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LITTLE MASTERPIECES OF SCIENCE
[Illustration: Charles R. Darwin.]
Little Masterpieces
of Science
Edited by George Iles
THE NATURALIST AS INTERPRETER AND SEER
_By_
Charles Darwin
Alfred R. Wallace
Thomas H. Huxley
Leland O. Howard
George Iles
NEW YORK
DOUBLEDAY, PAGE & COMPANY
1902
Copyright, 1902, by Doubleday, Page & Co.
Copyright, 1877, by D. Appleton & Co.
Copyright, 1901, by John Wanamaker
Copyright, 1895, by G. H. Buek & Co.
TRANSCRIBER'S NOTES: Obvious printer's errors have been silently
corrected. Hyphenated and accented words have been standardized. See
the end of this file for more information.
PREFACE
To gather stones and fallen boughs is soon to ask, what may be done with
them, can they be piled and fastened together for shelter? So begins
architecture, with the hut as its first step, with the Alhambra, St.
Peter's, the capitol at Washington, as its last. In like fashion the
amassing of fact suggests the ordering of fact: when observation is
sufficiently full and varied it comes to the reasons for what it sees.
The geologist delves from layer to layer of the earth beneath his tread,
he finds as he compares their fossils that the more recent forms of life
stand highest in the scale of being, that in the main the animals and
plants of one era are more allied to those immediately next than to
those of remoter times. He thus divines that he is but exploring the
proofs of lineal descent, and with this thought in his mind he finds
that the collections not only of his own district, but of every other,
take on a new meaning. The great seers of science do not await every jot
and tittle of evidence in such a case as this. They discern the drift of
a fact here, a disclosure there, and with both wisdom and boldness
assume that what they see is but a promise of what shall duly be
revealed. Thus it was that Darwin early in his studies became convinced
of the truth of organic evolution: the labours of a lifetime of all but
superhuman effort, a judicial faculty never exceeded among men, served
only to confirm his confidence that all the varied forms of life upon
earth have come to be what they are through an intelligible process,
mainly by "natural selection."
The present volume offers from the classic pages of Darwin his summary
of the argument of "The Origin of Species," his account of how that book
came to be written, and his recapitulation of "The Descent of Man." All
this affords a supreme lesson as to the value of observation with a
purpose. When Darwin was confronted with an organ or trait which puzzled
him, he was wont to ask, What use can it have had? And always the answer
was that every new peculiarity of plant, or beast, is seized upon and
held whenever it confers advantage in the unceasing conflict for place
and food. No hue of scale or plume, no curve of beak or note of song,
but has served a purpose in the plot of life, or advanced the action in
a drama where the penalty for failure is extinction.
As Charles Darwin stood first among the naturalists of the nineteenth
century, his advocacy of evolution soon wrought conviction among the
thinkers competent to follow his evidence and weigh his arguments. The
opposition to his theories though short was sharp, and here he found a
lieutenant of unflinching courage, of the highest expository power, in
Professor Huxley. This great teacher came to America in 1876, and
discoursed on the ancestry of the horse, as disclosed in fossils then
recently discovered in the Far West, maintaining that they afforded
unimpeachable proof of organic evolution. His principal lecture is here
given.
In a remarkable field of "natural selection" Bates, Wallace and Poulton
have explained the value of "mimicry" as an aid to beasts, birds,
insects, as they elude their enemies or lie unsuspected on the watch for
prey. The resemblances thus worked out through successive generations
attest the astonishing plasticity of bodily forms, a plasticity which
would be incredible were not its evidence under our eyes in every
quarter of the globe. Insects have high economic importance as agents of
destruction: we are learning how to pit one of them against another, so
as to leave a clear field to the farmer and the fruit grower. In this
department a leader is Professor Howard, who contributes a noteworthy
chapter on the successful fight against the pest which threatened with
ruin the orange groves of California.
To the every-day observer the most enticing field of natural history is
that in which common flowers and common insects work out their unending
co-partnery. A blossom by its scent, its beauty of tint, allures a moth
or bee and thus, in effect, is able to take flight and find a mate
across a county so as to perpetuate its race a hundred miles from home.
Our volume closes with a sketch of the singular ties which thus bind
together the fortunes of blossom and insect, so that at last the very
form of a flower may be cast in the mould of its winged ally. A word is
also spoken regarding the singular relations of late detected between
the world of vegetation and minute forms once deemed parasitic. The pea
and its kindred harbor on their rootlets certain tiny lodgers; the
tenants pay a liberal rent in the form of nitrogen compounds, a striking
interlacement of interests!
GEORGE ILES.
CONTENTS
DARWIN, CHARLES
THE ORIGIN OF SPECIES IN SUMMARY
Varieties merge gradually into species. Animals tend to
increase in geometrical ratio. Varieties diverge in consonance
with diversity of opportunity for life. In the struggle
for existence those which best accord with their surroundings
will survive and propagate their kind. Sexual selection
has put a premium on beauty. The causes which in brief
periods produce varieties, in long periods give rise to
species. Instincts, as of the hive bee, are slowly developed.
Geology supports the theory of Evolution: the changes in time
in the fossil record are gradual. Geographical distribution
lends its corroboration: in each region most of the inhabitants
in every great class are plainly related. A common ancestor
is suggested when we see the similarity of hand, wing and
fin. Embryos of birds, reptiles and fish are closely similar
and unlike adult forms. Slight changes in the course of
millions of years produce wide divergences. 3
DARWIN, CHARLES
HOW "THE ORIGIN OF SPECIES" CAME TO BE WRITTEN
During his voyage on the _Beagle_ Darwin saw fossil
armadillos like existing species, and on the islands of the
Galapagos group a gradually increased diversity of species of
every kind. All this suggested that species gradually become
modified. Notes gathered of facts bearing on the question.
Observes that it is the variation between one animal and
another which gives the breeder his opportunity. Reads
Malthus on Population, a work which points out the keen
struggle for existence and that favourable variations tend to
be preserved. In 1842 draws up a brief abstract of the theory
of "natural selection." In 1856 begins an elaborate work on
the same theme, but in 1858, hearing that Wallace has written
an essay advancing an independent theory of natural selection,
offers a summary of his argument to the Linnean Society
of London. Writes "The Origin of Species," which is published
most successfully, November, 1859. 35
DARWIN, CHARLES
THE DESCENT OF MAN: THE ARGUMENT IN BRIEF
Since evolution is probable for all other animals, it is
probable for man. The human form has so much in common with
the forms of other animals that community of descent is
strongly suggested. Man, like other creatures, is subject to
the struggle for existence. Evidence shows that it is likely
that man is descended from a tailed and hairy quadruped that
dwelt in trees. Man's mental power has been the chief factor
in his advance, especially in his development of language.
Conscience is due to social instincts, love of approbation,
memory, imagination and religious feeling. Sexual selection
in its effects upon human advancement. 45
WALLACE, ALFRED R.
MIMICRY AND OTHER PROTECTIVE RESEMBLANCES AMONG ANIMALS
The colours of animals are useful for concealment from their
prey, from the creatures upon which they prey. The lion is
scarcely visible as he crouches on the sand or among desert
rocks and stones. Larks, quails and many other birds are so
tinted and mottled that their detection is difficult. The
polar bear, living amid ice and snow, is white. Reptiles and
fish are so coloured as to be almost invisible in the grass
or gravel where they rest. Many beetles and other insects
are so like the leaves or bark on which they feed that
when motionless they cannot be discerned. Some butterflies
resemble dead, dry or decaying leaves so closely as to elude
discovery. Every individual better protected by colour than
others, has a better chance for life, and of transmitting his
hues. Harmless beetles and flies are so like wasps and bees
as to be left alone. 71
HUXLEY, THOMAS H.
EVOLUTION OF THE HORSE
The hoof of the horse is simply a greatly enlarged and
thickened nail: four of his five toes are reduced to mere
vestiges. His teeth are built of substances of varying
hardness: they wear away at different rates presenting uneven
grinding surfaces. Probable descent of the horse, link by
link, especially as traced in the fossils of North America.
Evolution has taken a long time: how long the physicist and
the astronomer must decide. 101
HOWARD, LELAND O.
FIGHTING PESTS WITH INSECT ALLIES
A scale insect threatened with ruin the orchards of California.
Professor C. V. Riley decided that the pest was a native
of Australia. Mr. A. Hoebele observes in Australia that
the pest is kept down by ladybirds. These are accordingly
sent to California where they destroy the scale insect and
restore prosperity among the fruit-growers. Another pest,
of olive trees, is devoured by an imported ladybird of
another species. This plan extended to Portugal and Egypt
with success. Grasshoppers killed by a fungus cultivated
for the purpose. Introduction into the United States of
the insect which fertilizes the Smyrna fig. 123
ILES, GEORGE
THE STRANGE STORY OF THE FLOWERS: A CHAPTER IN MODERN BOTANY
Dress is important, whether natural or artificial. Because
they catch dust on their clothes, bees, moths and butterflies
have brought about myriad espousals of flower with flower.
Colours and scents of blossoms attract insects. A flower
which in form, scent or hue varies gainfully is likely to
survive while others perish. All the parts of a flower are
leaves in disguise. Floral modes of repulsion and defence.
Plants which devour insects, a habit gradually acquired. The
mesquit tree tells of water. Plants believed to indicate
mineral veins. Seeds as emigrants equipped with wings or
hooks. Parasitic plants and their degradation. Tenants that
pay a liberal rent. The gardener as a creator of new flowers.
The modern sugar beet due to Mons. Vilmorin. 139
THE NATURALIST AS
INTERPRETER AND
SEER
THE ORIGIN OF SPECIES: THE ARGUMENT IN SUMMARY
CHARLES DARWIN
[Charles Darwin, one of the greatest men of all time, did
more to advance and prove the theory of evolution than
anybody else who ever lived. This he accomplished by virtue
of the highest gifts of observation, experiment, and
generalization. His truthfulness, patience, and calmness of
judgment have never been exceeded by mortal. His works are
published by D. Appleton & Co., New York, together with his
"Life and Letters," edited by his son Francis. From "The
Origin of Species" the argument in summary is here given.]
On the view that species are only strongly marked and permanent
varieties, and that each species first existed as a variety, we can see
why it is that no line of demarcation can be drawn between species,
commonly supposed to have been produced by special acts of creation, and
varieties which are acknowledged to have been produced by secondary
laws. On this same view we can understand how it is that in a region
where many species of a genus have been produced, and where they now
flourish, these same species should present many varieties; for where
the manufactory of species has been active, we might expect, as a
general rule, to find it still in action; and this is the case if
varieties be incipient species. Moreover, the species of the larger
genera, which afford the greater number of varieties or incipient
species, retain to a certain degree the character of varieties; for they
differ from each other by a less amount of difference than do the
species of smaller genera. The closely allied species also of a larger
genera apparently have restricted ranges, and in their affinities they
are clustered in little groups round other species--in both respects
resembling varieties. These are strange relations on the view that each
species was independently created, but are intelligible if each existed
first as a variety.
As each species tends by its geometrical rate of reproduction to
increase inordinately in number; and as the modified descendants of each
species will be enabled to increase by as much as they become more
diversified in habits and structure, so as to be able to seize on many
and widely different places in the economy of nature, there will be a
constant tendency in natural selection to preserve the most divergent
offspring of any one species. Hence, during a long-continued course of
modification, the slight differences of characteristic of varieties of
the same species, tend to be augmented into the greater differences
characteristic of the species of the same genus. New and improved
varieties will inevitably supplant and exterminate the older, less
improved, and intermediate varieties; and thus species are rendered to a
large extent defined and distinct objects. Dominant species belonging
to the larger groups within each class tend to give birth to new and
dominant forms; so that each large group tends to become still larger,
and at the same time more divergent in character. But as all groups
cannot thus go on increasing in size, for the world would not hold them,
the more dominant groups beat the less dominant. This tendency in the
large groups to go on increasing in size and diverging in character,
together with the inevitable contingency of much extinction, explains
the arrangement of all the forms of life in groups subordinate to
groups, all within a few great classes, which has prevailed throughout
all time. This grand fact of the grouping of all organic beings under
what is called the Natural System, is utterly inexplicable on the theory
of creation.
As natural selection acts solely by accumulating slight, successive,
favourable variations, it can produce no great or sudden modifications;
it can act only by short and slow steps. Hence, the canon of "Nature
makes no leaps," which every fresh addition to our knowledge tends to
confirm, is on this theory intelligible. We can see why throughout
nature the same general end is gained by an almost infinite diversity of
means, for every peculiarity when once acquired is long inherited, and
structures already modified in many different ways have to be adapted
for the same general purpose. We can, in short, see why nature is
prodigal in variety, though niggard in innovation. But why this should
be a law of nature if each species has been independently created no man
can explain.
Many other facts are, as it seems to me, explicable on this theory. How
strange it is that a bird, under the form of a woodpecker, should prey
on insects on the ground; that upland geese which rarely or never swim,
would possess webbed feet; that a thrush-like bird should dive and feed
on sub-aquatic insects; and that a petrel should have the habits and
structure fitting it for the life of an auk! and so in endless other
cases. But on the view of each species constantly trying to increase in
number, with natural selection always ready to adapt the slowly varying
descendants of each to any unoccupied or ill-occupied place in nature,
these facts cease to be strange, or might even have been anticipated.
We can to a certain extent understand how it is that there is so much
beauty throughout nature; for this may be largely attributed to the
agency of selection. That beauty, according to our sense of it, is not
universal, must be admitted by every one who will look at some venomous
snakes, at some fishes, and at certain hideous bats with a distorted
resemblance to the human face. Sexual selection has given the most
brilliant colours, elegant patterns, and other ornaments to the males,
and sometimes to both sexes of many birds, butterflies and other
animals. With birds it has often rendered the voice of the male musical
to the female, as well as to our ears. Flowers and fruit have been
rendered conspicuous by brilliant colours in contrast with the green
foliage, in order that the flowers may be easily seen, visited and
fertilized by insects, and the seeds disseminated by birds. How it comes
that certain colours, sounds and forms should give pleasure to man and
the lower animals, that is, how the sense of beauty in its simplest form
was first acquired, we do not know any more than how certain odours and
flavours were first rendered agreeable.
As natural selection acts by competition, it adopts and improves the
inhabitants of each country only in relation to their co-inhabitants; so
that we need feel no surprise at the species of any one country,
although on the ordinary view supposed to have been created and
specially adapted for that country, being beaten and supplanted by the
naturalized productions from another land. Nor ought we marvel if all
the contrivances in nature be not, as far as we can judge, absolutely
perfect, as in the case even of the human eye; or if some of them be
abhorrent to our ideas of fitness. We need not marvel at the sting of
the bee, when used against an enemy, causing the bee's own death; at
drones being produced in such great numbers for one single act, and
being then slaughtered by their sterile sisters; at the astonishing
waste of pollen by our fir trees; at the instinctive hatred of the queen
bee for her own fertile daughters; at ichneumonidae feeding within the
living bodies of caterpillars; or at other such cases. The wonder
indeed, is, on the theory of natural selection, that more cases of the
want of absolute perfection have not been detected.
The complex and little known laws governing production of varieties are
the same, as far as we can judge, with the laws which have governed the
production of distinct species. In both cases physical conditions seem
to have produced some direct and definite effect, but how much we cannot
say. Thus, when varieties enter any new station, they occasionally
assume some of the characters proper to the species of that station.
With both varieties and species, use and disuse seem to have produced a
considerable effect; for it is impossible to resist this conclusion when
we look, for instance, at the logger-headed duck, which has wings
incapable of flight, in nearly the same condition as in the domestic
duck; or when we look at the burrowing tucu-tucu, which is occasionally
blind, and then at certain moles, which are habitually blind and have
their eyes covered with skin; or when we look at the blind animals
inhabiting the dark caves of America and Europe. With varieties and
species, correlated variation seems to have played an important part, so
that when one part has been modified other parts have been necessarily
modified. With both varieties and species, reversions to long-lost
characters occasionally occur. How inexplicable on the theory of
creation is the occasional appearance of stripes on the shoulders and
legs of the several species of the horse-genus and of their hybrids! How
simply is this fact explained if we believe that these species are all
descended from a striped progenitor, in the same manner as the several
domestic breeds of the pigeon are descended from the blue and barred
rock pigeon!
On the ordinary view of each species having been independently created,
why should specific characters, or those by which the species of the
same genus differ from each other, be more variable than generic
characters in which they all agree? Why, for instance, should the colour
of a flower be more likely to vary in any one species of genus, if the
other species possess differently coloured flowers, than if all
possessed the same coloured flowers? If species are only well-marked
varieties, of which the characters have become in a high degree
permanent, we can understand this fact; for they have already varied
since they branched off from a common progenitor in certain characters,
by which they have come to be specifically different from each other;
therefore these same characters would be more likely again to vary than
the generic characters which have been inherited without change for an
immense period. It is inexplicable on the theory of creation why a part
developed in a very unusual manner in one species alone of a genus, and
therefore, as we may naturally infer, of great importance to that
species, should be eminently liable to variation; but, on our view,
this part has undergone, since the several species branched off from a
common progenitor, an unusual amount of variability and modification,
and therefore we might expect the part generally to be still variable.
But a part may be developed in the most unusual manner, like the wing of
a bat, and yet not be more variable than any other structure, if the
part be common to many subordinate forms, that is, if it has been
inherited for a very long period; for in this case it will have been
rendered constant by long-continued natural selection.
Glancing at instincts, marvellous as some are, they offer no greater
difficulty than do corporeal structures on the theory of the natural
selection of successive, slight, but profitable modifications. We can
thus understand why nature moves by graduated steps in endowing certain
animals of the same class with their several instincts. I have attempted
to show how much light the principle of gradation throws on the
admirable architectural powers of the hive-bee. Habit no doubt often
comes into play in modifying instincts; but it certainly is not
indispensable, as we see in the case of neuter insects, which leave no
progeny to inherit the effects of long-continued habit. On the view of
all the species of the same genus having descended from a common parent,
and having inherited much in common, we can understand how it is that
allied species, when placed under widely different conditions of life,
yet follow nearly the same instincts; why the thrushes of temperate and
tropical South America, for instance, line their nests with mud like our
British species. On the view of instincts having been slowly acquired
through natural selection, we need not marvel at some instincts being
not perfect and liable to mistakes, and at many instincts causing other
animals to suffer.
If species be only well-marked and permanent varieties, we can see at
once why their crossed offspring should follow the same complex laws in
their degrees and kinds of resemblance to their parents--in being
absorbed into each other by successive crosses, and in other such
points--as do the crossed offspring of acknowledged varieties. This
similarity would be a strange fact, if species had been independently
created and varieties had been produced through secondary laws.
If we admit that the geological record is imperfect to an extreme
degree, then the facts, which the record does give, strongly support the
theory of descent with modification. New species have come on the stage
slowly and at successive intervals; and the amount of change after equal
intervals of time, is widely different in different groups. The
extinction of species and of whole groups of species, which has played
so conspicuous a part in the history of the organic world, almost
inevitably follows from the principle of natural selection; for old
forms are supplanted by new and improved forms. Neither single species
nor groups of species reappear when the chain of ordinary generation is
once broken. The gradual diffusion of dominant forms, with the slow
modification of their descendants, causes the forms of life, after long
intervals of time, to appear as if they had changed simultaneously
throughout the world. The fact of the fossil remains of each formation
being in some degree intermediate in character between the fossils in
the formations above and below, is simply explained by their
intermediate position in the chain of descent. The grand fact that all
extinct beings can be classed with all recent beings, naturally follows
from the living and the extinct being the offspring of common parents.
As species have generally diverged in character during their long course
of descent and modification, we can understand why it is that the more
ancient forms, or early progenitors of each group, so often occupy a
position in some degree intermediate between existing groups. Recent
forms are generally looked upon as being, on the whole, higher in the
scale of organization than ancient forms; and they must be higher, in so
far as the later and more improved forms have conquered the older and
less improved forms in the struggle for life; they have also generally
had their organs more specialized for different functions. This fact is
perfectly compatible with numerous beings still retaining simple but
little improved structures, fitted for simple conditions of life; it is
likewise compatible with some forms having retrograded in organization,
by having become at each stage of descent better fitted for new and
degraded habits of life. Lastly, the wonderful law of the long endurance
of allied forms on the same continent--of marsupials [as kangaroos] in
Australia, of edentata [as armadillos, sloths, and anteaters] in
America, and other such cases--is intelligible, for within the same
country the existing and the extinct will be closely allied by descent.
Looking to geographical distribution, if we admit that there has been
during the long course of ages much migration from one part of the world
to another, owing to former climatical and geographical changes and to
the many occasional and unknown means of dispersal, then we can
understand, on the theory of descent with modification, most of the
great leading facts in distribution. We can see why there should be so
striking a parallelism in the distribution of organic beings throughout
space, and in their geological succession throughout time; for in both
cases the beings have been connected by the bond of ordinary generation,
and the means of modification have been the same. We see the full
meaning of the wonderful fact, which has struck every traveller, namely,
that on the same continent, under the most diverse conditions, under
heat and cold, on mountain and lowland, on deserts and marshes, most of
the inhabitants within each great class are plainly related; for they
are the descendants of the same progenitors and early colonists. On this
same principle of former migration, combined in most cases with
modification, we can understand by the aid of the Glacial period, the
identity of some few plants and the close alliance of many others, on
the most distant mountains, and in the northern and southern temperate
zones; and likewise the close alliance of some of the inhabitants of the
sea in the northern and southern temperate latitudes, though separated
by the whole inter-tropical ocean. Although two countries may present
physical conditions as closely similar as the same species ever acquire,
we need feel no surprise at their inhabitants being widely different, if
they have been for a long period completely sundered from each other;
for as the relation of organism to organism is the most important of all
relations, and as the two countries will have received colonists at
various periods and in different proportions, from some other country or
from each other, the course of modification in the two areas will
inevitably have been different.
On this view of migration, with subsequent modification, we see why
oceanic islands are inhabited by only few species, but of these, why
many are peculiar or endemic forms. We clearly see why species belonging
to those groups of animals which cannot cross wide spaces of the ocean,
as frogs and terrestrial mammals, do not inhabit oceanic islands; and
why, on the other hand, new and peculiar species of bats, animals which
can traverse the ocean, are often found on islands far distant from any
continent. Such cases as the presence of peculiar species of bats on
oceanic islands and the absence of all other terrestrial mammals, are
facts utterly inexplicable on the theory of independent acts of
creation.
The existence of closely allied representative species in any two areas,
implies on the theory of descent with modification, that the same
parent-forms formerly inhabited both areas: and we almost invariably
find that wherever many closely allied species inhabit two areas, some
identical species are still common to both. Wherever many closely allied
yet distant species occur, doubtful forms and varieties belonging to the
same groups likewise occur. It is a rule of high generality that the
inhabitants of each area are related to the inhabitants of the nearest
source whence immigrants might have been derived. We see this in the
striking relation of nearly all the plants and animals of the Galapagos
Archipelago, of Juan Fernandez, and of the other American islands, to
the plants and animals of the neighbouring American mainland; and of
those of the Cape Verde Archipelago, and of the other African islands to
the African mainland. It must be admitted that these facts receive no
explanation on the theory of creation.
The fact, as we have seen, that all past and present organic beings can
be arranged within a few great classes, in groups subordinate to groups,
and with the extinct groups often falling in between the recent groups,
is intelligible on the theory of natural selection with its
contingencies of extinction and divergence of character. On these same
principles we see how it is that the mutual affinities of the forms
within each class are so complex and circuitous. We see why certain
characters are far more serviceable than others for classification; why
adaptive characters derived from rudimentary parts, though of no service
to the beings, are often of high classificatory value; and why
embryological characters are often the most valuable of all. The real
affinities of all organic beings, in contradistinction to their adaptive
resemblances, are due to inheritance or community of descent. The
Natural System is a genealogical arrangement, with the acquired grades
of difference, marked by the terms, varieties, species, genera,
families, etc.; and we have to discover the lines of descent by the most
permanent characters, whatever they may be, and of however slight vital
importance.
The similar framework of bones in the hand of a man, wing of a bat, fin
of the porpoise, and leg of the horse--the same number of vertebrae
forming the neck of the giraffe and of the elephant--and innumerable
other such facts, at once explain themselves on the theory of descent
with slow and slight successive modifications. The similarity of pattern
in the wing and in the leg of a bat, though used for such different
purpose--in the jaws and legs of a crab--in the petals, stamens, and
pistils of a flower, is likewise, to a large extent, intelligible on
the view of the gradual modification of parts or organs, which were
aboriginally alike in an early progenitor in each of these classes. On
the principle of successive variations not always supervening at an
early age, and being inherited at a corresponding not early period of
life, we clearly see why the embryos of mammals, birds, reptiles, and
fishes should be so closely similar, and so unlike the adult forms. We
may cease marvelling at the embryo of an air-breathing mammal or bird
having branchial slits and arteries running in loops, like those of a
fish which has to breathe the air dissolved in water by the aid of
well-developed branchiae [gills].
Disuse, aided sometimes by natural selection, will often have reduced
organs when rendered useless under changed habits or conditions of life;
and we can understand on this view the meaning of rudimentary organs.
But disuse and selection will generally act on each creature, when it
has come to maturity and has to play its full part in the struggle for
existence, and will thus have little power in an organ during early
life; hence the organ will not be reduced or rendered rudimentary at
this early age. The calf, for instance, has inherited teeth, which never
cut through the gums of the upper jaw, from an early progenitor having
well-developed teeth; and we may believe, that the teeth in the mature
animal were formerly reduced by disuse, owing to the tongue and palate,
or lips, having become excellently fitted through natural selection to
browse without their aid; whereas in the calf, the teeth have been left
unaffected, and on the principle of inheritance at corresponding ages
have been inherited from a remote period to the present day. On the view
of each organism with all its separate parts having been specially
created, how utterly inexplicable is it that organs bearing the plain
stamp of inutility, such as the teeth in the embryonic calf or the
shrivelled wings under the soldered wing covers of many beetles, should
so frequently occur. Nature may be said to have taken pains to reveal
her scheme of modification, by means of rudimentary organs, of
embryological and homologous [corresponding] structures, but we are too
blind to understand her meaning.
I have now recapitulated the facts and considerations which have
thoroughly convinced me that species have been modified, during a long
course of descent. This has been effected chiefly through the natural
selection of numerous successive, slight, favourable variations; aided
in an important manner by the inherited effects of the use and disuse of
parts; and in an unimportant manner, that is, in relation to adaptive
structures, whether past or present, by the direct action of external
conditions, and by variations which seem to us in our ignorance to arise
spontaneously. It appears that I formerly underrated the frequency and
value of these latter forms of variation, as leading to permanent
modifications of structure independently of natural selection. But as
my conclusions have lately been much misrepresented, and it has been
stated that I attribute the modification of species exclusively to
natural selection, I may be permitted to remark that in the first
edition of this work, and subsequently, I placed in a most conspicuous,
position--namely, at the close of the Introduction--the following words:
"I am convinced that natural selection has been the main but not the
exclusive means of modification." This has been of no avail. Great is
the power of steady misrepresentation; but the history of science shows
that fortunately this power does not long endure.
It can hardly be supposed that a false theory would explain, in so
satisfactory a manner as does the theory of natural selection, the
several large classes of facts above specified. It has recently been
objected that this is an unsafe method of arguing; but it is a method
used in judging the common events of life, and has often been used by
the greatest natural philosophers. The undulatory theory of light has
thus been arrived at; and the belief in the revolution of the earth on
its own axis was until lately supported by hardly any direct evidence.
It is no valid objection that science as yet throws no light on the far
higher problems of the essence of the origin of life. Who can explain
what is the essence of the attraction of gravity? No one now objects to
following out the results consequent on this unknown element of
attraction; notwithstanding that Leibnitz formerly accused Newton of
introducing "occult qualities and miracles into philosophy."
I see no good reasons why the views given in this volume should shock
the religious feelings of any one. It is satisfactory, as showing how
transient such impressions are, to remember that the greatest discovery
ever made by man, namely, the law of the attraction of gravity, was also
attacked by Leibnitz, "as subversive of natural, and inferentially of
revealed religion." A celebrated author and divine has written to me
that "he has gradually learned to see that it is just as noble a
conception of the Deity to believe that He created a few original forms
capable of self-development into other and needful forms, as to believe
that He required a fresh act of creation to supply the voids caused by
the action of His laws."
Why, it may be asked, until recently did nearly all the most eminent
living naturalists and geologists disbelieve in the mutability of
species? It cannot be asserted that organic beings in a state of nature
are subject to no variation; it cannot be proved that the amount of
variation in the course of long ages is a limited quantity; no clear
distinction has been, or can be, drawn between species and well-marked
varieties. It cannot be maintained that species when intercrossed are
invariably sterile and varieties invariably fertile; or that sterility
is a special endowment and sign of creation. The belief that species
were immutable productions was almost unavoidable as long as the
history of the world was thought to be of short duration; and now that
we have acquired some idea of the lapse of time, we are too apt to
assume, without proof, that the geological record is so perfect that it
would have afforded us plain evidence of the mutation of species, if
they had undergone mutation.
But the chief cause of our natural unwillingness to admit that one
species has given birth to other and distinct species, is that we are
always slow in admitting great changes of which we do not see the steps.
The difficulty is the same as that felt by so many geologists, when
Lyell first insisted that long lines of inland cliffs had been formed,
and great valleys excavated, by the agencies which we still see at work.
The mind cannot possibly grasp the full meaning of the term of even a
million years; it cannot add up and perceive the full effects of many
slight variations, accumulated during an almost infinite number of
generations.
Although I am fully convinced of the truth of the views given in this
volume under the form of an abstract, I by no means expect to convince
experienced naturalists whose minds are stocked with a multitude of
facts all viewed, during a long course of years, from a point of view
directly opposite to mine. It is so easy to hide our ignorance under
such expressions as the "plan of creation," "unity of design," etc., and
to think that we give an explanation when we only restate a fact. Any
one whose disposition leads him to attach more weight to unexplained
difficulties than to the explanation of a certain number of facts will
certainly reject the theory. A few naturalists, endowed with much
flexibility of mind, and who have already begun to doubt the
immutability of species, may be influenced by this volume; but I look
with confidence to the future, to young and rising naturalists, who will
be able to view both sides of the question with impartiality. Whoever is
led to believe that species are mutable will do good service by
conscientiously expressing his conviction; for thus only can the load of
prejudice by which this subject is overwhelmed be removed.
Several eminent naturalists have of late published their belief that a
multitude of reputed species in each genus are not real species; but
that other species are real, that is, have been independently created.
This seems to me a strange conclusion to arrive at. They admit that a
multitude of forms, which till lately they themselves thought were
special creations, and which are still thus looked at by the majority of
naturalists, and which consequently have all the external characteristic
features of true species--they admit that these have been produced by
variation, but they refuse to extend the same view to other and slightly
different forms. Nevertheless, they do not pretend that they can define,
or even conjecture, which are the created forms of life, and which are
those produced by secondary laws. They admit variation as a true cause
in one case, they arbitrarily reject it in another, without assigning
any distinction in the two cases. The day will come when this will be
given as a curious illustration of the blindness of preconceived
opinion. These authors seem no more startled at a miraculous act of
creation than at an ordinary birth. But do they really believe that at
innumerable periods in the earth's history certain elemental atoms have
been commanded suddenly to flash into living tissues? Do they believe
that at each supposed act of creation one individual or many were
produced? Were all the infinite numerous kinds of animals and plants
created as eggs or seed, or as full grown? and in the case of mammals,
were they created bearing the false marks of nourishment from the
mother's womb? Undoubtedly some of these same questions cannot be
answered by those who believe in the appearance or creation of only a
few forms of life, or of some one form alone. It has been maintained by
several authors that it is as easy to believe in the creation of a
million beings as of one; but Maupertuis's philosophical axiom "of least
action" leads the mind more willingly to admit the smaller number; and
certainly we ought not to believe that innumerable beings within each
great class have been created with plain, but deceptive, marks of
descent from a single parent.
As a record of a former state of things, I have retained in the
foregoing paragraphs, and elsewhere, several sentences which imply that
naturalists believe in the separate creation of each species; and I have
been much censured for having thus expressed myself. But undoubtedly
this was the general belief when the first edition of the present work
appeared. I formerly spoke to very many naturalists on the subject of
evolution, and never once met with any sympathetic agreement. It is
probable that some did then believe in evolution, but they were either
silent or expressed themselves so ambiguously that it was not easy to
understand their meaning. Now, things are wholly changed, and almost
every naturalist admits the great principle of evolution. There are,
however, some who still think that species have suddenly given birth,
through quite unexplained means, to new and totally different forms.
But, as I have attempted to show, weighty evidence can be opposed to the
admission of great and abrupt modifications. Under a scientific point of
view, and as leading to further investigation, but little advantage is
gained by believing that new forms are suddenly developed in an
inexplicable manner from old and widely different forms, over the old
belief in the creation of species from the dust of the earth.
It may be asked how far I extend the doctrine of the modification of
species. The question is difficult to answer, because the more distinct
the forms are which we consider, by so much the arguments in favour of
community of descent become fewer in number and less in force. But some
arguments of the greatest weight extend very far. All the members of
whole classes are connected together by a chain of affinities, and all
can be classed on the same principle, in groups subordinate to groups.
Fossil remains sometimes tend to fill up very wide intervals between
existing orders.
Organs in a rudimentary condition plainly show that an early progenitor
had the organ in a fully developed condition, and this in some cases
implies an enormous amount of modification in the descendants.
Throughout whole classes various structures are formed on the same
pattern, and at a very early age the embryos closely resemble each
other. Therefore I cannot doubt that the theory of descent with
modification embraces all the members of the same great class or
kingdom. I believe that animals are descended from at most only four or
five progenitors, and plants from an equal or lesser number.
Analogy would lead me one step further, namely, to the belief that all
animals and plants are descended from some one prototype. But analogy
may be a deceitful guide. Nevertheless all living things have much in
common, in their chemical composition, their cellular structure, their
laws of growth, and their liability to injurious influences. We see this
even in so trifling a fact as that the same poison often similarly
affects plants and animals; or that the poison secreted by the gall-fly
produces monstrous growths on the wild rose or oak tree. With all
organic beings, excepting perhaps some of the very lowest, sexual
reproduction seems to be essentially similar. With all, as far as is at
present known, the germinal vesicle is the same; so that all organisms
start from a common origin. If we look even to the two main
divisions--namely, to the animal and vegetable kingdoms--certain low
forms are so far intermediate in character that naturalists have
disputed to which kingdom they should be referred. As Professor Asa Gray
has remarked, "the spores and other reproductive bodies of many of the
lower algae may claim to have first a characteristically animal, and then
an unequivocally vegetable existence." Therefore, on the principle of
natural selection with divergence of character, it does not seem
incredible that, from some such low and intermediate form, both animals
and plants may have been developed; and, if we admit this, we must
likewise admit that all the organic beings which have ever lived on this
earth may be descended from some one primordial form. But this inference
is chiefly grounded on analogy, and it is immaterial whether or not it
is accepted. No doubt it is possible, as Mr. G. H. Lewes has urged, that
at the first commencement of life many different forms were evolved; but
if so, we may conclude that only a very few have left modified
descendants. For, as I have recently remarked in regard to the members
of each great kingdom, such as the Vertebrata, Articulata, etc., we
have distinct evidence in their embryological, homologous, and
rudimentary structures, that within each kingdom all the members are
descended from a single progenitor.
When the views advanced by me in this volume, and by Mr. Wallace, or
when analogous views on the origin of species are generally admitted, we
can dimly foresee that there will be a considerable revolution in
natural history. Systematists will be able to pursue their labours as at
present; but they will not be incessantly haunted by the shadowy doubt
whether this or that form be a true species. This, I feel sure and I
speak after experience, will be no slight relief. The endless disputes
whether or not some fifty species of British brambles are good species
will cease. Systematists will have only to decide (not that this will be
easy) whether any form be sufficiently constant and distinct from other
forms, to be capable of definition; and if definable, whether the
differences be sufficiently important to deserve a specific name. This
latter point will become a far more essential consideration than it is
at present; for differences, however slight, between any two forms, if
not blended by intermediate gradations, are looked at by most
naturalists as sufficient to raise both forms to the rank of species.