Alfred Russel Wallace, 'On the Law which has Regulated The Introduction of New Species', Volume 16 (2nd Series), Annals and Magazine of Natural History, September 1855.
Text prepared by David Clifford, Ph.D., Cambridge University.
Geographical 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
1. 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.
2. 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.
3. 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.
4. 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
5. The distribution of the organic world in time is very similar to its present
distribution in space.
6. Most of the larger and some small groups extend through several geological
periods.
7. In each period, however, there are peculiar groups, found nowhere else, and
extending through one or several formations.
8. Species of one genus, or genera of one family occurring in the same geological
time, are more closely allied than those separated in time.
9. 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.
10. 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 there occur. 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.
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
Last modified 2001