Introduction of New Species E-book

                                 1855

    ON THE LAW WHICH HAS REGULATED THE INTRODUCTION OF NEW SPECIES

                       by Alfred Russel Wallace

                            February, 1855

                (also known as the Sarawack Law. ed.)




Electronically Enhanced Text (c) Copyright 1996, World Library(R)



       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 phaenomenon 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 brown-backed 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 oeconomy, 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.
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  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.
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                               THE END