On the Law which has Regulated the Introduction of New Species (1855)
by Alfred Russel Wallace
eographical Distribution Dependent on Geologic Changes.
Every naturalist who has directed his attention to the
subject of the geographical distribution of animals and plants, must have been
interested in the singular facts which it presents. Many of these facts are
quite different from what would have been anticipated, and have hitherto
been considered as highly curious, but quite inexplicable. None of the
explanations attempted from the time of Linnaeus are now considered at all
satisfactory; none of them have given a cause sufficient to account for the
facts known at the time, or comprehensive enough to include all the new
facts which have since been, and are daily being added. Of late years,
however, a great light has been thrown upon the subject by geological
investigations, which have shown that the present state of the earth and of
the organisms now inhabiting it, is but the last stage of a long and
uninterrupted series of changes which it has undergone, and consequently,
that to endeavour to explain and account for its present condition without
any reference to those changes (as has frequently been done) must lead to
very imperfect and erroneous conclusions.
The facts proved by geology are briefly these:—That
during an immense, but unknown period, the surface of the earth has
undergone successive changes; land has sunk beneath the ocean, while fresh
land has risen up from it; mountain chains have been elevated; islands have
been formed into continents, and continents submerged till they have become
islands; and these changes have taken place, not once merely, but perhaps
hundreds, perhaps thousands of times:—That all these operations have
been more or less continuous, but unequal in their progress, and during the
whole series the organic life of the earth has undergone a corresponding
alteration. This alteration also has been gradual, but complete; after a certain
interval not a single species existing which had lived at the commencement of
the period. This complete renewal of the forms of life also appears to have
occurred several times:—That from the last of the geological epochs to
the present or historical epoch, the change of organic life has been gradual:
the first appearance of animals now existing can in many cases be traced,
their numbers gradually increasing in the more recent formations, while other
species continually die out and disappear, so that the present condition of the
organic world is clearly derived by a natural process of gradual extinction and
creation of species from that of the latest geological periods. We may
therefore safely infer a like gradation and natural sequence from one
geological epoch to another.
Now, taking this as a fair statement of the results of
geological inquiry, we see that the present geographical distribution of life
upon the earth must be the result of all the previous changes, both of the
surface of the earth itself and of its inhabitants. Many causes, no doubt, have
operated of which we must ever remain in ignorance, and we may, therefore,
expect to find many details very difficult of explanation, and in attempting to
give one, must allow ourselves to call into our service geological changes
which it is highly probable may have occurred, though we have no direct
evidence of their individual operation.
The great increase of our knowledge within the last twenty
years, both of the present and past history of the organic world, has
accumulated a body of facts which should afford a sufficient foundation for a
comprehensive law embracing and explaining them all, and giving a direction to
new researches. It is about ten years since the idea of such a law suggested
itself to the writer of this essay, and he has since taken every opportunity of
testing it by all the newly-ascertained facts with which he has become
acquainted, or has been able to observe himself. These have all served to
convince him of the correctness of his hypothesis. Fully to enter into such a
subject would occupy much space, and it is only in consequence of some
views having been lately promulgated, he believes, in a wrong direction, that
he now ventures to present his ideas to the public, with only such obvious
illustrations of the arguments and results as occur to him in a place far
removed from all means of reference and exact information.
A Law deduced from well-known Geographical and
Geological Facts.
The following propositions in Organic Geography and
Geology give the main facts on which the hypothesis is founded.
GEOGRAPHY
- Large groups, such as classes and orders, are generally
spread over the whole earth, while smaller ones, such as families and genera,
are frequently confined to one portion, often to a very limited district.
- In widely distributed families the genera are often limited
in range; in widely distributed genera, well marked groups of species are
peculiar to each geographical district.
- When a group is confined to one district, and is rich in
species, it is almost invariably the case that the most closely allied species
are found in the same locality or in closely adjoining localities, and that
therefore the natural sequence of the species by affinity is also
geographical.
- In countries of a similar climate, but separated by a wide
sea or lofty mountains, the families, genera and species of the one are often
represented by closely allied families, genera and species peculiar to the
other.
GEOLOGY
- The distribution of the organic world in time is very similar
to its present distribution in space.
- Most of the larger and some small groups extend through
several geological periods.
- In each period, however, there are peculiar groups, found
nowhere else, and extending through one or several formations.
- Species of one genus, or genera of one family occurring in
the same geological time, are more closely allied than those separated in time.
- As generally in geography no species or genus occurs in
two very distant localities without being also found in intermediate places, so
in geology the life of a species or genus has not been interrupted. In other
words, no group or species has come into existence twice.
- The following law may be deduced from these
facts:—Every species has come into existence coincident both in space
and time with a pre-existing closely allied species.
This law agrees with, explains and illustrates all the facts
connected with the following branches of the subject:—1st. The system
of natural affinities. 2nd. The distribution of animals and plants in space. 3rd.
The same in time, including all the phaenomena of representative groups, and
those which Professor Forbes supposed to manifest polarity. 4th. The
phaenomena of rudimentary organs. We will briefly endeavour to show its
bearing upon each of these.
The Form of a true system of Classification determined by
this Law.
If the law above enunciated be true, it follows that the
natural series of affinities will also represent the order in which the several
species came into existence, each one having had for its immediate antitype a
closely allied species existing at the time of its origin. It is evidently possible
that two or three distinct species may have had a common antitype, and that
each of these may again have become the antitypes from which other closely
allied species were created. The effect of this would be, that so long as each
species has had but one new species formed on its model, the line of affinities
will be simple, and may be represented by placing the several species in direct
succession in a straight line. But if two or more species have been
independently formed on the plan of a common antitype, then the series of
affinities will be compound, and can only be represented by a forked or many
branched line. Now, all attempts at a Natural classification and arrangement of
organic beings show, that both these plans have obtained in creation.
Sometimes the series of affinities can be well represented for a space by a
direct progression from species to species or from group to group, but it is
generally found impossible so to continue. There constantly occur two or more
modifications of an organ or modifications of two distinct organs, leading us on
to two distinct series of species, which at length differ so much from each
other as to form distinct genera or families.
These are the parallel series or representative groups of
naturalists, and they often occur in different countries, or are found fossil in
different formations. They are said to have an analogy to each other when
they are so far removed from their common antitype as to differ in many
important points of structure, while they still preserve a family resemblance.
We thus see how difficult it is to determine in every case whether a given
relation is an analogy or an affinity, for it is evident that as we go back along
the parallel or divergent series, towards the common antitype, the analogy
which existed between the two groups becomes an affinity. We are also made
aware of the difficulty of arriving at a true classification, even in a small and
perfect group;—in the actual state of nature it is almost impossible, the
species being so numerous and the modifications of form and structure so
varied, arising probably from the immense number of species which have
served as antitype for the existing species, and thus produced a complicated
branching of the lines of affinity, as intricate as the twigs of a gnarled oak or
the vascular system of the human body. Again, if we consider that we have
only fragments of this vast system, the stem and main branches being
represented by extinct species of which we have no knowledge, while a vast
mass of limbs and boughs and minute twigs and scattered leaves is what we
have to place in order, and determine the true position each originally
occupied with regard to the others, the whole difficulty of the true Natural
System of classification becomes apparent to us.
We shall thus find ourselves obliged to reject all those
systems of classification which arrange species or groups in circles, as well as
those which fix a definite number for the divisions of each group. The latter
class have been very generally rejected by naturalists, as contrary to nature,
notwithstanding the ability with which they have been advocated; but the
circular system of affinities seems to have obtained a deeper hold, many
eminent naturalists having to some extent adopted it. We have, however,
never been able to find a case in which the circle has been closed by a direct
and close affinity. In most cases a palpable analogy has been substituted, in
others the affinity is very obscure or altogether doubtful. The complicated
branching of the lines of affinities in extensive groups must also afford great
facilities for giving a show of probability to any such purely artificial
arrangements. Their death-blow was given by the admirable paper of the
lamented Mr. Strickland, published in the "Annals of Natural History," in which
he so cleverly showed the true synthetical method of discovering the Natural
System.
Geographical Distribution of Organisms.
If we now consider the geographical distribution of animals
and plants upon the earth, we shall find all the facts beautifully in accordance
with, and readily explained by, the present hypothesis. A country having
species, genera, and whole families peculiar to it, will be the necessary result
of its having been isolated for a long period, sufficient for many series of
species to have been created on the type of pre-existing ones, which, as well
as many of the earlier-formed species, have become extinct, and thus made
the groups appear isolated. If in any case the antitype had an extensive range,
two or more groups of species might have been formed, each varying from it in
a different manner, and thus producing several representative or analogous
groups. The Sylviadae of Europe and the Sylvicolidae of North America, the
Heliconidae of South America and the Euploeas of the East, the group of
Trogons inhabiting Asia, and that peculiar to South America, are examples that
may be accounted for in this manner.
Such phaenomena as are exhibited by the Galapagos
Islands, which contain little groups of plants and animals peculiar to
themselves, but most nearly allied to those of South America, have not
hitherto received any, even a conjectural explanation. The Galapagos are a
volcanic group of high antiquity, and have probably never been more closely
connected with the continent than they are at present. They must have been
first peopled, like other newly-formed islands, by the action of winds and
currents, and at a period sufficiently remote to have had the original species
die out, and the modified prototypes only remain. In the same way we can
account for the separate islands having each their peculiar species, either on
the supposition that the same original emigration peopled the whole of the
islands with the same species from which differently modified prototypes were
created, or that the islands were successively peopled from each other, but
that new species have been created in each on the plan of the pre-existing
ones. St. Helena is a similar case of a very ancient island having obtained an
entirely peculiar, though limited, flora. On the other hand, no example is known
of an island which can be proved geologically to be of very recent origin (late
in the Tertiary, for instance), and yet possess generic or family groups, or
even many species peculiar to itself.
When a range of mountains has attained a great elevation,
and has so remained during a long geological period, the species of the two
sides at and near their bases will be often very different, representative
species of some genera occurring, and even whole genera being peculiar to
one side, as is remarkably seen in the case of the Andes and Rocky Mountains.
A similar phaenomena occurs when an island has been separated from a
continent at a very early period. The shallow sea between the Peninsula of
Malacca, Java, Sumatra and Borneo was probably a continent or large island
at an early epoch, and may have become submerged as the volcanic ranges
of Java and Sumatra were elevated. The organic results we see in the very
considerable number of species of animals common to some or all of these
countries, while at the same time a number of closely allied representative
species exist peculiar to each, showing that a considerable period has elapsed
since their separation. The facts of geographical distribution and of geology
may thus mutually explain each other in doubtful cases, should the principles
here advocated be clearly established.
In all those cases in which an island has been separated
from a continent, or raised by volcanic or coralline action from the sea, or in
which a mountain-chain has been elevated in a recent geological epoch, the
phaenomena of peculiar groups or even of single representative species will
not exist. Our own island is an example of this, its separation from the
continent being geologically very recent, and we have consequently scarcely
a species which is peculiar to it; while the Alpine range, one of the most
recent mountain elevations, separates faunas and floras which scarcely differ
more than may be due to climate and latitude alone.
The series of facts alluded to in Proposition (3), of closely
allied species in rich groups being found geographically near each other, is
most striking and important. Mr. Lovell Reeve has well exemplified it in his able
and interesting paper on the Distribution of the Bulimi. It is also seen in the
Hummingbirds and Toucans, little groups of two or three closely allied species
being often found in the same or closely adjoining districts, as we have had
the good fortune of personally verifying. Fishes give evidence of a similar kind:
each great river has its peculiar genera, and in more extensive genera its
groups of closely allied species. But it is the same throughout Nature; every
class and order of animals will contribute similar facts. Hitherto no attempt has
been made to explain these singular phaenomena, or to show how they have
arisen. Why are the genera of Palms and of Orchids in almost every case
confined to one hemisphere? Why are the closely allied species of
brownbacked Trogons all found in the East, and the green-backed in the
West? Why are the Macaws and the Cockatoos similarly restricted? Insects
furnish a countless number of analogous examples;—the Goliathi of Africa,
the Ornithopterae of the Indian Islands, the Heliconidae of South America, the
Danaidae of the East, and in all, the most closely allied species found in
geographical proximity. The question forces itself upon every thinking mind,
why are these things so? They could not be as they are had no law regulated
their creation and dispersion. The law here enunciated not merely explains, but
necessitates the facts we see to exist, while the vast and long-continued
geological changes of the earth readily account for the exceptions and
apparent discrepancies that here and thereoccur. The writer's object in
putting forward his views in the present imperfect manner is to submit them to
the test of other minds, and to be made aware of all the facts supposed to be
inconsistent with them. As his hypothesis is one which claims acceptance
solely as explaining and connecting facts which exist in nature, he expects
facts alone to be brought to disprove it, not a priori arguments against its
probability.
Geological Distribution of the Forms of Life.
The phaenomena of geological distribution are exactly
analogous to those of geography. Closely allied species are found associated
in the same beds, and the change from species to species appears to have
been as gradual in time as in space. Geology, however, furnishes us with
positive proof of the extinction and production of species, though it does not
inform us how either has taken place. The extinction of species, however,
offers but little difficulty, and the modus operandi has been well illustrated by
Sir C. Lyell in his admirable
"Principles." Geological changes, however
gradual, must occasionally have modified external conditions to such an extent
as to have rendered the existence of certain species impossible. The
extinction would in most cases be effected by a gradual dying-out, but in
some instances there might have been a sudden destruction of a species of
limited range. To discover how the extinct species have from time to time
been replaced by new ones down to the very latest geological period, is the
most difficult, and at the same time the most interesting problem in the
natural history of the earth. The present inquiry, which seeks to eliminate from
known facts a law which has determined, to a certain degree, what species
could and did appear at a given epoch, may, it is hoped, be considered as one
step in the right direction towards a complete solution of it.
High Organization of very ancient Animals consistent with
this Law.
Much discussion has of late years taken place on the
question, whether the succession of life upon the globe has been from a lower
to a higher degree of organization. The admitted facts seem to show that
there has been a general, but not a detailed progression. Mollusca and
Radiata existed before Vertebrata, and the progression from Fishes to Reptiles
and Mammalia, and also from the lower mammals to the higher, is indisputable.
On the other hand, it is said that the Mollusca and Radiata of the very earliest
periods were more highly organized than the great mass of those now existing,
and that the very first fishes that have been discovered are by no means the
lowest organised of the class. Now it is believed the present hypothesis will
harmonize with all these facts, and in a great measure serve to explain them;
for though it may appear to some readers essentially a theory of progression,
it is in reality only one of gradual change. It is, however, by no means difficult
to show that a real progression in the scale of organization is perfectly
consistent with all the appearances, and even with apparent retrogression,
should such occur.
Returning to the analogy of a branching tree, as the best
mode of representing the natural arrangement of species and their successive
creation, let us suppose that at an early geological epoch any group (say a
class of the Mollusca) has attained to a great richness of species and a high
organization. Now let this great branch of allied species, by geological
mutations, be completely or partially destroyed. Subsequently a new branch
springs from the same trunk, that is to say, new species are successively
created, having for their antitypes the same lower organized species which
had served as the antitypes for the former group, but which have survived the
modified conditions which destroyed it. This new group being subject to these
altered conditions, has modifications of structure and organization given to it,
and becomes the representative group of the former one in another geological
formation. It may, however, happen, that though later in time, the new series
of species may never attain to so high a degree of organization as those
preceding it, but in its turn become extinct, and give place to yet another
modification from the same root, which may be of higher or lower organization,
more or less numerous in species, and more or less varied in form and
structure than either of those which preceded it. Again, each of these groups
may not have become totally extinct, but may have left a few species, the
modified prototypes of which have existed in each succeeding period, a faint
memorial of their former grandeur and luxuriance. Thus every case of
apparent retrogression may be in reality a progress, though an interrupted one:
when some monarch of the forest loses a limb, it may be replaced by a feeble
and sickly substitute. The foregoing remarks appear to apply to the case of
the Mollusca, which, at a very early period, had reached a high organization
and a great development of forms and species in the testaceous Cephalopoda.
In each succeeding age modified species and genera replaced the former ones
which had become extinct, and as we approach the present aera, but few and
small representatives of the group remain, while the Gasteropods and Bivalves
have acquired an immense preponderance. In the long series of changes the
earth has undergone, the process of peopling it with organic beings has been
continually going on, and whenever any of the higher groups have become
nearly or quite extinct, the lower forms which have better resisted the
modified physical conditions have served as the antitypes on which to found
the new races. In this manner alone, it is believed, can the representative
groups at successive periods, and the rising and fallings in the scale of
organization, be in every case explained.
Objections to Forbes' Theory of Polarity.
The hypothesis of polarity, recently put forward by
Professor Edward Forbes to account for the abundance of generic forms at a
very early period and at present, while in the intermediate epochs there is a
gradual diminution and impoverishment, till the minimum occurred at the
confines of the Palaeozoic and Secondary epochs, appears to us quite
unnecessary, as the facts may be readily accounted for on the principles
already laid down. Between the Palaeozoic and Neozoic periods of Professor
Forbes, there is scarcely a species in common, and the greater part of the
genera and families also disappear to be replaced by new ones. It is almost
universally admitted that such a change in the organic world must have
occupied a vast period of time. Of this interval we have no record; probably
because the whole area of the early formations now exposed to our
researches was elevated at the end of the Palaeozoic period, and remained so
through the interval required for the organic changes which resulted in the
fauna and flora of the Secondary period. The records of this interval are
buried beneath the ocean which covers three-fourths of the globe. Now it
appears highly probable that a long period of quiescence or stability in the
physical conditions of a district would be most favourable to the existence of
organic life in the greatest abundance, both as regards individuals and also as
to variety of species and generic group, just as we now find that the places
best adapted to the rapid growth and increase of individuals also contain the
greatest profusion of species and the greatest variety of forms,—the
tropics in comparison with the temperate and arctic regions. On the other
hand, it seems no less probable that a change in the physical conditions of a
district, even small in amount if rapid, or even gradual if to a great amount,
would be highly unfavourable to the existence of individuals, might cause the
extinction of many species, and would probably be equally unfavourable to the
creation of new ones. In this too we may find an analogy with the present
state of our earth, for it has been shown to be the violent extremes and rapid
changes of physical conditions, rather than the actual mean state in the
temperate and frigid zones, which renders them less prolific than the tropical
regions, as exemplified by the great distance beyond the tropics to which
tropical forms penetrate when the climate is equable, and also by the
richness in species and forms of tropical mountain regions which principally
differ from the temperate zone in the uniformity of their climate. However this
may be, it seems a fair assumption that during a period of geological repose
the new species which we know to have been created would have appeared,
that the creations would then exceed in number the extinctions, and
therefore the number of species would increase. In a period of geological
activity, on the other hand, it seems probable that the extinctions might
exceed the creations, and the number of species consequently diminish. That
such effects did take place in connexion with the causes to which we have
imputed them, is shown in the case of the Coal formation, the faults and
contortions of which show a period of great activity and violent convulsions,
and it is in the formation immediately succeeding this that the poverty of
forms of life is most apparent. We have then only to suppose a long period of
somewhat similar action during the vast unknown interval at the termination
of the Palaeozoic period, and then a decreasing violence or rapidity through
the Secondary period, to allow for the gradual repopulation of the earth with
varied forms, and the whole of the facts are explained. We thus have a clue
to the increase of the forms of life during certain periods, and their decrease
during others, without recourse to any causes but these we know to have
existed, and to effects fairly deducible from them. The precise manner in
which the geological changes of the early formations were effected is so
extremely obscure, that when we can explain important facts by a
retardation at one time and an acceleration at another of a process which we
know from its nature and from observation to have been unequal,—a
cause so simple may surely be preferred to one so obscure and hypothetical
as polarity.
I would also venture to suggest some reasons against the
very nature of the theory of Professor Forbes. Our knowledge of the organic
world during any geological epoch is necessarily very imperfect. Looking at
the vast numbers of species and groups that have been discovered by
geologists, this may be doubted; but we should compare their numbers not
merely with those that now exist upon the earth, but with a far larger amount.
We have no reason for believing that the number of species on the earth at
any former period was much less than at present; at all events the aquatic
portion, with which geologists have most acquaintance, was probably often
as great or greater. Now we know that there have been many complete
changes of species; new sets of organisms have many times been introduced
in place of old ones which have become extinct, so that the total amount
which have existed on the earth from the earliest geological period must have
borne about the same proportion to those now living, as the whole human
race who have lived and died upon the earth, to the population at the
present time. Again, at each epoch, the whole earth was no doubt, as now,
more or less the theatre of life, and as the successive generations of each
species died, their exuviae and preservable parts would be deposited over
every portion of the then existing seas and oceans, which we have reason for
supposing to have been more, rather than less, extensive than at present. In
order then to understand our possible knowledge of the early world and its
inhabitants, we must compare, not the area of the whole field of our
geological researches with the earth's surface, but the area of the examined
portion of each formation separately with the whole earth. For example,
during the Silurian period all the earth was Silurian, and animals were living
and dying, and depositing their remains more or less over the whole area of
the globe, and they were probably (the species at least) nearly as varied in
different latitudes and longitudes as at present. What proportion do the
Silurian districts bear to the whole surface of the globe, land and sea (for far
more extensive Silurian districts probably exist beneath the ocean than above
it), and what portion of the known Silurian districts has been actually
examined for fossils? Would the area of rock actually laid open to the eye be
the thousandth or the ten-thousandth part of the earth's surface? Ask the
same question with regard to the Oolite or the Chalk, or even to particular
beds of these when they differ considerably in their fossils, and you may then
get some notion of how small a portion of the whole we know.
But yet more important is the probability, nay almost the
certainty, that whole formations containing the records of vast geological
periods are entirely buried beneath the ocean, and for ever beyond our reach.
Most of the gaps in the geological series may thus be filled up, and vast
numbers of unknown and unimaginable animals, which might help to elucidate
the affinities of the numerous isolated groups which are a perpetual puzzle to
the zoologist, may there be buried, until future revolutions may raise them in
their turn above the waters, to afford materials for the study of whatever
race of intelligent beings may then have succeeded us. These considerations
must lead us to the conclusion, that our knowledge of the whole series of the
former inhabitants of the earth is necessarily most imperfect and
fragmentary,—as much so as our knowledge of the present organic
world would be, were we forced to make our collections and observations only
in spots equally limited in area and in number with those actually laid open for
the collection of fossils. Now, the hypothesis of Professor Forbes is essentially
one that assumes to a great extent the completeness of our knowledge of the
whole series of organic beings which have existed on the earth. This
appears to be a fatal objection to it, independently of all other considerations.
It may be said that the same objections exist against every theory on such a
subject, but this is not necessarily the case. The hypothesis put forward in
this paper depends in no degree upon the completeness of our knowledge of
the former condition of the organic world, but takes what facts we have as
fragments of a vast whole, and deduces from them something of the nature
and proportions of that whole which we can never know in detail. It is
founded upon isolated groups of facts, recognizes their isolation, and
endeavours to deduce from them the nature of the intervening portions.
Rudimentary Organs
Another important series of facts, quite in accordance with,
and even necessary deductions from, the law now developed, are those of
rudimentary organs. That these really do exist, and in most cases have no
special function in the animal economy, is admitted by the first authorities in
comparative anatomy. The minute limbs hidden beneath the skin in many of
the snake-like lizards, the anal hooks of the boa constrictor, the complete
series of jointed finger-bones in the paddle of the Manatus and whale, are a
few of the most familiar instances. In botany a similar class of facts has long
been recognised. Abortive stamens, rudimentary floral envelopes and
undeveloped carpels, are of the most frequent occurrence. To every
thoughtful naturalist the question must arise, What are these for? What have
they to do with the great laws of creation? Do they not teach us something
of the system of Nature? If each species has been created independently,
and without any necessary relations with pre-existing species, what do these
rudiments, these apparent imperfections mean? There must be a cause for
them; they must be the necessary results of some great natural law. Now, if,
as it has been endeavoured to be shown, the great law which has regulated
the peopling of the earth with animal and vegetable life is, that every change
shall be gradual; that no new creature shall be formed widely differing from
anything before existing; that in this, as in everything else in Nature, there
shall be gradation and harmony,—then these rudimentary organs are
necessary, and are an essential part of the system of Nature. Ere the higher
Vertebrata were formed, for instance, many steps were required, and many
organs had to undergo modifications from the rudimental condition in which
only they had as yet existed. We still see remaining an antitypal sketch of a
wing adapted for flight in the scaly flapper of the penguin, and limbs first
concealed beneath the skin, and then weakly protruding from it, were the
necessary gradations before others should be formed fully adapted for
locomotion. Many more of these modifications should we behold, and more
complete series of them, had we a view of all the forms which have ceased
to live. The great gaps that exist between fishes, reptiles, birds, and
mammals would then, no doubt, be softened down by intermediate groups,
and the whole organic world would be seen to be an unbroken and
harmonious system.
Conclusion
It has now been shown, though most briefly and imperfectly,
how the law that "Every species has come into existence coincident both
in time and space with a pre-existing closely allied species,"
connects together and renders intelligible a vast number of independent and
hitherto unexplained facts. The natural system of arrangement of organic
beings, their geographical distribution, their geological sequence, the
phaenomena of representative and substituted groups in all their
modifications, and the most singular peculiarities of anatomical structure, are
all explained and illustrated by it, in perfect accordance with the vast mass
of facts which the researches of modern naturalists have brought together,
and, it is believed, not materially opposed to any of them. It also claims a
superiority over previous hypotheses, on the ground that it not merely
explains, but necessitates what exists. Granted the law, and many of the
most important facts in Nature could not have been otherwise, but are almost
as necessary deductions from it, as are the elliptic orbits of the planets from
the law of gravitation.
Sarawak, Borneo, Feb 1855
[ Alfred Russel Wallace, "On the Law which has
Regulated the Introduction of New Species," Annals and
Magazine of Natural History 16 (September): 184-196. ]
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