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_CONTRIBUTIONS TO_ THE THEORY OF NATURAL SELECTION.
A Series of Essays.
BY
ALFRED RUSSEL WALLACE,
AUTHOR OF
"THE MALAY ARCHIPELAGO," ETC., ETC.
_SECOND EDITION, WITH CORRECTIONS AND ADDITIONS._
New York:
MACMILLAN AND CO.
1871.
[_The Right of Translation and Reproduction is reserved._]
LONDON:
PRINTED BY HEAD, HOLE & CO., FARRINGDON STREET,
AND IVY LANE, E.C.
PREFACE.
The present volume consists of essays which I have contributed to
various periodicals, or read before scientific societies during the last
fifteen years, with others now printed for the first time. The two first
of the series are printed without alteration, because, having gained me
the reputation of being an independent originator of the theory of
"natural selection," they may be considered to have some historical
value. I have added to them one or two very short explanatory notes, and
have given headings to subjects, to make them uniform with the rest of
the book. The other essays have been carefully corrected, often
considerably enlarged, and in some cases almost rewritten, so as to
express more fully and more clearly the views which I hold at the
present time; and as most of them originally appeared in publications
which have a very limited circulation, I believe that the larger portion
of this volume will be new to many of my friends and to most of my
readers.
I now wish to say a few words on the reasons which have led me to
publish this work. The second essay, especially when taken in connection
with the first, contains an outline sketch of the theory of the
origin of species (by means of what was afterwards termed by Mr.
Darwin--"natural selection,") as conceived by me before I had the least
notion of the scope and nature of Mr. Darwin's labours. They were
published in a way not likely to attract the attention of any but
working naturalists, and I feel sure that many who have heard of them,
have never had the opportunity of ascertaining how much or how little
they really contain. It therefore happens, that, while some writers give
me more credit than I deserve, others may very naturally class me with
Dr. Wells and Mr. Patrick Matthew, who, as Mr. Darwin has shown in the
historical sketch given in the 4th and 5th Editions of the "Origin of
Species," certainly propounded the fundamental principle of "natural
selection" before himself, but who made no further use of that
principle, and failed to see its wide and immensely important
applications.
The present work will, I venture to think, prove, that I both saw at the
time the value and scope of the law which I had discovered, and have
since been able to apply it to some purpose in a few original lines of
investigation. But here my claims cease. I have felt all my life, and I
still feel, the most sincere satisfaction that Mr. Darwin had been at
work long before me, and that it was not left for me to attempt to write
"The Origin of Species." I have long since measured my own strength, and
know well that it would be quite unequal to that task. Far abler men
than myself may confess, that they have not that untiring patience in
accumulating, and that wonderful skill in using, large masses of facts
of the most varied kind,--that wide and accurate physiological
knowledge,--that acuteness in devising and skill in carrying out
experiments,--and that admirable style of composition, at once clear,
persuasive and judicial,--qualities, which in their harmonious
combination mark out Mr. Darwin as the man, perhaps of all men now
living, best fitted for the great work he has undertaken and
accomplished.
My own more limited powers have, it is true, enabled me now and then to
seize on some conspicuous group of unappropriated facts, and to search
out some generalization which might bring them under the reign of known
law; but they are not suited to that more scientific and more laborious
process of elaborate induction, which in Mr. Darwin's hands has led to
such brilliant results.
Another reason which has led me to publish this volume at the present
time is, that there are some important points on which I differ from Mr.
Darwin, and I wish to put my opinions on record in an easily accessible
form, before the publication of his new work, (already announced,) in
which I believe most of these disputed questions will be fully
discussed.
I will now give the date and mode of publication of each of the essays
in this volume, as well as the amount of alteration they have undergone.
I.--ON THE LAW WHICH HAS REGULATED THE INTRODUCTION OF NEW SPECIES.
First published in the "Annals and Magazine of Natural History,"
September, 1855. Reprinted without alteration of the text.
II.--ON THE TENDENCY OF VARIETIES TO DEPART INDEFINITELY FROM THE
ORIGINAL TYPE.
First published in the "Journal of the Proceedings of the Linnaean
Society," August, 1858. Reprinted without alteration of the text, except
one or two grammatical emendations.
III.--MIMICRY AND OTHER PROTECTIVE RESEMBLANCES AMONG ANIMALS.
First published in the "Westminster Review," July, 1867. Reprinted with
a few corrections and some important additions, among which I may
especially mention Mr. Jenner Weir's observations and experiments on the
colours of the caterpillars eaten or rejected by birds.
IV.--THE MALAYAN PAPILIONIDAE, OR SWALLOW-TAILED BUTTERFLIES, AS
ILLUSTRATIVE OF THE THEORY OF NATURAL SELECTION.
First published in the "Transactions of the Linnaean Society," Vol. XXV.
(read March, 1864), under the title, "On the Phenomena of Variation and
Geographical Distribution, as illustrated by the Papilionidae of the
Malayan Region."
The introductory part of this essay is now reprinted, omitting tables,
references to plates, &c., with some additions, and several corrections.
Owing to the publication of Dr. Felder's "Voyage of the Novara"
(Lepidoptera) in the interval between the reading of my paper and its
publication, several of my new species must have their names changed for
those given to them by Dr. Felder, and this will explain the want of
agreement in some cases between the names used in this volume and those
of the original paper.
V.--ON INSTINCT IN MAN AND ANIMALS.
Not previously published.
VI.--THE PHILOSOPHY OF BIRDS' NESTS.
First published in the "Intellectual Observer," July, 1867. Reprinted
with considerable emendations and additions.
VII.--A THEORY OF BIRDS' NESTS; SHOWING THE RELATION OF CERTAIN
DIFFERENCES OF COLOUR IN BIRDS TO THEIR MODE OF NIDIFICATION.
First published in the "Journal of Travel and Natural History" (No. 2),
1868. Now reprinted with considerable emendations and additions, by
which I have endeavoured more clearly to express, and more fully to
illustrate, my meaning in those parts which have been misunderstood by
my critics.
VIII.--CREATION BY LAW.
First published in the "Quarterly Journal of Science," October, 1867.
Now reprinted with a few alterations and additions.
IX.--THE DEVELOPMENT OF HUMAN RACES UNDER THE LAW OF NATURAL SELECTION.
First published in the "Anthropological Review," May, 1864. Now
reprinted with a few important alterations and additions. I had intended
to have considerably extended this essay, but on attempting it I found
that I should probably weaken the effect without adding much to the
argument. I have therefore preferred to leave it as it was first
written, with the exception of a few ill-considered passages which never
fully expressed my meaning. As it now stands, I believe it contains the
enunciation of an important truth.
X.--THE LIMITS OF NATURAL SELECTION AS APPLIED TO MAN.
This is the further development of a few sentences at the end of an
article on "Geological Time and the Origin of Species," which appeared
in the "Quarterly Review," for April, 1869. I have here ventured to
touch on a class of problems which are usually considered to be beyond
the boundaries of science, but which, I believe, will one day be brought
within her domain.
* * * * *
For the convenience of those who are acquainted with any of my essays in
their original form, I subjoin references to the more important
additions and alterations now made to them.
_ADDITIONS AND CORRECTIONS TO THE ESSAYS AS ORIGINALLY PUBLISHED_.
Essays I. and II. are unaltered, but short notes are added at pp. 19,
24, 29, and 40.
III.--_Mimicry, and other Protective Resemblances among Animals._
PAGE
53 Additional illustration of protective colouring in the case of
the wood-dove and the robin.
63 On moths resembling bird's dung and mortar.
86 Correction of some names of African Papilios and a reference to
Mr. Trimen's observations.
89 Mr. Jenner Weir's observation on birds which refused to eat
_Spilosoma menthrasti_.
102 An additional case of snake mimicry in _Oxyrhopus trigeminus_.
107 Mr. Salvin's case of mimicry among hawks.
113 Name, _Diadema anomala_, added.
117 to 122. Use of gay colours in caterpillars, with an account of
Mr. Jenner Weir's and Mr. Butler's observations.
IV.--_The Malayan Papilionidae or Swallow-tailed Butterflies, as
illustrative of the Theory of Natural Selection._
135 to 140. Additions to the discussion on the rank of the
Papilionidae, and on the principles which determine the
comparative rank of groups in the animal kingdom.
164 Illustration of variability from Mr. Baker's revision of the
British Roses.
173 Additional facts, on local variations of colour.
196 Additional genus of birds (Ceycopsis) peculiar to Celebes.
199, 200. Concluding remarks.
VI.--_The Philosophy of Birds' Nests._
218 On nesting of Terns and Gulls, rewritten.
220 to 222. Daines Barrington, and others, on the song of birds.
223 On young birds learning to build, by memory and imitation.
224 Levaillant, on mode of nest-building.
229 On imperfect adaptation in birds' nests.
VII.--_A Theory of Birds' Nests._
231, 232. Introductory passages modified, with some omissions.
233 How modifications of organization would affect the form of the
nest.
235 Illustration from the habits of children and savages.
235, 236. Objection to term "hereditary habit" answered.
237 Passage rewritten, on more or less variable characters in
relation to nidification.
248 On males choosing or rejecting females, and on the various modes
in which colour may be acquired by female birds.
249 On probable ancestral colours of female birds.
255 Protective colouring of the Waxwing.
VIII.--_Creation by Law._
293 Amount of variation in dogs.
296, 297. The "Times" on Natural Selection.
298 to 300. On intermediate or generalized forms of extinct animals
as an indication of transmutation or development.
302 Tabular demonstration of the Origin of Species by Natural
Selection.
IX.--_The development of Human Races, under the law of Natural
Selection._
316 On colour as perhaps correlated with immunity from disease in
man.
326, 327. On the probable future development of man.
330 Concluding paragraph rewritten.
_London, March, 1870._
PREFACE TO THE SECOND EDITION.
The flattering reception of my Essays by the public and the press having
led to a second edition being called for within a year of its first
publication, I have taken the opportunity to make a few necessary
corrections. I have also added a few passages to the 6th and 7th Essays,
and have given two notes, explanatory of some portions of the last
chapter which appear to have been not always understood. These additions
are as follows:--
+-------------------------------------------------------------------+
| _To avoid altering the paging the additional pages now given have |
| been lettered._ |
+---------+---------+-----------------------------------------------+
| 1st Ed. | 2nd Ed. | |
+---------+---------+ |
| 221 | 221 | Additional facts as to birds acquiring |
| | | the song of other species. |
| | | |
| 223 | 223A } | Mr. Spruce's remarks on young birds |
| | 223B } | pairing with old. |
| | | |
| 228 | 228A } | Pouchet's observations on a change |
| | 228B } | in the nests of swallows. |
| | | |
| 229 | -- | Passage omitted about nest of Golden |
| | | Crested Warbler, which had been |
| | | inserted on Rennie's authority, but |
| | | has not been confirmed by any later |
| | | observers. |
| | | |
| 261 | 261 | Daines Barrington, on importance of |
| | | protection to the female bird. |
| | | |
| | 372 | Note A. |
| | | |
| | 372B | Note B. |
+---------+---------+-----------------------------------------------+
CONTENTS.
I.--_On the Law which has regulated the introduction of New
Species._ Pp. 1-25
Geographical distribution dependent on Geologic Changes
A Law deduced from well-known Geographical and Geological facts
The form of a true system of Classification determined by this Law
Geographical Distribution of Organisms
Geological Distribution of the forms of Life
High Organization of very ancient Animals consistent with this Law
Objections to Forbes' Theory of Polarity
Rudimentary Organs
Conclusion
II.--_On the Tendency of Varieties to depart indefinitely from the
Original Type._ Pp. 26-44
Instability of Varieties supposed to prove the permanent
distinctness of Species
The Struggle for Existence
The Law of Population of Species
The Abundance or Rarity of a Species dependent upon its more or less
perfect Adaptation to the Conditions of Existence
Useful Variations will tend to Increase, useless or hurtful
Variations to Diminish
Superior Varieties will ultimately extirpate the Original Species
The Partial Reversion of Domesticated Varieties explained
Lamarck's Hypothesis very different from that now advanced
Conclusion
III.--_Mimicry, and other Protective Resemblances among
Animals._ Pp. 45-129
Test of true and false Theories
Importance of the Principle of Utility
Popular Theories of Colour in Animals
Importance of Concealment as influencing Colour
Special modifications of Colour
Theory of Protective Colouring
Objection that Colour as being dangerous should not exist in Nature
Mimicry
Mimicry among Lepidoptera
Lepidoptera mimicking other Insects
Mimicry among Beetles
Beetles mimicking other Insects
Insects mimicking Species of other Orders
Cases of Mimicry among the Vertebrata
Mimicry among Snakes
Mimicry among Birds
Mimicry among Mammals
Objections to Mr. Bates' Theory of Mimicry
Mimicry by Female Insects only
Cause of the dull Colours of Female Birds
Use of the gaudy Colours of many Caterpillars
Summary
General deductions as to Colour in Nature
Conclusion
IV.--_The Malayan Papilionidae, or Swallow-tailed Butterflies, as
illustrative of the Theory of Natural Selection._ Pp. 130-200
Special value of the Diurnal Lepidoptera for inquiries of this
Nature
Question of the rank of the Papilionidae
Distribution of the Papilionidae
Definition of the word Species
Laws and Modes of Variation
Simple Variability
Polymorphism or Dimorphism
Local form or variety
Co-existing Variety
Race or Subspecies
Species
Variation as specially influenced by Locality
Local Variation of Size
Local Variation of Form
Local Variations of Colour
Remarks on the facts of Local Variation
Mimicry
Concluding Remarks on Variation in Lepidoptera
Arrangement and Geographical Distribution of the Malayan Papilionidae
Arrangement
Geographical Distribution
Range of the Groups of Malayan Papilionidae
Remarkable peculiarities of the island of Celebes
Concluding Remarks
V.--_On Instinct in Man and Animals._ Pp. 201-210
How Instinct may be best Studied
Definition of Instinct
Does Man possess Instincts?
How Indians travel through unknown and trackless Forests
VI.--_The Philosophy of Birds' Nests._ Pp. 211-230
Instinct or Reason in the Construction of Birds' Nests
Do Men build by Reason or by Imitation?
Why does each Bird build a peculiar kind of Nest?
How do young Birds learn to build their first Nest?
Do Birds sing by Instinct or by Imitation?
Man's Works mainly Imitative
How young Birds may learn to build Nests.
Birds do Alter and Improve their Nests when altered conditions
require it
Conclusion
VII.--_A Theory of Birds' Nests; showing the relation of certain
differences of colour in female birds to their mode of
nidification._ Pp. 231-263
Changed Conditions and persistent Habits as influencing Nidification
Classification of Nests
Sexual differences of Colour in Birds
The Law which connects the Colours of Female Birds with the mode of
Nidification
What the Facts Teach us Colour more variable than Structure or
Habits, and therefore the Character which has generally been
modified
Exceptional cases confirmatory of the above Explanation
Real or apparent exceptions to the Law stated at p. 240
Various modes of Protection of Animals
Females of some groups require and obtain more Protection than the
Males
Conclusion
VIII.--_Creation by Law._ Pp. 264-301
Laws from which the Origin of Species may be deduced
Mr. Darwin's Metaphors liable to Misconception
A case of Orchis-structure explained by Natural Selection
Adaptation brought about by General Laws
Beauty in Nature
How new Forms are produced by Variation and Selection
The Objection that there are Limits to Variation
Objection to the argument from Classification
The _Times_ on Natural Selection
Intermediate or generalized forms of Extinct Animals an indication
of Transmutation or Development
Conclusion
A Demonstration of the Origin of Species
IX.--_The Development of Human Races under the Law of Natural
Selection_. Pp. 302-331
Wide difference of Opinion as to Man's Origin
Outline of the Theory of Natural Selection
Different effects of Natural Selection on Animals and on Man
Influence of External Nature in the development of the Human Mind
Extinction of Lower Races
The Origin of the Races of Man
The Bearing of these views on the Antiquity of Man
Their Bearing on the Dignity and Supremacy of Man
Their Bearing on the future Development of Man
Summary
Conclusion
X.--_The Limits of Natural Selection as applied to Man._ Pp. 333--371
What Natural Selection can Not do
The Brain of the Savage shown to be Larger than he Needs it to be
Size of Brain an important Element of Mental Power
Comparison of the Brains of Man and of Anthropoid Apes
Range of intellectual power in Man
Intellect of Savages and of Animals compared
The use of the Hairy Covering of Mammalia
The Constant absence of Hair from certain parts of Man's body a
remarkable Phenomenon
Savage Man feels the want of this Hairy Covering
Man's Naked Skin could not have been produced by Natural Selection
Feet and Hands of Man considered as Difficulties on the Theory of
Natural Selection
The Origin of Some of Man's Mental Faculties, by the preservation of
Useful Variations, not possible
Difficulty as to the Origin of the Moral Sense
Summary of the Argument as to the Insufficiency of Natural Selection
to account for the Development of Man
The Origin of Consciousness
The Nature of Matter
Matter is Force
All Force is probably Will-force
Conclusion
I.
ON THE LAW WHICH HAS REGULATED THE INTRODUCTION OF NEW SPECIES.[A]
+--------------------------------------------------------------+
| [A] Written at Sarawak in February, 1855, and published in |
| the "Annals and Magazine of Natural History," September, |
| 1855. |
+--------------------------------------------------------------+
_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 antitypes 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 these systems of
classification which arrange species or groups in circles, as well as
these 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
clearly 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 possesses 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 only, 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 Humming-birds 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 phenomena, 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
risings 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.[B] 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 those 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.
+--------------------------------------------------------------+
| [B] Professor Ramsay has since shown that a glacial epoch |
| probably occurred at the time of the Permian formation, |
| which will more satisfactorily account for the comparative |
| poverty of species. |
+--------------------------------------------------------------+