientific morphology, lies in the fact that they show us how,
within the narrowest anatomical sphere, and without profound changes of
the essential internal organization, the greatest variety in external
bodily forms can be attained. The reason of this is evidently their
flying mode of life and their free locomotion. In consequence of this
birds, as well as insects, have spread very rapidly over the whole
surface of the earth, have settled in all possible localities
inaccessible to other animals, and variously modified their specific
form by superficial adaptation to particular local relations.
Next to the flying animals, those animals, of course, have spread most
quickly and furthest which were next best able to migrate, that is, the
best runners among the inhabitants of the land, and the best swimmers
among the inhabitants of the water. However, the power of such active
migrations is not confined to those animals which throughout life enjoy
free locomotion. For the fixed animals also, such as corals, tubicolous
worms, sea-squirts, lily encrinites, sea-acorns, barnacles, and many
other lower animals which adhere to seaweeds, stones, etc., enjoy, at
least at an early period of life, free locomotion. They all migrate
before they adhere to anything. Their first free locomotive condition of
early life is generally that of a “ciliated” larva, a roundish, cellular
corpuscle, which, by means of a garb of movable “flimmer-hairs,” (Latin,
“cilia”) swarms about in the water and bears the name of Planula.
But the power of free locomotion, and hence, also, of active migration,
is not confined to animals alone, but many plants likewise enjoy it.
Many lower aquatic plants, especially the class of the Tangles (Algæ),
swim about freely in the water in early life, like the lower animals
just mentioned, by means of a vibratile hairy coat, a vibrating whip, or
a covering of tremulous fringes, and only at a later period adhere to
objects. Even in the case of many higher plants, which we designate as
creepers and climbing plants, we may speak of active migration. Their
elongated stalks and perennial roots creep or climb during their long
process of growth to new positions, and by means of their widespread
branches they acquire new habitations, to which they attach themselves
by buds, and bring forth new colonies of individuals of their species.
Influential as these active migrations of most animals and many plants
are, yet alone they would by no means be sufficient to explain the
chorology of organisms. _Passive migrations_ have ever been by far the
more important, and of far greater influence, in the case of most plants
and in that of many animals. Such passive changes of locality are
produced by extremely numerous causes. Air and water in their eternal
motion, wind and waves with their manifold currents, play the chief
part. The wind in all places and at all times raises light organisms,
small animals and plants, but especially their young germs, animal eggs
and plant seeds, and carries them far over land and seas. Where they
fall into the water they are seized by currents or waves and carried to
other places. It is well known, from numerous examples, how far in many
cases trunks of trees, hard shelled fruits, and other not readily
perishable portions of plants are carried away from their original home
by the course of rivers and by the currents of the sea. Trunks of palm
trees from the West Indies are brought by the Gulf Stream to the British
and Norwegian coasts. All large rivers bring down driftwood from the
mountains, and frequently alpine plants are carried from their home at
the source of the river into the plains, and even further, down to the
sea. Frequently numerous inhabitants live between the roots of the
plants thus carried down, and between the branches of the trees thus
washed away there are various inhabitants which have to take part in the
passive migration. The bark of the tree is covered with mosses, lichens,
and parasitic insects. Other insects, spiders, etc., even small
reptiles and mammals, are hidden within the hollow trunk or cling to the
branches. In the earth adhering to the fibres of the roots, in the dust
lying in the cracks of the bark, there are innumerable germs of smaller
animals and plants. Now, if the trunk thus washed away lands safely on a
foreign shore or on a distant island, the guests who had to take part in
the involuntary voyage can leave their boat and settle in the new
country. A very remarkable kind of water-transport is formed by the
floating icebergs which annually become loosened from the eternal ice of
the Polar Sea. Although these cold regions are thinly peopled, yet many
of their inhabitants, who were accidentally upon an iceberg while it was
becoming loosened, are carried away with it by the currents, and landed
on warmer shores. In this manner, by means of loosened blocks of ice
from the northern Polar Sea, often whole populations of small animals
and plants have been carried to the northern shores of Europe and
America. Nay, even polar foxes and polar bears have been carried in this
way to Iceland and to the British Isles.
Transport by air is no less important than transport by water in this
matter of passive migration. The dust covering our streets and roofs,
the earth lying on dry fields and dried-up pools, the light moist soil
of forests, in short, the whole surface of the globe contains millions
of small organisms and their germs. Many of these small animals and
plants can without injury become completely dried up, and awake again to
life as soon as they are moistened. Every gust of wind raises up with
the dust innumerable little creatures of this kind, and often carries
them away to other places miles off. But even larger organisms, and
especially their germs, may often make distant passive journeys through
the air. The seeds of many plants are provided with light feathery
processes, which act as parachutes and facilitate their flight in the
air, and prevent their falling. Spiders make journeys of many miles
through the air on their fine filaments, their so-called gossamer
threads. Young frogs are frequently raised by whirlwinds into the air by
thousands, and fall down in a distant part as a “shower of frogs.”
Storms may carry birds and insects across half the earth’s
circumference. They drop in the United States, having risen in England.
Starting from California, they only come to rest in China. But, again,
many other organisms may make the journey from one continent to another
together with the birds and insects. Of course all parasites, the number
of which is legion, fleas, lice, mites, moulds, etc., migrate with the
organisms upon which they live. In the earth which often remains
sticking to the claws of birds there are also small animals and plants
or their germs. Thus the voluntary or involuntary migration of a single
larger organism may carry a whole small flora and fauna from one part of
the earth to another.
Besides the means of transport here mentioned, there are many others
which explain the distribution of animal and vegetable species over the
large tracts of the earth’s surface, and especially the general
distribution of the so-called cosmopolitan species. But these alone
would not nearly be sufficient to explain all chorological facts. How is
it, for example, that many inhabitants of fresh water live in various
rivers or lakes far away and quite apart from one another? How is it
that many inhabitants of mountains, which cannot exist in plains, are
found upon entirely separated and far distant chains of mountains? It
is difficult to believe, and in many cases quite inconceivable, that
these inhabitants of fresh water should have in any way, actively or
passively, migrated over the land lying between the lakes, or that the
inhabitants of mountains in any way, actively or passively, crossed the
plains lying between their mountain homes. But here geology comes to our
help, as a mighty ally, and completely solves these difficult problems
for us.
The history of the earth’s development shows us that the distribution of
land and water on its surface is ever and continually changing. In
consequence of geological changes of the earth’s crust, _elevations_ and
_depressions_ of the ground take place everywhere, sometimes more
strongly marked in one place, sometimes in another. Even if they happen
so slowly that in the course of centuries the seashore rises or sinks
only a few inches, or even only a few lines, still they nevertheless
effect great results in the course of long periods of time. And
long—immeasurably long—periods of time have not been wanting in the
earth’s history. During the course of many millions of years, ever since
organic life existed on the earth, land and water have perpetually
struggled for supremacy. Continents and islands have sunk into the sea,
and new ones have arisen out of its bosom. Lakes and seas have slowly
been raised and dried up, and new water basins have arisen by the
sinking of the ground. Peninsulas have become islands by the narrow neck
of land which connected them with the mainland sinking into the water.
The islands of an archipelago have become the peaks of a continuous
chain of mountains by the whole floor of their sea being considerably
raised.
Thus the Mediterranean at one time was an inland sea, when, in the place
of the Straits of Gibraltar, an isthmus connected Africa with Spain.
England, even during the more recent history of the earth, when man
already existed, has repeatedly been connected with the European
continent and been repeatedly separated from it. Nay, even Europe and
North America have been directly connected. The South Sea at one time
formed a large Pacific Continent, and the numerous little islands which
now lie scattered in it were simply the highest peaks of the mountains
covering that continent. The Indian Ocean formed a continent which
extended from the Sunda Islands along the southern coast of Asia to the
east coast of Africa. This large continent of former times Sclater, an
Englishman, has called _Lemuria_, from the monkey-like animals which
inhabited it, and it is at the same time of great importance from being
the probable cradle of the human race, which in all likelihood here
first developed out of anthropoid apes. The important proof which Alfred
Wallace has furnished,(36) by the help of chorological facts, that the
present Malayan Archipelago consists in reality of two completely
different divisions, is particularly interesting. The western division,
the Indo-Malayan Archipelago, comprising the large islands of Borneo,
Java, and Sumatra, was formerly connected by Malacca with the Asiatic
continent, and probably also with the Lemurian continent just mentioned.
The eastern division, on the other hand, the Austro-Malayan Archipelago,
comprising Celebes, the Moluccas, New Guinea, Solomon’s Islands, etc.,
was formerly directly connected with Australia. Both divisions were
formerly two continents separated by a strait, but they have now for
the most part sunk below the level of the sea. Wallace, solely on the
ground of his accurate chorological observations, has been able in the
most acute manner to determine the position of this former strait, the
south end of which passes between Balij and Lombok.
Thus, ever since liquid water existed on the earth, the boundaries of
water and land have eternally changed, and we may assert that the
outlines of continents and islands have never remained for an hour, nay,
even for a minute, exactly the same. For the waves eternally and
perpetually break on the edge of the coast, and whatever the land in
these places loses in extent, it gains in other places by the
accumulation of mud, which condenses into solid stone and again rises
above the level of the sea as new land. Nothing can be more erroneous
than the idea of a firm and unchangeable outline of our continents, such
as is impressed upon us in early youth by defective lessons on
geography, which are devoid of a geological basis.
I need hardly draw attention to the fact that these geological changes
of the earth’s surface have ever been exceedingly important to the
migrations of organisms, and consequently to their Chorology. From them
we learn to understand how it is that the same or nearly related species
of animals and plants can occur on different islands, although they
could not have passed through the water separating them, and how other
species living in fresh water can inhabit different enclosed
water-basins, although they could not have crossed the land lying
between them. These islands were formerly mountain peaks of a connected
continent, and these lakes were once directly connected with one
another. The former were separated by geological depressions, the latter
by elevations. Now, if we further consider how often and how unequally
these alternating elevations and depressions occur on the different
parts of the earth, and how, in consequence of this, the boundaries of
the geographical tracts of distribution of species become changed, and
if we further consider in what exceedingly various ways the active and
passive migrations of organisms must have been influenced by them, then
we shall be in a position to completely understand the great variety of
the picture which is at present offered to us by the distribution of
animal and vegetable species.
There is yet another important circumstance to be mentioned here, which
is likewise of great importance for a complete explanation of this
varied geographical picture, and which throws light upon many very
obscure facts, which, without its help, we should not be able to
comprehend. I mean the gradual _change of climate_ which has taken place
during the long course of the organic history of the earth. As we saw in
our last chapter, at the beginning of organic life on the earth a much
higher and more equal temperature must have generally prevailed than at
present. The differences of zones, which in our time are so very
striking, did not exist at all in those times. It is probable that for
many millions of years but one climate prevailed over the whole earth,
which very closely resembled, or even surpassed, the hottest tropical
climate of the present day. The highest north which man has yet reached
was then covered with palms and other tropical plants, the fossil
remains of which are still found there. The temperature of this climate
at a later period gradually decreased; but still the poles remained so
warm that the whole surface of the earth could be inhabited by
organisms. It was only at a comparatively very recent period of the
earth’s history, namely, at the beginning of the tertiary period, that
there occurred, as it seems, the first perceptible cooling of the
earth’s crust at the poles, and through this the first differentiation
or separation of the different zones of temperature or climatic zones.
But the slow and gradual decrease of temperature continued to extend
more and more within the tertiary period, until at last, at both poles
of the earth, the first permanent ice caps were formed.
I need scarcely point out in detail how very much this change of climate
must have affected the geographical distribution of organisms, and the
origin of numerous new species. The animal and vegetable species, which,
down to the tertiary period, had found an agreeable tropical climate all
over the earth, even as far as the poles, were now forced either to
adapt themselves to the intruding cold, or to flee from it. Those
species which adapted and accustomed themselves to the decreasing
temperature became new species simply by this very acclimatization,
under the influence of natural selection. The other species, which fled
from the cold, had to emigrate and seek a milder climate in lower
latitudes. The tracts of distribution which had hitherto existed must by
this have been vastly changed.
However, during the last great period of the earth’s history, during the
quaternary period (or diluvial period) succeeding the tertiary one, the
decrease of the heat of the earth from the poles did not by any means
remain stationary. The temperature fell lower and lower, nay, even far
below the present degree. Northern and Central Asia, Europe, and North
America from the north pole, were covered to a great extent by a
connected sheet of ice, which in our part of the earth seems to have
reached the Alps. In a similar manner the cold also advancing from the
south pole covered a large portion of the southern hemisphere, which is
now free from it, with a rigid sheet of ice. Thus, between these vast
lifeless ice continents there remained only a narrow zone to which the
life of the organic world had to withdraw. This period, during which
man, or at least the human ape, already existed, and which forms the
first period of the so-called _diluvial epoch_, is now universally known
as the _ice_ or _glacial period_.
The ingenious Carl Schimper is the first naturalist who clearly
conceived the idea of the ice period, and proved the great extent of the
former glaciation of Central Europe by the help of the so-called
boulders, or erratic blocks of stone, as also by the “glacier tables.”
Louis Agassiz, stimulated by him, and considerably supported by the
independent investigations of the eminent geologist Charpentier,
afterwards undertook the task of carrying out the theory of the ice
period. In England, the geologist Forbes distinguished himself in this
matter, and also was the first to apply it to the theory of migrations
and the geographical distribution of species dependent upon migration.
Agassiz, however, afterwards injured the theory by his one-sided
exaggeration, inasmuch as, from his partiality to Cuvier’s theory of
cataclysms, he endeavoured to attribute the destruction of the whole
animate creation then existing, to the sudden coming on of the cold of
the ice period and the “revolution” connected with it.
It is unnecessary here to enter into detail as to the ice period itself,
and into investigations about its limits, and I may omit this all the
more reasonably since the whole of our recent geological literature is
full of it. It will be found discussed in detail in the works of
Cotta,(31) Lyell,(30) Vogt,(27) Zittel,(32) etc. Its great importance to
us here is that it helps us to explain the most difficult chorological
problems, as Darwin has correctly perceived.
For there can be no doubt that this glaciation of the present temperate
zones must have exercised an exceedingly important influence on the
geographical and topographical distribution of organisms, and that it
must have entirely changed it. While the cold slowly advanced from the
poles towards the equator, and covered land and sea with a connected
sheet of ice, it must of course have driven the whole living world
before it. Animals and plants had to migrate if they wished to escape
being frozen. But as at that time the temperate and tropical zones were
probably no less densely peopled with animals and plants than at
present, there must have arisen a fearful struggle for life between the
latter and the intruders coming from the poles. During this struggle,
which certainly lasted many thousands of years, many species must have
perished and many become modified and been transformed into new species.
The hitherto existing tracts of distribution of species must have become
completely changed, and the struggle have been continued, nay, indeed,
must have broken out anew and been carried on in new forms, when the ice
period had reached and gone beyond its furthest point, and when in the
post-glacial period the temperature again increased, and organisms began
to migrate back again towards the poles.
In any case this great change of climate, whether a greater or less
importance be ascribed to it, is one of those occurrences in the history
of the earth which have most powerfully influenced the distribution of
organic forms. But more especially one important and difficult
chorological circumstance is explained by it in the simplest manner,
namely, the specific agreement of many of our Alpine inhabitants with
some of those living in polar regions. There is a great number of
remarkable animal and vegetable forms which are common to these two far
distant parts of the earth, and which are found nowhere in the wide
plains lying between them. Their migration from the polar lands to the
Alpine heights, or _vice versa_, would be inconceivable under the
present climatic circumstances, or could be assumed at least only in a
few rare instances. But such a migration could take place, nay, was
obliged to take place, during the gradual advance and retreat of the
ice-sheet. As the glaciation encroached from Northern Europe towards our
Alpine chains, the polar inhabitants retreating before it—gentian,
saxifrage, polar foxes, and polar hares—must have peopled Germany, in
fact all Central Europe. When the temperature again increased, only a
portion of these Arctic inhabitants returned with the retreating ice to
the Arctic zones. Another portion of them climbed up the mountains of
the Alpine chain instead, and there found the cold climate suited to
them. The problem is thus solved in a most simple manner.
We have hitherto principally considered the _theory of the migrations_
of organisms in so far as it explains the radiation of every animal and
vegetable species from a single primæval home, from a “central point of
creation,” and the dispersion of these species over a greater or less
portion of the earth’s surface. But these migrations are also of great
importance to the theory of development, because we can perceive in them
a very important means for the _origin of new species_. When animals and
plants migrate they meet in their new home, in the same way as do human
emigrants, with conditions which are more or less different from those
which they have inherited throughout generations, and to which they have
been accustomed. The emigrants must either submit and adapt themselves
to these new conditions of life or they perish. By adaptation their
peculiar specific character becomes the more changed the greater the
difference between the new and the old home. The new climate, the new
food, but above all, new neighbours in the forms of other animals and
plants, influence and tend to modify the inherited character of the
immigrant species, and if it is not hardy enough to resist the
influences, then sooner or later a new species must arise out of it. In
most cases this transformation of an immigrant species takes place so
quickly under the influence of the altered struggle for life, that even
after a few generations a new species arises from it.
Migration has an especial influence in this way on all organisms with
separate sexes. For in them the origin of new species by natural
selection is always rendered difficult, or delayed, by the fact that the
modified descendants occasionally again mix sexually with the unchanged
original form, and thus by crossing return to the first form. But if
such varieties have migrated, if great distances or barriers to
migration—seas, mountains, etc.—have separated them from the old home,
then the danger of a mingling with the primary form is prevented, and
the isolation of the emigrant form, which becomes a new species by
adaptation, prevents its breeding with the old stock, and hence prevents
its return in this way to the original form.
The importance of migration for the isolation of newly-originating
species and the prevention of a speedy return to the primary form has
been especially pointed out by the philosophic traveller, Moritz Wagner,
of Munich. In a special treatise on “Darwin’s Theory and the Law of the
Migration of Organisms,”(40) Wagner gives from his own rich experience a
great number of striking examples which confirm the theory of migration
set forth by Darwin in the eleventh and twelfth chapters of his book,
where he especially discusses the effect of the complete isolation of
emigrant organisms in the origin of new species. Wagner sets forth the
simple causes which have “locally bounded the form and founded its
typical difference,” in the following three propositions:—1. The
greater the total amount of change in the hitherto existing conditions
of life which the emigrating individuals find on entering a new
territory, the more intensely must the innate variability of every
organism manifest itself. 2. The less this increased individual
variability of organisms is disturbed in the peaceful process of
reproduction by the mingling of numerous subsequent immigrants of the
same species, the more frequently will nature succeed, by
intensification and transmission of the new characteristics, in forming
a new variety or race, that is, a commencing species. 3. The more
advantageous the changes experienced by the individual organs are to the
variety, the more readily will it be able to adapt itself to the
surrounding conditions; and the longer the undisturbed breeding of a
commencing variety of colonists in a new territory continues without its
mingling with subsequent immigrants of the same species, the oftener a
new species will arise out of the variety.
Every one will agree with these three propositions of Moritz Wagner’s.
But we must consider his view, that the migration and the subsequent
isolation of the emigrant individuals is a _necessary_ condition for the
origin of new species, to be completely erroneous. Wagner says, “without
a long-enduring separation of colonists from their former species, the
formation of a new race cannot succeed—selection, in fact, cannot take
place. Unlimited crossing, unhindered sexual mingling of all individuals
of a species will always produce uniformity, and drive varieties, whose
characteristics have not been fixed throughout a series of generations,
back to the primary form.”
This sentence, in which Wagner himself comprises the main result of his
investigations, he would be able to defend only if all organisms were of
separate sexes, if every origin of new individuals were possible only by
the mingling of male and female individuals. But this is by no means the
case. Curiously enough, Wagner says nothing of the numerous
hermaphrodites which, possessing both the sexual organs, are capable of
self-fructification, and likewise nothing of the countless organisms
which are not sexually differentiated.
Now, from the earliest times of the organic history of the earth, there
have existed thousands of organic species (thousands of which still
exist) in which no difference of sex whatever exists, and, in fact, in
which no sexual propagation takes place, and which exclusively reproduce
themselves in a non-sexual manner by division, budding, formation of
spores, etc. All the great mass of Protista, the Monera, Amœbæ,
Myxomycetes, Rhizopoda, etc., in short, all the lower organisms which we
shall have to enumerate in the domain of Protista, standing midway
between the animal and vegetable kingdoms, propagate themselves
_exclusively in a non-sexual manner_. And this domain comprises a class
of organisms which is one of the richest in forms, nay, even in a
certain respect the richest of all in forms, as all possible geometrical
fundamental forms are represented in it. I allude to the wonderful class
of the Rhizopoda, or Ray-streamers, to which the lime-shelled Acyttaria
and the flint-shelled Radiolaria belong. (Compare chapter xvi.)
It is self-evident, therefore, that Wagner’s theory is quite
inapplicable to all these non-sexual organisms. Moreover, the same
applies to all those hermaphrodites in which every individual possesses
both male and female organs and is capable of self-fructification. This
is the case, for instance, in the Flat-worms, flukes, and tape-worms,
further in the important Sack-worms (Tunicates), the invertebrate
relatives of the vertebrate animals, and in very many other organisms of
different groups. Many of these species have arisen by natural
selection, without a “crossing” of the originating species with its
primary form having been possible.
As I have already shown in the eighth chapter, the origin of the two
sexes, and consequently sexual propagation in general, must be
considered as a process which began only in later periods of the organic
history of the earth, being the result of differentiation or _division
of labour_. The most ancient terrestrial organisms can have propagated
themselves only in the simplest non-sexual manner. Even now all
Protista, as well as all the countless forms of cells, which constitute
the body of higher organisms, multiply themselves only by non-sexual
generation. And yet there arise here “new species” by differentiation in
consequence of natural selection.
But even if we were to take into consideration the animal and vegetable
species with separate sexes, in this case too we should have to oppose
Wagner’s chief proposition, that “the _migration_ of organisms and their
formation of colonies is the _necessary condition of natural
selection_.” August Weismann, in his treatise on the “Influence of
Isolation upon the Formation of Species,”(24) has already sufficiently
refuted that proposition, and has shown that even in one and the same
district one bi-sexual species may divide itself into several species by
natural selection. In relation to this question, I must again call to
mind the great influence which _division of labour, or differentiation_,
possesses, being one of the necessary results of natural selection. All
the different kinds of cells constituting the body of the higher
organisms, the nerve cells, muscle cells, gland cells, etc., all these
“good species,” these “bonæ species” of elementary organisms, have
arisen solely by division of labour, in consequence of natural
selection, although they not only never were locally isolated, but ever
since their origin have always existed in the closest local relations
one with another. Now, the same reasoning that applies to these
elementary organisms, or “individuals of the first order,” applies also
to the many-celled organisms of a higher order which only at a later
date have arisen as “good species” from among their fellows.
We are therefore of the same opinion as Darwin and Wallace, that the
migration of organisms and their isolation in their new home is a very
advantageous condition for the origin of new species; but we cannot
admit, as Wagner asserts, that it is a _necessary_ condition, and that
without it no species can arise. Wagner sets up this opinion, “that
migration is a necessary condition for natural selection,” as a special
“_law of migration_”; but we consider it sufficiently refuted by the
above-mentioned facts. We have, moreover, already pointed out that in
reality the origin of new species by natural selection is a
_mathematical and logical necessity_ which, without anything else,
follows from the simple combination of three great facts. These three
fundamental facts are—the Struggle for Life, the Adaptability, and the
Hereditivity of organisms.
We cannot here enter into detail concerning the numerous interesting
phenomena furnished by the geographical and topographical distribution
of organic species, which are all wonderfully explained by the theory of
selection and migration. For these I refer to the writings of Darwin,(1)
Wallace,(36) and Moritz Wagner,(40) in which the important doctrine of
the _limits of distribution_—seas, rivers, and mountains—is
excellently discussed and illustrated by numerous examples. Only three
other phenomena must be mentioned here on account of their special
importance. First, the close relation of forms, that is, the striking
“family likeness” existing between the characteristic local forms of
every part of the globe, and their extinct fossil ancestors in the same
part of the globe; secondly, the no less striking “family likeness”
between the inhabitants of island groups and those of the neighbouring
continent from which the islands were peopled; lastly and thirdly, the
peculiar character presented in general by the flora and fauna of
islands taken as a whole.
All these chorological facts given by Darwin, Wallace, and
Wagner—especially the remarkable phenomena of the limited local fauna
and flora, the relations of insular to continental inhabitants, the wide
distribution of the so-called “cosmopolitan species,” the close
relationship of the local species of the present day with the extinct
species of the same limited territory, the demonstrable radiation of
every species from a single central point of creation—all these, and
all other phenomena furnished to us by the geographical and the
topographical distribution of organisms, are explained in a simple and
thorough manner by the theory of selection and migration, while without
it they are simply incomprehensible. Consequently, in the whole of this
series of phenomena we find a new and weighty proof of the truth of the
Theory of Descent.
[1] The world is perfect save where Man
Comes in with his strife.
[2] Archebiosis (Bastian), Abiogenesis (Huxley).
[3] Alle Glieder bilden sich aus nach ew’gen Gesetzen,
Und die seltenste Form bewahrt im Geheimniss das Urbild.
Also bestimmt die Gestalt die Lebensweise des Thieres.
Und die Weise zu leben, sie wirkt auf alle Gestalten
Mächtig zurück. So zeiget sich fest die geordnete Bildung,
Welche zum Wechsel sich neigt durch äusserlich wirkende Wesen.
[4] “Einstweilen bis den Bau der Welt
Philosophie zusammenhält,
Erhält sich ihr Getriebe
Durch Hunger und durch Liebe.”
[5] “Allgemeine Naturgeschichte und Theorie des Himmels.”
[6] We must wait for fuller information on the subject of
Bathybius, at the hands of the naturalists of the _Challenger_
expedition, before accepting it finally as a distinct
organism.—_Editor._
END OF VOL. I.
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