On the Origin of Species, 6th Edition
by
Charles Darwin
Part 9 out of 11
sphinx-moth, the curious folded one of a bee or bug, and the great jaws of
a beetle? Yet all these organs, serving for such widely different
purposes, are formed by infinitely numerous modifications of an upper lip,
mandibles, and two pairs of maxillae. The same law governs the
construction of the mouths and limbs of crustaceans. So it is with the
flowers of plants.
Nothing can be more hopeless than to attempt to explain this similarity of
pattern in members of the same class, by utility or by the doctrine of
final causes. The hopelessness of the attempt has been expressly admitted
by Owen in his most interesting work on the "Nature of Limbs." On the
ordinary view of the independent creation of each being, we can only say
that so it is; that it has pleased the Creator to construct all the animals
and plants in each great class on a uniform plan; but this is not a
scientific explanation.
The explanation is to a large extent simple, on the theory of the selection
of successive slight modifications, each being profitable in some way to
the modified form, but often affecting by correlation other parts of the
organisation. In changes of this nature, there will be little or no
tendency to alter the original pattern, or to transpose the parts. The
bones of a limb might be shortened and flattened to any extent, becoming at
the same time enveloped in thick membrane, so as to serve as a fin; or a
webbed hand might have all its bones, or certain bones, lengthened to any
extent, with the membrane connecting them increased, so as to serve as a
wing; yet all these modifications would not tend to alter the framework of
the bones or the relative connexion of the parts. If we suppose that an
early progenitor--the archetype, as it may be called--of all mammals, birds
and reptiles, had its limbs constructed on the existing general pattern,
for whatever purpose they served, we can at once perceive the plain
signification of the homologous construction of the limbs throughout the
class. So with the mouths of insects, we have only to suppose that their
common progenitor had an upper lip, mandibles, and two pairs of maxillae,
these parts being perhaps very simple in form; and then natural selection
will account for the infinite diversity in structure and function of the
mouths of insects. Nevertheless, it is conceivable that the general
pattern of an organ might become so much obscured as to be finally lost, by
the reduction and ultimately by the complete abortion of certain parts, by
the fusion of other parts, and by the doubling or multiplication of others,
variations which we know to be within the limits of possibility. In the
paddles of the gigantic extinct sea-lizards, and in the mouths of certain
suctorial crustaceans, the general pattern seems thus to have become
partially obscured.
There is another and equally curious branch of our subject; namely, serial
homologies, or the comparison of the different parts or organs in the same
individual, and not of the same parts or organs in different members of the
same class. Most physiologists believe that the bones of the skull are
homologous--that is, correspond in number and in relative connexion--with
the elemental parts of a certain number of vertebrae. The anterior and
posterior limbs in all the higher vertebrate classes are plainly
homologous. So it is with the wonderfully complex jaws and legs of
crustaceans. It is familiar to almost every one, that in a flower the
relative position of the sepals, petals, stamens, and pistils, as well as
their intimate structure, are intelligible on the view that they consist of
metamorphosed leaves, arranged in a spire. In monstrous plants, we often
get direct evidence of the possibility of one organ being transformed into
another; and we can actually see, during the early or embryonic stages of
development in flowers, as well as in crustaceans and many other animals,
that organs, which when mature become extremely different are at first
exactly alike.
How inexplicable are the cases of serial homologies on the ordinary view of
creation! Why should the brain be enclosed in a box composed of such
numerous and such extraordinarily shaped pieces of bone apparently
representing vertebrae? As Owen has remarked, the benefit derived from the
yielding of the separate pieces in the act of parturition by mammals, will
by no means explain the same construction in the skulls of birds and
reptiles. Why should similar bones have been created to form the wing and
the leg of a bat, used as they are for such totally different purposes,
namely flying and walking? Why should one crustacean, which has an
extremely complex mouth formed of many parts, consequently always have
fewer legs; or conversely, those with many legs have simpler mouths? Why
should the sepals, petals, stamens, and pistils, in each flower, though
fitted for such distinct purposes, be all constructed on the same pattern?
On the theory of natural selection, we can, to a certain extent, answer
these questions. We need not here consider how the bodies of some animals
first became divided into a series of segments, or how they became divided
into right and left sides, with corresponding organs, for such questions
are almost beyond investigation. It is, however, probable that some serial
structures are the result of cells multiplying by division, entailing the
multiplication of the parts developed from such cells. It must suffice for
our purpose to bear in mind that an indefinite repetition of the same part
or organ is the common characteristic, as Owen has remarked, of all low or
little specialised forms; therefore the unknown progenitor of the
Vertebrata probably possessed many vertebrae; the unknown progenitor of the
Articulata, many segments; and the unknown progenitor of flowering plants,
many leaves arranged in one or more spires. We have also formerly seen
that parts many times repeated are eminently liable to vary, not only in
number, but in form. Consequently such parts, being already present in
considerable numbers, and being highly variable, would naturally afford the
materials for adaptation to the most different purposes; yet they would
generally retain, through the force of inheritance, plain traces of their
original or fundamental resemblance. They would retain this resemblance
all the more, as the variations, which afforded the basis for their
subsequent modification through natural selection, would tend from the
first to be similar; the parts being at an early stage of growth alike, and
being subjected to nearly the same conditions. Such parts, whether more or
less modified, unless their common origin became wholly obscured, would be
serially homologous.
In the great class of molluscs, though the parts in distinct species can be
shown to be homologous, only a few serial homologies; such as the valves of
Chitons, can be indicated; that is, we are seldom enabled to say that one
part is homologous with another part in the same individual. And we can
understand this fact; for in molluscs, even in the lowest members of the
class, we do not find nearly so much indefinite repetition of any one part
as we find in the other great classes of the animal and vegetable kingdoms.
But morphology is a much more complex subject than it at first appears, as
has lately been well shown in a remarkable paper by Mr. E. Ray Lankester,
who has drawn an important distinction between certain classes of cases
which have all been equally ranked by naturalists as homologous. He
proposes to call the structures which resemble each other in distinct
animals, owing to their descent from a common progenitor with subsequent
modification, "homogenous"; and the resemblances which cannot thus be
accounted for, he proposes to call "homoplastic". For instance, he
believes that the hearts of birds and mammals are as a whole homogenous--
that is, have been derived from a common progenitor; but that the four
cavities of the heart in the two classes are homoplastic--that is, have
been independently developed. Mr. Lankester also adduces the close
resemblance of the parts on the right and left sides of the body, and in
the successive segments of the same individual animal; and here we have
parts commonly called homologous which bear no relation to the descent of
distinct species from a common progenitor. Homoplastic structures are the
same with those which I have classed, though in a very imperfect manner, as
analogous modifications or resemblances. Their formation may be attributed
in part to distinct organisms, or to distinct parts of the same organism,
having varied in an analogous manner; and in part to similar modifications,
having been preserved for the same general purpose or function, of which
many instances have been given.
Naturalists frequently speak of the skull as formed of metamorphosed
vertebrae; the jaws of crabs as metamorphosed legs; the stamens and pistils
in flowers as metamorphosed leaves; but it would in most cases be more
correct, as Professor Huxley has remarked, to speak of both skull and
vertebrae, jaws and legs, etc., as having been metamorphosed, not one from
the other, as they now exist, but from some common and simpler element.
Most naturalists, however, use such language only in a metaphorical sense:
they are far from meaning that during a long course of descent, primordial
organs of any kind--vertebrae in the one case and legs in the other--have
actually been converted into skulls or jaws. Yet so strong is the
appearance of this having occurred that naturalists can hardly avoid
employing language having this plain signification. According to the views
here maintained, such language may be used literally; and the wonderful
fact of the jaws, for instance, of a crab retaining numerous characters,
which they probably would have retained through inheritance, if they had
really been metamorphosed from true though extremely simple legs, is in
part explained.
DEVELOPMENT AND EMBRYOLOGY.
This is one of the most important subjects in the whole round of natural
history. The metamorphoses of insects, with which every one is familiar,
are generally effected abruptly by a few stages; but the transformations
are in reality numerous and gradual, though concealed. A certain
ephemerous insect (Chloeon) during its development, moults, as shown by Sir
J. Lubbock, above twenty times, and each time undergoes a certain amount of
change; and in this case we see the act of metamorphosis performed in a
primary and gradual manner. Many insects, and especially certain
crustaceans, show us what wonderful changes of structure can be effected
during development. Such changes, however, reach their acme in the so-
called alternate generations of some of the lower animals. It is, for
instance, an astonishing fact that a delicate branching coralline, studded
with polypi, and attached to a submarine rock, should produce, first by
budding and then by transverse division, a host of huge floating jelly-
fishes; and that these should produce eggs, from which are hatched swimming
animalcules, which attach themselves to rocks and become developed into
branching corallines; and so on in an endless cycle. The belief in the
essential identity of the process of alternate generation and of ordinary
metamorphosis has been greatly strengthened by Wagner's discovery of the
larva or maggot of a fly, namely the Cecidomyia, producing asexually other
larvae, and these others, which finally are developed into mature males and
females, propagating their kind in the ordinary manner by eggs.
It may be worth notice that when Wagner's remarkable discovery was first
announced, I was asked how was it possible to account for the larvae of
this fly having acquired the power of a sexual reproduction. As long as
the case remained unique no answer could be given. But already Grimm has
shown that another fly, a Chironomus, reproduces itself in nearly the same
manner, and he believes that this occurs frequently in the order. It is
the pupa, and not the larva, of the Chironomus which has this power; and
Grimm further shows that this case, to a certain extent, "unites that of
the Cecidomyia with the parthenogenesis of the Coccidae;" the term
parthenogenesis implying that the mature females of the Coccidae are
capable of producing fertile eggs without the concourse of the male.
Certain animals belonging to several classes are now known to have the
power of ordinary reproduction at an unusually early age; and we have only
to accelerate parthenogenetic reproduction by gradual steps to an earlier
and earlier age--Chironomus showing us an almost exactly intermediate
stage, viz., that of the pupa--and we can perhaps account for the
marvellous case of the Cecidomyia.
It has already been stated that various parts in the same individual, which
are exactly alike during an early embryonic period, become widely different
and serve for widely different purposes in the adult state. So again it
has been shown that generally the embryos of the most distinct species
belonging to the same class are closely similar, but become, when fully
developed, widely dissimilar. A better proof of this latter fact cannot be
given than the statement by Von Baer that "the embryos of mammalia, of
birds, lizards and snakes, probably also of chelonia, are in the earliest
states exceedingly like one another, both as a whole and in the mode of
development of their parts; so much so, in fact, that we can often
distinguish the embryos only by their size. In my possession are two
little embryos in spirit, whose names I have omitted to attach, and at
present I am quite unable to say to what class they belong. They may be
lizards or small birds, or very young mammalia, so complete is the
similarity in the mode of formation of the head and trunk in these animals.
The extremities, however, are still absent in these embryos. But even if
they had existed in the earliest stage of their development we should learn
nothing, for the feet of lizards and mammals, the wings and feet of birds,
no less than the hands and feet of man, all arise from the same fundamental
form." The larvae of most crustaceans, at corresponding stages of
development, closely resemble each other, however different the adults may
become; and so it is with very many other animals. A trace of the law of
embryonic resemblance occasionally lasts till a rather late age: thus
birds of the same genus, and of allied genera, often resemble each other in
their immature plumage; as we see in the spotted feathers in the young of
the thrush group. In the cat tribe, most of the species when adult are
striped or spotted in lines; and stripes or spots can be plainly
distinguished in the whelp of the lion and the puma. We occasionally,
though rarely, see something of the same kind in plants; thus the first
leaves of the ulex or furze, and the first leaves of the phyllodineous
acacias, are pinnate or divided like the ordinary leaves of the
leguminosae.
The points of structure, in which the embryos of widely different animals
within the same class resemble each other, often have no direct relation to
their conditions of existence. We cannot, for instance, suppose that in
the embryos of the vertebrata the peculiar loop-like courses of the
arteries near the branchial slits are related to similar conditions--in the
young mammal which is nourished in the womb of its mother, in the egg of
the bird which is hatched in a nest, and in the spawn of a frog under
water. We have no more reason to believe in such a relation than we have
to believe that the similar bones in the hand of a man, wing of a bat, and
fin of a porpoise, are related to similar conditions of life. No one
supposes that the stripes on the whelp of a lion, or the spots on the young
blackbird, are of any use to these animals.
The case, however, is different when an animal, during any part of its
embryonic career, is active, and has to provide for itself. The period of
activity may come on earlier or later in life; but whenever it comes on,
the adaptation of the larva to its conditions of life is just as perfect
and as beautiful as in the adult animal. In how important a manner this
has acted, has recently been well shown by Sir J. Lubbock in his remarks on
the close similarity of the larvae of some insects belonging to very
different orders, and on the dissimilarity of the larvae of other insects
within the same order, according to their habits of life. Owing to such
adaptations the similarity of the larvae of allied animals is sometimes
greatly obscured; especially when there is a division of labour during the
different stages of development, as when the same larva has during one
stage to search for food, and during another stage has to search for a
place of attachment. Cases can even be given of the larvae of allied
species, or groups of species, differing more from each other than do the
adults. In most cases, however, the larvae, though active, still obey,
more or less closely, the law of common embryonic resemblance. Cirripedes
afford a good instance of this: even the illustrious Cuvier did not
perceive that a barnacle was a crustacean: but a glance at the larva shows
this in an unmistakable manner. So again the two main divisions of
cirripedes, the pedunculated and sessile, though differing widely in
external appearance, have larvae in all their stages barely
distinguishable.
The embryo in the course of development generally rises in organisation. I
use this expression, though I am aware that it is hardly possible to define
clearly what is meant by organisation being higher or lower. But no one
probably will dispute that the butterfly is higher than the caterpillar.
In some cases, however, the mature animal must be considered as lower in
the scale than the larva, as with certain parasitic crustaceans. To refer
once again to cirripedes: the larvae in the first stage have three pairs
of locomotive organs, a simple single eye, and a probosciformed mouth, with
which they feed largely, for they increase much in size. In the second
stage, answering to the chrysalis stage of butterflies, they have six pairs
of beautifully constructed natatory legs, a pair of magnificent compound
eyes, and extremely complex antennae; but they have a closed and imperfect
mouth, and cannot feed: their function at this stage is, to search out by
their well-developed organs of sense, and to reach by their active powers
of swimming, a proper place on which to become attached and to undergo
their final metamorphosis. When this is completed they are fixed for life:
their legs are now converted into prehensile organs; they again obtain a
well-constructed mouth; but they have no antennae, and their two eyes are
now reconverted into a minute, single, simple eye-spot. In this last and
complete state, cirripedes may be considered as either more highly or more
lowly organised than they were in the larval condition. But in some genera
the larvae become developed into hermaphrodites having the ordinary
structure, or into what I have called complemental males; and in the latter
the development has assuredly been retrograde; for the male is a mere sack,
which lives for a short time and is destitute of mouth, stomach, and every
other organ of importance, excepting those for reproduction.
We are so much accustomed to see a difference in structure between the
embryo and the adult, that we are tempted to look at this difference as in
some necessary manner contingent on growth. But there is no reason why,
for instance, the wing of a bat, or the fin of a porpoise, should not have
been sketched out with all their parts in proper proportion, as soon as any
part became visible. In some whole groups of animals and in certain
members of other groups this is the case, and the embryo does not at any
period differ widely from the adult: thus Owen has remarked in regard to
cuttle-fish, "there is no metamorphosis; the cephalopodic character is
manifested long before the parts of the embryo are completed." Land-shells
and fresh-water crustaceans are born having their proper forms, while the
marine members of the same two great classes pass through considerable and
often great changes during their development. Spiders, again, barely
undergo any metamorphosis. The larvae of most insects pass through a
worm-like stage, whether they are active and adapted to diversified habits,
or are inactive from being placed in the midst of proper nutriment, or from
being fed by their parents; but in some few cases, as in that of Aphis, if
we look to the admirable drawings of the development of this insect, by
Professor Huxley, we see hardly any trace of the vermiform stage.
Sometimes it is only the earlier developmental stages which fail. Thus,
Fritz Muller has made the remarkable discovery that certain shrimp-like
crustaceans (allied to Penoeus) first appear under the simple nauplius-
form, and after passing through two or more zoea-stages, and then through
the mysis-stage, finally acquire their mature structure: now in the whole
great malacostracan order, to which these crustaceans belong, no other
member is as yet known to be first developed under the nauplius-form,
though many appear as zoeas; nevertheless Muller assigns reasons for his
belief, that if there had been no suppression of development, all these
crustaceans would have appeared as nauplii.
How, then, can we explain these several facts in embryology--namely, the
very general, though not universal, difference in structure between the
embryo and the adult; the various parts in the same individual embryo,
which ultimately become very unlike, and serve for diverse purposes, being
at an early period of growth alike; the common, but not invariable,
resemblance between the embryos or larvae of the most distinct species in
the same class; the embryo often retaining, while within the egg or womb,
structures which are of no service to it, either at that or at a later
period of life; on the other hand, larvae which have to provide for their
own wants, being perfectly adapted to the surrounding conditions; and
lastly, the fact of certain larvae standing higher in the scale of
organisation than the mature animal into which they are developed? I
believe that all these facts can be explained as follows.
It is commonly assumed, perhaps from monstrosities affecting the embryo at
a very early period, that slight variations or individual differences
necessarily appear at an equally early period. We have little evidence on
this head, but what we have certainly points the other way; for it is
notorious that breeders of cattle, horses and various fancy animals, cannot
positively tell, until some time after birth, what will be the merits and
demerits of their young animals. We see this plainly in our own children;
we cannot tell whether a child will be tall or short, or what its precise
features will be. The question is not, at what period of life any
variation may have been caused, but at what period the effects are
displayed. The cause may have acted, and I believe often has acted, on one
or both parents before the act of generation. It deserves notice that it
is of no importance to a very young animal, as long as it is nourished and
protected by its parent, whether most of its characters are acquired a
little earlier or later in life. It would not signify, for instance, to a
bird which obtained its food by having a much-curved beak whether or not
while young it possessed a beak of this shape, as long as it was fed by its
parents.
I have stated in the first chapter, that at whatever age any variation
first appears in the parent, it tends to reappear at a corresponding age in
the offspring. Certain variations can only appear at corresponding ages;
for instance, peculiarities in the caterpillar, cocoon, or imago states of
the silk-moth; or, again, in the full-grown horns of cattle. But
variations which, for all that we can see might have appeared either
earlier or later in life, likewise tend to reappear at a corresponding age
in the offspring and parent. I am far from meaning that this is invariably
the case, and I could give several exceptional cases of variations (taking
the word in the largest sense) which have supervened at an earlier age in
the child than in the parent.
These two principles, namely, that slight variations generally appear at a
not very early period of life, and are inherited at a corresponding not
early period, explain, as I believe, all the above specified leading facts
in embryology. But first let us look to a few analogous cases in our
domestic varieties. Some authors who have written on Dogs maintain that
the greyhound and bull-dog, though so different, are really closely allied
varieties, descended from the same wild stock, hence I was curious to see
how far their puppies differed from each other. I was told by breeders
that they differed just as much as their parents, and this, judging by the
eye, seemed almost to be the case; but on actually measuring the old dogs
and their six-days-old puppies, I found that the puppies had not acquired
nearly their full amount of proportional difference. So, again, I was told
that the foals of cart and race-horses--breeds which have been almost
wholly formed by selection under domestication--differed as much as the
full-grown animals; but having had careful measurements made of the dams
and of three-days-old colts of race and heavy cart-horses, I find that this
is by no means the case.
As we have conclusive evidence that the breeds of the Pigeon are descended
from a single wild species, I compared the young pigeons within twelve
hours after being hatched. I carefully measured the proportions (but will
not here give the details) of the beak, width of mouth, length of nostril
and of eyelid, size of feet and length of leg, in the wild parent species,
in pouters, fantails, runts, barbs, dragons, carriers, and tumblers. Now,
some of these birds, when mature, differ in so extraordinary a manner in
the length and form of beak, and in other characters, that they would
certainly have been ranked as distinct genera if found in a state of
nature. But when the nestling birds of these several breeds were placed in
a row, though most of them could just be distinguished, the proportional
differences in the above specified points were incomparably less than in
the full-grown birds. Some characteristic points of difference--for
instance, that of the width of mouth--could hardly be detected in the
young. But there was one remarkable exception to this rule, for the young
of the short-faced tumbler differed from the young of the wild rock-pigeon,
and of the other breeds, in almost exactly the same proportions as in the
adult stage.
These facts are explained by the above two principles. Fanciers select
their dogs, horses, pigeons, etc., for breeding, when nearly grown up.
They are indifferent whether the desired qualities are acquired earlier or
later in life, if the full-grown animal possesses them. And the cases just
given, more especially that of the pigeons, show that the characteristic
differences which have been accumulated by man's selection, and which give
value to his breeds, do not generally appear at a very early period of
life, and are inherited at a corresponding not early period. But the case
of the short-faced tumbler, which when twelve hours old possessed its
proper characters, proves that this is not the universal rule; for here the
characteristic differences must either have appeared at an earlier period
than usual, or, if not so, the differences must have been inherited, not at
a corresponding, but at an earlier age.
Now, let us apply these two principles to species in a state of nature.
Let us take a group of birds, descended from some ancient form and modified
through natural selection for different habits. Then, from the many slight
successive variations having supervened in the several species at a not
early age, and having been inherited at a corresponding age, the young will
have been but little modified, and they will still resemble each other much
more closely than do the adults, just as we have seen with the breeds of
the pigeon. We may extend this view to widely distinct structures and to
whole classes. The fore-limbs, for instance, which once served as legs to
a remote progenitor, may have become, through a long course of
modification, adapted in one descendant to act as hands, in another as
paddles, in another as wings; but on the above two principles the
fore-limbs will not have been much modified in the embryos of these several
forms; although in each form the fore-limb will differ greatly in the adult
state. Whatever influence long continued use or disuse may have had in
modifying the limbs or other parts of any species, this will chiefly or
solely have affected it when nearly mature, when it was compelled to use
its full powers to gain its own living; and the effects thus produced will
have been transmitted to the offspring at a corresponding nearly mature
age. Thus the young will not be modified, or will be modified only in a
slight degree, through the effects of the increased use or disuse of parts.
With some animals the successive variations may have supervened at a very
early period of life, or the steps may have been inherited at an earlier
age than that at which they first occurred. In either of these cases the
young or embryo will closely resemble the mature parent-form, as we have
seen with the short-faced tumbler. And this is the rule of development in
certain whole groups, or in certain sub-groups alone, as with cuttle-fish,
land-shells, fresh-water crustaceans, spiders, and some members of the
great class of insects. With respect to the final cause of the young in
such groups not passing through any metamorphosis, we can see that this
would follow from the following contingencies: namely, from the young
having to provide at a very early age for their own wants, and from their
following the same habits of life with their parents; for in this case it
would be indispensable for their existence that they should be modified in
the same manner as their parents. Again, with respect to the singular fact
that many terrestrial and fresh-water animals do not undergo any
metamorphosis, while marine members of the same groups pass through various
transformations, Fritz Muller has suggested that the process of slowly
modifying and adapting an animal to live on the land or in fresh water,
instead of in the sea, would be greatly simplified by its not passing
through any larval stage; for it is not probable that places well adapted
for both the larval and mature stages, under such new and greatly changed
habits of life, would commonly be found unoccupied or ill-occupied by other
organisms. In this case the gradual acquirement at an earlier and earlier
age of the adult structure would be favoured by natural selection; and all
traces of former metamorphoses would finally be lost.
If, on the other hand, it profited the young of an animal to follow habits
of life slightly different from those of the parent-form, and consequently
to be constructed on a slightly different plan, or if it profited a larva
already different from its parent to change still further, then, on the
principle of inheritance at corresponding ages, the young or the larvae
might be rendered by natural selection more and more different from their
parents to any conceivable extent. Differences in the larva might, also,
become correlated with successive stages of its development; so that the
larva, in the first stage, might come to differ greatly from the larva in
the second stage, as is the case with many animals. The adult might also
become fitted for sites or habits, in which organs of locomotion or of the
senses, etc., would be useless; and in this case the metamorphosis would be
retrograde.
>From the remarks just made we can see how by changes of structure in the
young, in conformity with changed habits of life, together with inheritance
at corresponding ages, animals might come to pass through stages of
development, perfectly distinct from the primordial condition of their
adult progenitors. Most of our best authorities are now convinced that the
various larval and pupal stages of insects have thus been acquired through
adaptation, and not through inheritance from some ancient form. The
curious case of Sitaris--a beetle which passes through certain unusual
stages of development--will illustrate how this might occur. The first
larval form is described by M. Fabre, as an active, minute insect,
furnished with six legs, two long antennae, and four eyes. These larvae
are hatched in the nests of bees; and when the male bees emerge from their
burrows, in the spring, which they do before the females, the larvae spring
on them, and afterwards crawl on to the females while paired with the
males. As soon as the female bee deposits her eggs on the surface of the
honey stored in the cells, the larvae of the Sitaris leap on the eggs and
devour them. Afterwards they undergo a complete change; their eyes
disappear; their legs and antennae become rudimentary, and they feed on
honey; so that they now more closely resemble the ordinary larvae of
insects; ultimately they undergo a further transformation, and finally
emerge as the perfect beetle. Now, if an insect, undergoing
transformations like those of the Sitaris, were to become the progenitor of
a whole new class of insects, the course of development of the new class
would be widely different from that of our existing insects; and the first
larval stage certainly would not represent the former condition of any
adult and ancient form.
On the other hand it is highly probable that with many animals the
embryonic or larval stages show us, more or less completely, the condition
of the progenitor of the whole group in its adult state. In the great
class of the Crustacea, forms wonderfully distinct from each other, namely,
suctorial parasites, cirripedes, entomostraca, and even the malacostraca,
appear at first as larvae under the nauplius-form; and as these larvae live
and feed in the open sea, and are not adapted for any peculiar habits of
life, and from other reasons assigned by Fritz Muller, it is probable that
at some very remote period an independent adult animal, resembling the
Nauplius, existed, and subsequently produced, along several divergent lines
of descent, the above-named great Crustacean groups. So again, it is
probable, from what we know of the embryos of mammals, birds, fishes and
reptiles, that these animals are the modified descendants of some ancient
progenitor, which was furnished in its adult state with branchiae, a swim-
bladder, four fin-like limbs, and a long tail, all fitted for an aquatic
life.
As all the organic beings, extinct and recent, which have ever lived, can
be arranged within a few great classes; and as all within each class have,
according to our theory, been connected together by fine gradations, the
best, and, if our collections were nearly perfect, the only possible
arrangement, would be genealogical; descent being the hidden bond of
connexion which naturalists have been seeking under the term of the Natural
System. On this view we can understand how it is that, in the eyes of most
naturalists, the structure of the embryo is even more important for
classification than that of the adult. In two or more groups of animals,
however much they may differ from each other in structure and habits in
their adult condition, if they pass through closely similar embryonic
stages, we may feel assured that they are all descended from one parent-
form, and are therefore closely related. Thus, community in embryonic
structure reveals community of descent; but dissimilarity in embryonic
development does not prove discommunity of descent, for in one of two
groups the developmental stages may have been suppressed, or may have been
so greatly modified through adaptation to new habits of life as to be no
longer recognisable. Even in groups, in which the adults have been
modified to an extreme degree, community of origin is often revealed by the
structure of the larvae; we have seen, for instance, that cirripedes,
though externally so like shell-fish, are at once known by their larvae to
belong to the great class of crustaceans. As the embryo often shows us
more or less plainly the structure of the less modified and ancient
progenitor of the group, we can see why ancient and extinct forms so often
resemble in their adult state the embryos of existing species of the same
class. Agassiz believes this to be a universal law of nature; and we may
hope hereafter to see the law proved true. It can, however, be proved true
only in those cases in which the ancient state of the progenitor of the
group has not been wholly obliterated, either by successive variations
having supervened at a very early period of growth, or by such variations
having been inherited at an earlier age than that at which they first
appeared. It should also be borne in mind, that the law may be true, but
yet, owing to the geological record not extending far enough back in time,
may remain for a long period, or for ever, incapable of demonstration. The
law will not strictly hold good in those cases in which an ancient form
became adapted in its larval state to some special line of life, and
transmitted the same larval state to a whole group of descendants; for such
larval state will not resemble any still more ancient form in its adult
state.
Thus, as it seems to me, the leading facts in embryology, which are second
to none in importance, are explained on the principle of variations in the
many descendants from some one ancient progenitor, having appeared at a not
very early period of life, and having been inherited at a corresponding
period. Embryology rises greatly in interest, when we look at the embryo
as a picture, more or less obscured, of the progenitor, either in its adult
or larval state, of all the members of the same great class.
RUDIMENTARY, ATROPHIED, AND ABORTED ORGANS.
Organs or parts in this strange condition, bearing the plain stamp of
inutility, are extremely common, or even general, throughout nature. It
would be impossible to name one of the higher animals in which some part or
other is not in a rudimentary condition. In the mammalia, for instance,
the males possess rudimentary mammae; in snakes one lobe of the lungs is
rudimentary; in birds the "bastard-wing" may safely be considered as a
rudimentary digit, and in some species the whole wing is so far rudimentary
that it cannot be used for flight. What can be more curious than the
presence of teeth in foetal whales, which when grown up have not a tooth in
their heads; or the teeth, which never cut through the gums, in the upper
jaws of unborn calves?
Rudimentary organs plainly declare their origin and meaning in various
ways. There are beetles belonging to closely allied species, or even to
the same identical species, which have either full-sized and perfect wings,
or mere rudiments of membrane, which not rarely lie under wing-covers
firmly soldered together; and in these cases it is impossible to doubt,
that the rudiments represent wings. Rudimentary organs sometimes retain
their potentiality: this occasionally occurs with the mammae of male
mammals, which have been known to become well developed and to secrete
milk. So again in the udders of the genus Bos, there are normally four
developed and two rudimentary teats; but the latter in our domestic cows
sometimes become well developed and yield milk. In regard to plants, the
petals are sometimes rudimentary, and sometimes well developed in the
individuals of the same species. In certain plants having separated sexes
Kolreuter found that by crossing a species, in which the male flowers
included a rudiment of a pistil, with an hermaphrodite species, having of
course a well-developed pistil, the rudiment in the hybrid offspring was
much increased in size; and this clearly shows that the rudimentary and
perfect pistils are essentially alike in nature. An animal may possess
various parts in a perfect state, and yet they may in one sense be
rudimentary, for they are useless: thus the tadpole of the common
salamander or water-newt, as Mr. G.H. Lewes remarks, "has gills, and passes
its existence in the water; but the Salamandra atra, which lives high up
among the mountains, brings forth its young full-formed. This animal never
lives in the water. Yet if we open a gravid female, we find tadpoles
inside her with exquisitely feathered gills; and when placed in water they
swim about like the tadpoles of the water-newt. Obviously this aquatic
organisation has no reference to the future life of the animal, nor has it
any adaptation to its embryonic condition; it has solely reference to
ancestral adaptations, it repeats a phase in the development of its
progenitors."
An organ, serving for two purposes, may become rudimentary or utterly
aborted for one, even the more important purpose, and remain perfectly
efficient for the other. Thus, in plants, the office of the pistil is to
allow the pollen-tubes to reach the ovules within the ovarium. The pistil
consists of a stigma supported on the style; but in some Compositae, the
male florets, which of course cannot be fecundated, have a rudimentary
pistil, for it is not crowned with a stigma; but the style remains well
developed and is clothed in the usual manner with hairs, which serve to
brush the pollen out of the surrounding and conjoined anthers. Again, an
organ may become rudimentary for its proper purpose, and be used for a
distinct one: in certain fishes the swim-bladder seems to be rudimentary
for its proper function of giving buoyancy, but has become converted into a
nascent breathing organ or lung. Many similar instances could be given.
Useful organs, however little they may be developed, unless we have reason
to suppose that they were formerly more highly developed, ought not to be
considered as rudimentary. They may be in a nascent condition, and in
progress towards further development. Rudimentary organs, on the other
hand, are either quite useless, such as teeth which never cut through the
gums, or almost useless, such as the wings of an ostrich, which serve
merely as sails. As organs in this condition would formerly, when still
less developed, have been of even less use than at present, they cannot
formerly have been produced through variation and natural selection, which
acts solely by the preservation of useful modifications. They have been
partially retained by the power of inheritance, and relate to a former
state of things. It is, however, often difficult to distinguish between
rudimentary and nascent organs; for we can judge only by analogy whether a
part is capable of further development, in which case alone it deserves to
be called nascent. Organs in this condition will always be somewhat rare;
for beings thus provided will commonly have been supplanted by their
successors with the same organ in a more perfect state, and consequently
will have become long ago extinct. The wing of the penguin is of high
service, acting as a fin; it may, therefore, represent the nascent state of
the wing: not that I believe this to be the case; it is more probably a
reduced organ, modified for a new function: the wing of the Apteryx, on
the other hand, is quite useless, and is truly rudimentary. Owen considers
the simple filamentary limbs of the Lepidosiren as the "beginnings of
organs which attain full functional development in higher vertebrates;"
but, according to the view lately advocated by Dr. Gunther, they are
probably remnants, consisting of the persistent axis of a fin, with the
lateral rays or branches aborted. The mammary glands of the
Ornithorhynchus may be considered, in comparison with the udders of a cow,
as in a nascent condition. The ovigerous frena of certain cirripedes,
which have ceased to give attachment to the ova and are feebly developed,
are nascent branchiae.
Rudimentary organs in the individuals of the same species are very liable
to vary in the degree of their development and in other respects. In
closely allied species, also, the extent to which the same organ has been
reduced occasionally differs much. This latter fact is well exemplified in
the state of the wings of female moths belonging to the same family.
Rudimentary organs may be utterly aborted; and this implies, that in
certain animals or plants, parts are entirely absent which analogy would
lead us to expect to find in them, and which are occasionally found in
monstrous individuals. Thus in most of the Scrophulariaceae the fifth
stamen is utterly aborted; yet we may conclude that a fifth stamen once
existed, for a rudiment of it is found in many species of the family, and
this rudiment occasionally becomes perfectly developed, as may sometimes be
seen in the common snap-dragon. In tracing the homologies of any part in
different members of the same class, nothing is more common, or, in order
fully to understand the relations of the parts, more useful than the
discovery of rudiments. This is well shown in the drawings given by Owen
of the leg bones of the horse, ox, and rhinoceros.
It is an important fact that rudimentary organs, such as teeth in the upper
jaws of whales and ruminants, can often be detected in the embryo, but
afterwards wholly disappear. It is also, I believe, a universal rule, that
a rudimentary part is of greater size in the embryo relatively to the
adjoining parts, than in the adult; so that the organ at this early age is
less rudimentary, or even cannot be said to be in any degree rudimentary.
Hence rudimentary organs in the adult are often said to have retained their
embryonic condition.
I have now given the leading facts with respect to rudimentary organs. In
reflecting on them, every one must be struck with astonishment; for the
same reasoning power which tells us that most parts and organs are
exquisitely adapted for certain purposes, tells us with equal plainness
that these rudimentary or atrophied organs are imperfect and useless. In
works on natural history, rudimentary organs are generally said to have
been created "for the sake of symmetry," or in order "to complete the
scheme of nature." But this is not an explanation, merely a restatement of
the fact. Nor is it consistent with itself: thus the boa-constrictor has
rudiments of hind limbs and of a pelvis, and if it be said that these bones
have been retained "to complete the scheme of nature," why, as Professor
Weismann asks, have they not been retained by other snakes, which do not
possess even a vestige of these same bones? What would be thought of an
astronomer who maintained that the satellites revolve in elliptic courses
round their planets "for the sake of symmetry," because the planets thus
revolve round the sun? An eminent physiologist accounts for the presence
of rudimentary organs, by supposing that they serve to excrete matter in
excess, or matter injurious to the system; but can we suppose that the
minute papilla, which often represents the pistil in male flowers, and
which is formed of mere cellular tissue, can thus act? Can we suppose that
rudimentary teeth, which are subsequently absorbed, are beneficial to the
rapidly growing embryonic calf by removing matter so precious as phosphate
of lime? When a man's fingers have been amputated, imperfect nails have
been known to appear on the stumps, and I could as soon believe that these
vestiges of nails are developed in order to excrete horny matter, as that
the rudimentary nails on the fin of the manatee have been developed for
this same purpose.
On the view of descent with modification, the origin of rudimentary organs
is comparatively simple; and we can understand to a large extent the laws
governing their imperfect development. We have plenty of cases of
rudimentary organs in our domestic productions, as the stump of a tail in
tailless breeds, the vestige of an ear in earless breeds of sheep--the
reappearance of minute dangling horns in hornless breeds of cattle, more
especially, according to Youatt, in young animals--and the state of the
whole flower in the cauliflower. We often see rudiments of various parts
in monsters; but I doubt whether any of these cases throw light on the
origin of rudimentary organs in a state of nature, further than by showing
that rudiments can be produced; for the balance of evidence clearly
indicates that species under nature do not undergo great and abrupt
changes. But we learn from the study of our domestic productions that the
disuse of parts leads to their reduced size; and that the result is
inherited.
It appears probable that disuse has been the main agent in rendering organs
rudimentary. It would at first lead by slow steps to the more and more
complete reduction of a part, until at last it became rudimentary--as in
the case of the eyes of animals inhabiting dark caverns, and of the wings
of birds inhabiting oceanic islands, which have seldom been forced by
beasts of prey to take flight, and have ultimately lost the power of
flying. Again, an organ, useful under certain conditions, might become
injurious under others, as with the wings of beetles living on small and
exposed islands; and in this case natural selection will have aided in
reducing the organ, until it was rendered harmless and rudimentary.
Any change in structure and function, which can be effected by small
stages, is within the power of natural selection; so that an organ
rendered, through changed habits of life, useless or injurious for one
purpose, might be modified and used for another purpose. An organ might,
also, be retained for one alone of its former functions. Organs,
originally formed by the aid of natural selection, when rendered useless
may well be variable, for their variations can no longer be checked by
natural selection. All this agrees well with what we see under nature.
Moreover, at whatever period of life either disuse or selection reduces an
organ, and this will generally be when the being has come to maturity and
to exert its full powers of action, the principle of inheritance at
corresponding ages will tend to reproduce the organ in its reduced state at
the same mature age, but will seldom affect it in the embryo. Thus we can
understand the greater size of rudimentary organs in the embryo relatively
to the adjoining parts, and their lesser relative size in the adult. If,
for instance, the digit of an adult animal was used less and less during
many generations, owing to some change of habits, or if an organ or gland
was less and less functionally exercised, we may infer that it would become
reduced in size in the adult descendants of this animal, but would retain
nearly its original standard of development in the embryo.
There remains, however, this difficulty. After an organ has ceased being
used, and has become in consequence much reduced, how can it be still
further reduced in size until the merest vestige is left; and how can it be
finally quite obliterated? It is scarcely possible that disuse can go on
producing any further effect after the organ has once been rendered
functionless. Some additional explanation is here requisite which I cannot
give. If, for instance, it could be proved that every part of the
organisation tends to vary in a greater degree towards diminution than
toward augmentation of size, then we should be able to understand how an
organ which has become useless would be rendered, independently of the
effects of disuse, rudimentary and would at last be wholly suppressed; for
the variations towards diminished size would no longer be checked by
natural selection. The principle of the economy of growth, explained in a
former chapter, by which the materials forming any part, if not useful to
the possessor, are saved as far as is possible, will perhaps come into play
in rendering a useless part rudimentary. But this principle will almost
necessarily be confined to the earlier stages of the process of reduction;
for we cannot suppose that a minute papilla, for instance, representing in
a male flower the pistil of the female flower, and formed merely of
cellular tissue, could be further reduced or absorbed for the sake of
economising nutriment.
Finally, as rudimentary organs, by whatever steps they may have been
degraded into their present useless condition, are the record of a former
state of things, and have been retained solely through the power of
inheritance--we can understand, on the genealogical view of classification,
how it is that systematists, in placing organisms in their proper places in
the natural system, have often found rudimentary parts as useful as, or
even sometimes more useful than, parts of high physiological importance.
Rudimentary organs may be compared with the letters in a word, still
retained in the spelling, but become useless in the pronunciation, but
which serve as a clue for its derivation. On the view of descent with
modification, we may conclude that the existence of organs in a
rudimentary, imperfect, and useless condition, or quite aborted, far from
presenting a strange difficulty, as they assuredly do on the old doctrine
of creation, might even have been anticipated in accordance with the views
here explained.
SUMMARY.
In this chapter I have attempted to show that the arrangement of all
organic beings throughout all time in groups under groups--that the nature
of the relationships by which all living and extinct organisms are united
by complex, radiating, and circuitous lines of affinities into a few grand
classes--the rules followed and the difficulties encountered by naturalists
in their classifications--the value set upon characters, if constant and
prevalent, whether of high or of the most trifling importance, or, as with
rudimentary organs of no importance--the wide opposition in value between
analogical or adaptive characters, and characters of true affinity; and
other such rules--all naturally follow if we admit the common parentage of
allied forms, together with their modification through variation and
natural selection, with the contingencies of extinction and divergence of
character. In considering this view of classification, it should be borne
in mind that the element of descent has been universally used in ranking
together the sexes, ages, dimorphic forms, and acknowledged varieties of
the same species, however much they may differ from each other in
structure. If we extend the use of this element of descent--the one
certainly known cause of similarity in organic beings--we shall understand
what is meant by the Natural System: it is genealogical in its attempted
arrangement, with the grades of acquired difference marked by the terms,
varieties, species, genera, families, orders, and classes.
On this same view of descent with modification, most of the great facts in
Morphology become intelligible--whether we look to the same pattern
displayed by the different species of the same class in their homologous
organs, to whatever purpose applied, or to the serial and lateral
homologies in each individual animal and plant.
On the principle of successive slight variations, not necessarily or
generally supervening at a very early period of life, and being inherited
at a corresponding period, we can understand the leading facts in
embryology; namely, the close resemblance in the individual embryo of the
parts which are homologous, and which when matured become widely different
in structure and function; and the resemblance of the homologous parts or
organs in allied though distinct species, though fitted in the adult state
for habits as different as is possible. Larvae are active embryos, which
have become specially modified in a greater or less degree in relation to
their habits of life, with their modifications inherited at a corresponding
early age. On these same principles, and bearing in mind that when organs
are reduced in size, either from disuse or through natural selection, it
will generally be at that period of life when the being has to provide for
its own wants, and bearing in mind how strong is the force of
inheritance--the occurrence of rudimentary organs might even have been
anticipated. The importance of embryological characters and of rudimentary
organs in classification is intelligible, on the view that a natural
arrangement must be genealogical.
Finally, the several classes of facts which have been considered in this
chapter, seem to me to proclaim so plainly, that the innumerable species,
genera and families, with which this world is peopled, are all descended,
each within its own class or group, from common parents, and have all been
modified in the course of descent, that I should without hesitation adopt
this view, even if it were unsupported by other facts or arguments.
CHAPTER XV.
RECAPITULATION AND CONCLUSION.
Recapitulation of the objections to the theory of Natural Selection --
Recapitulation of the general and special circumstances in its favour --
Causes of the general belief in the immutability of species -- How far the
theory of Natural Selection may be extended -- Effects of its adoption on
the study of Natural History -- Concluding remarks.
As this whole volume is one long argument, it may be convenient to the
reader to have the leading facts and inferences briefly recapitulated.
That many and serious objections may be advanced against the theory of
descent with modification through variation and natural selection, I do not
deny. I have endeavoured to give to them their full force. Nothing at
first can appear more difficult to believe than that the more complex
organs and instincts have been perfected, not by means superior to, though
analogous with, human reason, but by the accumulation of innumerable slight
variations, each good for the individual possessor. Nevertheless, this
difficulty, though appearing to our imagination insuperably great, cannot
be considered real if we admit the following propositions, namely, that all
parts of the organisation and instincts offer, at least individual
differences--that there is a struggle for existence leading to the
preservation of profitable deviations of structure or instinct--and,
lastly, that gradations in the state of perfection of each organ may have
existed, each good of its kind. The truth of these propositions cannot, I
think, be disputed.
It is, no doubt, extremely difficult even to conjecture by what gradations
many structures have been perfected, more especially among broken and
failing groups of organic beings, which have suffered much extinction; but
we see so many strange gradations in nature, that we ought to be extremely
cautious in saying that any organ or instinct, or any whole structure,
could not have arrived at its present state by many graduated steps. There
are, it must be admitted, cases of special difficulty opposed to the theory
of natural selection; and one of the most curious of these is the existence
in the same community of two or three defined castes of workers or sterile
female ants; but I have attempted to show how these difficulties can be
mastered.
With respect to the almost universal sterility of species when first
crossed, which forms so remarkable a contrast with the almost universal
fertility of varieties when crossed, I must refer the reader to the
recapitulation of the facts given at the end of the ninth chapter, which
seem to me conclusively to show that this sterility is no more a special
endowment than is the incapacity of two distinct kinds of trees to be
grafted together; but that it is incidental on differences confined to the
reproductive systems of the intercrossed species. We see the truth of this
conclusion in the vast difference in the results of crossing the same two
species reciprocally--that is, when one species is first used as the father
and then as the mother. Analogy from the consideration of dimorphic and
trimorphic plants clearly leads to the same conclusion, for when the forms
are illegitimately united, they yield few or no seed, and their offspring
are more or less sterile; and these forms belong to the same undoubted
species, and differ from each other in no respect except in their
reproductive organs and functions.
Although the fertility of varieties when intercrossed, and of their mongrel
offspring, has been asserted by so many authors to be universal, this
cannot be considered as quite correct after the facts given on the high
authority of Gartner and Kolreuter. Most of the varieties which have been
experimented on have been produced under domestication; and as
domestication (I do not mean mere confinement) almost certainly tends to
eliminate that sterility which, judging from analogy, would have affected
the parent-species if intercrossed, we ought not to expect that
domestication would likewise induce sterility in their modified descendants
when crossed. This elimination of sterility apparently follows from the
same cause which allows our domestic animals to breed freely under
diversified circumstances; and this again apparently follows from their
having been gradually accustomed to frequent changes in their conditions of
life.
A double and parallel series of facts seems to throw much light on the
sterility of species, when first crossed, and of their hybrid offspring.
On the one side, there is good reason to believe that slight changes in the
conditions of life give vigour and fertility to all organic beings. We
know also that a cross between the distinct individuals of the same
variety, and between distinct varieties, increases the number of their
offspring, and certainly gives to them increased size and vigour. This is
chiefly owing to the forms which are crossed having been exposed to
somewhat different conditions of life; for I have ascertained by a
labourious series of experiments that if all the individuals of the same
variety be subjected during several generations to the same conditions, the
good derived from crossing is often much diminished or wholly disappears.
This is one side of the case. On the other side, we know that species
which have long been exposed to nearly uniform conditions, when they are
subjected under confinement to new and greatly changed conditions, either
perish, or if they survive, are rendered sterile, though retaining perfect
health. This does not occur, or only in a very slight degree, with our
domesticated productions, which have long been exposed to fluctuating
conditions. Hence when we find that hybrids produced by a cross between
two distinct species are few in number, owing to their perishing soon after
conception or at a very early age, or if surviving that they are rendered
more or less sterile, it seems highly probable that this result is due to
their having been in fact subjected to a great change in their conditions
of life, from being compounded of two distinct organisations. He who will
explain in a definite manner why, for instance, an elephant or a fox will
not breed under confinement in its native country, whilst the domestic pig
or dog will breed freely under the most diversified conditions, will at the
same time be able to give a definite answer to the question why two
distinct species, when crossed, as well as their hybrid offspring, are
generally rendered more or less sterile, while two domesticated varieties
when crossed and their mongrel offspring are perfectly fertile.
Turning to geographical distribution, the difficulties encountered on the
theory of descent with modification are serious enough. All the
individuals of the same species, and all the species of the same genus, or
even higher group, are descended from common parents; and therefore, in
however distant and isolated parts of the world they may now be found, they
must in the course of successive generations have travelled from some one
point to all the others. We are often wholly unable even to conjecture how
this could have been effected. Yet, as we have reason to believe that some
species have retained the same specific form for very long periods of time,
immensely long as measured by years, too much stress ought not to be laid
on the occasional wide diffusion of the same species; for during very long
periods there will always have been a good chance for wide migration by
many means. A broken or interrupted range may often be accounted for by
the extinction of the species in the intermediate regions. It cannot be
denied that we are as yet very ignorant as to the full extent of the
various climatical and geographical changes which have affected the earth
during modern periods; and such changes will often have facilitated
migration. As an example, I have attempted to show how potent has been the
influence of the Glacial period on the distribution of the same and of
allied species throughout the world. We are as yet profoundly ignorant of
the many occasional means of transport. With respect to distinct species
of the same genus, inhabiting distant and isolated regions, as the process
of modification has necessarily been slow, all the means of migration will
have been possible during a very long period; and consequently the
difficulty of the wide diffusion of the species of the same genus is in
some degree lessened.
As according to the theory of natural selection an interminable number of
intermediate forms must have existed, linking together all the species in
each group by gradations as fine as our existing varieties, it may be
asked, Why do we not see these linking forms all around us? Why are not
all organic beings blended together in an inextricable chaos? With respect
to existing forms, we should remember that we have no right to expect
(excepting in rare cases) to discover DIRECTLY connecting links between
them, but only between each and some extinct and supplanted form. Even on
a wide area, which has during a long period remained continuous, and of
which the climatic and other conditions of life change insensibly in
proceeding from a district occupied by one species into another district
occupied by a closely allied species, we have no just right to expect often
to find intermediate varieties in the intermediate zones. For we have
reason to believe that only a few species of a genus ever undergo change;
the other species becoming utterly extinct and leaving no modified progeny.
Of the species which do change, only a few within the same country change
at the same time; and all modifications are slowly effected. I have also
shown that the intermediate varieties which probably at first existed in
the intermediate zones, would be liable to be supplanted by the allied
forms on either hand; for the latter, from existing in greater numbers,
would generally be modified and improved at a quicker rate than the
intermediate varieties, which existed in lesser numbers; so that the
intermediate varieties would, in the long run, be supplanted and
exterminated.
On this doctrine of the extermination of an infinitude of connecting links,
between the living and extinct inhabitants of the world, and at each
successive period between the extinct and still older species, why is not
every geological formation charged with such links? Why does not every
collection of fossil remains afford plain evidence of the gradation and
mutation of the forms of life? Although geological research has
undoubtedly revealed the former existence of many links, bringing numerous
forms of life much closer together, it does not yield the infinitely many
fine gradations between past and present species required on the theory,
and this is the most obvious of the many objections which may be urged
against it. Why, again, do whole groups of allied species appear, though
this appearance is often false, to have come in suddenly on the successive
geological stages? Although we now know that organic beings appeared on
this globe, at a period incalculably remote, long before the lowest bed of
the Cambrian system was deposited, why do we not find beneath this system
great piles of strata stored with the remains of the progenitors of the
Cambrian fossils? For on the theory, such strata must somewhere have been
deposited at these ancient and utterly unknown epochs of the world's
history.
I can answer these questions and objections only on the supposition that
the geological record is far more imperfect than most geologists believe.
The number of specimens in all our museums is absolutely as nothing
compared with the countless generations of countless species which have
certainly existed. The parent form of any two or more species would not be
in all its characters directly intermediate between its modified offspring,
any more than the rock-pigeon is directly intermediate in crop and tail
between its descendants, the pouter and fantail pigeons. We should not be
able to recognise a species as the parent of another and modified species,
if we were to examine the two ever so closely, unless we possessed most of
the intermediate links; and owing to the imperfection of the geological
record, we have no just right to expect to find so many links. If two or
three, or even more linking forms were discovered, they would simply be
ranked by many naturalists as so many new species, more especially if found
in different geological substages, let their differences be ever so slight.
Numerous existing doubtful forms could be named which are probably
varieties; but who will pretend that in future ages so many fossil links
will be discovered, that naturalists will be able to decide whether or not
these doubtful forms ought to be called varieties? Only a small portion of
the world has been geologically explored. Only organic beings of certain
classes can be preserved in a fossil condition, at least in any great
number. Many species when once formed never undergo any further change but
become extinct without leaving modified descendants; and the periods during
which species have undergone modification, though long as measured by
years, have probably been short in comparison with the periods during which
they retained the same form. It is the dominant and widely ranging species
which vary most frequently and vary most, and varieties are often at first
local--both causes rendering the discovery of intermediate links in any one
formation less likely. Local varieties will not spread into other and
distant regions until they are considerably modified and improved; and when
they have spread, and are discovered in a geological formation, they appear
as if suddenly created there, and will be simply classed as new species.
Most formations have been intermittent in their accumulation; and their
duration has probably been shorter than the average duration of specific
forms. Successive formations are in most cases separated from each other
by blank intervals of time of great length, for fossiliferous formations
thick enough to resist future degradation can, as a general rule, be
accumulated only where much sediment is deposited on the subsiding bed of
the sea. During the alternate periods of elevation and of stationary level
the record will generally be blank. During these latter periods there will
probably be more variability in the forms of life; during periods of
subsidence, more extinction.
With respect to the absence of strata rich in fossils beneath the Cambrian
formation, I can recur only to the hypothesis given in the tenth chapter;
namely, that though our continents and oceans have endured for an enormous
period in nearly their present relative positions, we have no reason to
assume that this has always been the case; consequently formations much
older than any now known may lie buried beneath the great oceans. With
respect to the lapse of time not having been sufficient since our planet
was consolidated for the assumed amount of organic change, and this
objection, as urged by Sir William Thompson, is probably one of the gravest
as yet advanced, I can only say, firstly, that we do not know at what rate
species change, as measured by years, and secondly, that many philosophers
are not as yet willing to admit that we know enough of the constitution of
the universe and of the interior of our globe to speculate with safety on
its past duration.
That the geological record is imperfect all will admit; but that it is
imperfect to the degree required by our theory, few will be inclined to
admit. If we look to long enough intervals of time, geology plainly
declares that species have all changed; and they have changed in the manner
required by the theory, for they have changed slowly and in a graduated
manner. We clearly see this in the fossil remains from consecutive
formations invariably being much more closely related to each other than
are the fossils from widely separated formations.
Such is the sum of the several chief objections and difficulties which may
justly be urged against the theory; and I have now briefly recapitulated
the answers and explanations which, as far as I can see, may be given. I
have felt these difficulties far too heavily during many years to doubt
their weight. But it deserves especial notice that the more important
objections relate to questions on which we are confessedly ignorant; nor do
we know how ignorant we are. We do not know all the possible transitional
gradations between the simplest and the most perfect organs; it cannot be
pretended that we know all the varied means of Distribution during the long
lapse of years, or that we know how imperfect is the Geological Record.
Serious as these several objections are, in my judgment they are by no
means sufficient to overthrow the theory of descent with subsequent
modification.
Now let us turn to the other side of the argument. Under domestication we
see much variability, caused, or at least excited, by changed conditions of
life; but often in so obscure a manner, that we are tempted to consider the
variations as spontaneous. Variability is governed by many complex laws,
by correlated growth, compensation, the increased use and disuse of parts,
and the definite action of the surrounding conditions. There is much
difficulty in ascertaining how largely our domestic productions have been
modified; but we may safely infer that the amount has been large, and that
modifications can be inherited for long periods. As long as the conditions
of life remain the same, we have reason to believe that a modification,
which has already been inherited for many generations, may continue to be
inherited for an almost infinite number of generations. On the other hand
we have evidence that variability, when it has once come into play, does
not cease under domestication for a very long period; nor do we know that
it ever ceases, for new varieties are still occasionally produced by our
oldest domesticated productions.
Variability is not actually caused by man; he only unintentionally exposes
organic beings to new conditions of life and then nature acts on the
organisation and causes it to vary. But man can and does select the
variations given to him by nature, and thus accumulates them in any desired
manner. He thus adapts animals and plants for his own benefit or pleasure.
He may do this methodically, or he may do it unconsciously by preserving
the individuals most useful or pleasing to him without any intention of
altering the breed. It is certain that he can largely influence the
character of a breed by selecting, in each successive generation,
individual differences so slight as to be inappreciable except by an
educated eye. This unconscious process of selection has been the great
agency in the formation of the most distinct and useful domestic breeds.
That many breeds produced by man have to a large extent the character of
natural species, is shown by the inextricable doubts whether many of them
are varieties or aboriginally distinct species.
There is no reason why the principles which have acted so efficiently under
domestication should not have acted under nature. In the survival of
favoured individuals and races, during the constantly recurrent Struggle
for Existence, we see a powerful and ever-acting form of Selection. The
struggle for existence inevitably follows from the high geometrical ratio
of increase which is common to all organic beings. This high rate of
increase is proved by calculation--by the rapid increase of many animals
and plants during a succession of peculiar seasons, and when naturalised in
new countries. More individuals are born than can possibly survive. A
grain in the balance may determine which individuals shall live and which
shall die--which variety or species shall increase in number, and which
shall decrease, or finally become extinct. As the individuals of the same
species come in all respects into the closest competition with each other,
the struggle will generally be most severe between them; it will be almost
equally severe between the varieties of the same species, and next in
severity between the species of the same genus. On the other hand the
struggle will often be severe between beings remote in the scale of nature.
The slightest advantage in certain individuals, at any age or during any
season, over those with which they come into competition, or better
adaptation in however slight a degree to the surrounding physical
conditions, will, in the long run, turn the balance.
With animals having separated sexes, there will be in most cases a struggle
between the males for the possession of the females. The most vigorous
males, or those which have most successfully struggled with their
conditions of life, will generally leave most progeny. But success will
often depend on the males having special weapons or means of defence or
charms; and a slight advantage will lead to victory.
As geology plainly proclaims that each land has undergone great physical
changes, we might have expected to find that organic beings have varied
under nature, in the same way as they have varied under domestication. And
if there has been any variability under nature, it would be an
unaccountable fact if natural selection had not come into play. It has
often been asserted, but the assertion is incapable of proof, that the
amount of variation under nature is a strictly limited quantity. Man,
though acting on external characters alone and often capriciously, can
produce within a short period a great result by adding up mere individual
differences in his domestic productions; and every one admits that species
present individual differences. But, besides such differences, all
naturalists admit that natural varieties exist, which are considered
sufficiently distinct to be worthy of record in systematic works. No one
has drawn any clear distinction between individual differences and slight
varieties; or between more plainly marked varieties and subspecies and
species. On separate continents, and on different parts of the same
continent, when divided by barriers of any kind, and on outlying islands,
what a multitude of forms exist, which some experienced naturalists rank as
varieties, others as geographical races or sub species, and others as
distinct, though closely allied species!
If, then, animals and plants do vary, let it be ever so slightly or slowly,
why should not variations or individual differences, which are in any way
beneficial, be preserved and accumulated through natural selection, or the
survival of the fittest? If man can by patience select variations useful
to him, why, under changing and complex conditions of life, should not
variations useful to nature's living products often arise, and be preserved
or selected? What limit can be put to this power, acting during long ages
and rigidly scrutinising the whole constitution, structure, and habits of
each creature, favouring the good and rejecting the bad? I can see no
limit to this power, in slowly and beautifully adapting each form to the
most complex relations of life. The theory of natural selection, even if
we look no further than this, seems to be in the highest degree probable.
I have already recapitulated, as fairly as I could, the opposed
difficulties and objections: now let us turn to the special facts and
arguments in favour of the theory.
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 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 "Natura non
facit saltum," 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 fertilised 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 adapts 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 naturalised
productions from another land. Nor ought we to 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 the 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; and 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 the 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 the 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 a 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 distinct 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 different
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 tropical and temperate 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 at once see
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
organisation 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 specialised for different functions. This fact is perfectly
compatible with numerous beings still retaining simple and but little
improved structures, fitted for simple conditions of life; it is likewise
compatible with some forms having retrograded in organisation, 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 in Australia, of edentata 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 intertropical ocean. Although two
countries may present physical conditions as closely similar as the same
species ever require, 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
distinct 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 de 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, though
of paramount importance to the beings, are of hardly any importance in
classification; why 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.
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 on 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 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 of
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 problem of the essence or
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 any 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 vera causa 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 infinitely 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' 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 be
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.
Hereafter we shall be compelled to acknowledge that the only distinction
between species and well-marked varieties is, that the latter are known, or
believed to be connected at the present day by intermediate gradations,
whereas species were formerly thus connected. Hence, without rejecting the
consideration of the present existence of intermediate gradations between
any two forms, we shall be led to weigh more carefully and to value higher
the actual amount of difference between them. It is quite possible that
forms now generally acknowledged to be merely varieties may hereafter be
thought worthy of specific names; and in this case scientific and common
language will come into accordance. In short, we shall have to treat
species in the same manner as those naturalists treat genera, who admit
that genera are merely artificial combinations made for convenience. This
may not be a cheering prospect; but we shall at least be freed from the
vain search for the undiscovered and undiscoverable essence of the term
species.
The other and more general departments of natural history will rise greatly
in interest. The terms used by naturalists, of affinity, relationship,
community of type, paternity, morphology, adaptive characters, rudimentary
and aborted organs, etc., will cease to be metaphorical and will have a
plain signification. When we no longer look at an organic being as a
savage looks at a ship, as something wholly beyond his comprehension; when
we regard every production of nature as one which has had a long history;
when we contemplate every complex structure and instinct as the summing up
of many contrivances, each useful to the possessor, in the same way as any
great mechanical invention is the summing up of the labour, the experience,
the reason, and even the blunders of numerous workmen; when we thus view
each organic being, how far more interesting--I speak from experience--does
the study of natural history become!
A grand and almost untrodden field of inquiry will be opened, on the causes
and laws of variation, on correlation, on the effects of use and disuse, on
the direct action of external conditions, and so forth. The study of
domestic productions will rise immensely in value. A new variety raised by
man will be a far more important and interesting subject for study than one
more species added to the infinitude of already recorded species. Our
classifications will come to be, as far as they can be so made,
genealogies; and will then truly give what may be called the plan of
creation. The rules for classifying will no doubt become simpler when we
have a definite object in view. We possess no pedigree or armorial
bearings; and we have to discover and trace the many diverging lines of
descent in our natural genealogies, by characters of any kind which have
long been inherited. Rudimentary organs will speak infallibly with respect
to the nature of long-lost structures. Species and groups of species which
are called aberrant, and which may fancifully be called living fossils,
will aid us in forming a picture of the ancient forms of life. Embryology
will often reveal to us the structure, in some degree obscured, of the
prototypes of each great class.
When we can feel assured that all the individuals of the same species, and
all the closely allied species of most genera, have, within a not very
remote period descended from one parent, and have migrated from some one
birth-place; and when we better know the many means of migration, then, by
the light which geology now throws, and will continue to throw, on former
changes of climate and of the level of the land, we shall surely be enabled
to trace in an admirable manner the former migrations of the inhabitants of
the whole world. Even at present, by comparing the differences between the
inhabitants of the sea on the opposite sides of a continent, and the nature
of the various inhabitants of that continent in relation to their apparent
means of immigration, some light can be thrown on ancient geography.
The noble science of geology loses glory from the extreme imperfection of
the record. The crust of the earth, with its embedded remains, must not be
looked at as a well-filled museum, but as a poor collection made at hazard
and at rare intervals. The accumulation of each great fossiliferous
formation will be recognised as having depended on an unusual occurrence of
favourable circumstances, and the blank intervals between the successive
stages as having been of vast duration. But we shall be able to gauge with
some security the duration of these intervals by a comparison of the
preceding and succeeding organic forms. We must be cautious in attempting
to correlate as strictly contemporaneous two formations, which do not
include many identical species, by the general succession of the forms of
life. As species are produced and exterminated by slowly acting and still
existing causes, and not by miraculous acts of creation; and as the most
important of all causes of organic change is one which is almost
independent of altered and perhaps suddenly altered physical conditions,
namely, the mutual relation of organism to organism--the improvement of one
organism entailing the improvement or the extermination of others; it
follows, that the amount of organic change in the fossils of consecutive
formations probably serves as a fair measure of the relative, though not
actual lapse of time. A number of species, however, keeping in a body
might remain for a long period unchanged, whilst within the same period,
several of these species, by migrating into new countries and coming into
competition with foreign associates, might become modified; so that we must
not overrate the accuracy of organic change as a measure of time.
In the future I see open fields for far more important researches.
Psychology will be securely based on the foundation already well laid by
Mr. Herbert Spencer, that of the necessary acquirement of each mental power
and capacity by gradation. Much light will be thrown on the origin of man
and his history.
Authors of the highest eminence seem to be fully satisfied with the view
that each species has been independently created. To my mind it accords
better with what we know of the laws impressed on matter by the Creator,
that the production and extinction of the past and present inhabitants of
the world should have been due to secondary causes, like those determining
the birth and death of the individual. When I view all beings not as
special creations, but as the lineal descendants of some few beings which
lived long before the first bed of the Cambrian system was deposited, they
seem to me to become ennobled. Judging from the past, we may safely infer
that not one living species will transmit its unaltered likeness to a
distinct futurity. And of the species now living very few will transmit
progeny of any kind to a far distant futurity; for the manner in which all
organic beings are grouped, shows that the greater number of species in
each genus, and all the species in many genera, have left no descendants,
but have become utterly extinct. We can so far take a prophetic glance
into futurity as to foretell that it will be the common and widely spread
species, belonging to the larger and dominant groups within each class,
which will ultimately prevail and procreate new and dominant species. As
all the living forms of life are the lineal descendants of those which
lived long before the Cambrian epoch, we may feel certain that the ordinary
succession by generation has never once been broken, and that no cataclysm
has desolated the whole world. Hence, we may look with some confidence to
a secure future of great length. And as natural selection works solely by
and for the good of each being, all corporeal and mental endowments will
tend to progress towards perfection.
It is interesting to contemplate a tangled bank, clothed with many plants
of many kinds, with birds singing on the bushes, with various insects
flitting about, and with worms crawling through the damp earth, and to
reflect that these elaborately constructed forms, so different from each
other, and dependent upon each other in so complex a manner, have all been
produced by laws acting around us. These laws, taken in the largest sense,
being Growth with reproduction; Inheritance which is almost implied by
reproduction; Variability from the indirect and direct action of the
conditions of life, and from use and disuse; a Ratio of Increase so high as
to lead to a Struggle for Life, and as a consequence to Natural Selection,
entailing Divergence of Character and the Extinction of less improved
forms. Thus, from the war of nature, from famine and death, the most
exalted object which we are capable of conceiving, namely, the production
of the higher animals, directly follows. There is grandeur in this view of
life, with its several powers, having been originally breathed by the
Creator into a few forms or into one; and that, whilst this planet has gone
circling on according to the fixed law of gravity, from so simple a
beginning endless forms most beautiful and most wonderful have been, and
are being evolved.
GLOSSARY OF THE PRINCIPAL SCIENTIFIC TERMS USED IN THE PRESENT VOLUME.
(I am indebted to the kindness of Mr. W.S. Dallas for this Glossary, which
has been given because several readers have complained to me that some of
the terms used were unintelligible to them. Mr. Dallas has endeavoured to
give the explanations of the terms in as popular a form as possible.)
ABERRANT.--Forms or groups of animals or plants which deviate in important
characters from their nearest allies, so as not to be easily included in
the same group with them, are said to be aberrant.
ABERRATION (in Optics).--In the refraction of light by a convex lens the
rays passing through different parts of the lens are brought to a focus at
slightly different distances--this is called SPHERICAL ABERRATION; at the
same time the coloured rays are separated by the prismatic action of the
lens and likewise brought to a focus at different distances--this is
CHROMATIC ABERRATION.
ABNORMAL.--Contrary to the general rule.
ABORTED.--An organ is said to be aborted, when its development has been
arrested at a very early stage.
ALBINISM.--Albinos are animals in which the usual colouring matters
characteristic of the species have not been produced in the skin and its
appendages. Albinism is the state of being an albino.
ALGAE.--A class of plants including the ordinary sea-weeds and the
filamentous fresh-water weeds.
ALTERNATION OF GENERATIONS.--This term is applied to a peculiar mode of
reproduction which prevails among many of the lower animals, in which the
egg produces a living form quite different from its parent, but from which
the parent-form is reproduced by a process of budding, or by the division
of the substance of the first product of the egg.
AMMONITES.--A group of fossil, spiral, chambered shells, allied to the
existing pearly Nautilus, but having the partitions between the chambers
waved in complicated patterns at their junction with the outer wall of the
shell.
ANALOGY.--That resemblance of structures which depends upon similarity of
function, as in the wings of insects and birds. Such structures are said
to be ANALOGOUS, and to be ANALOGUES of each other.
ANIMALCULE.--A minute animal: generally applied to those visible only by
the microscope.
ANNELIDS.--A class of worms in which the surface of the body exhibits a
more or less distinct division into rings or segments, generally provided
with appendages for locomotion and with gills. It includes the ordinary
marine worms, the earth-worms, and the leeches.
ANTENNAE.--Jointed organs appended to the head in Insects, Crustacea and
Centipedes, and not belonging to the mouth.
ANTHERS.--The summits of the stamens of flowers, in which the pollen or
fertilising dust is produced.
APLACENTALIA, APLACENTATA or APLACENTAL MAMMALS.--See MAMMALIA.
ARCHETYPAL.--Of or belonging to the Archetype, or ideal primitive form upon
which all the beings of a group seem to be organised.
ARTICULATA.--A great division of the Animal Kingdom characterised generally
by having the surface of the body divided into rings called segments, a
greater or less number of which are furnished with jointed legs (such as
Insects, Crustaceans and Centipedes).
ASYMMETRICAL.--Having the two sides unlike.
ATROPHIED.--Arrested in development at a very early stage.
BALANUS.--The genus including the common Acorn-shells which live in
abundance on the rocks of the sea-coast.
BATRACHIANS.--A class of animals allied to the Reptiles, but undergoing a
peculiar metamorphosis, in which the young animal is generally aquatic and
breathes by gills. (Examples, Frogs, Toads, and Newts.)
BOULDERS.--Large transported blocks of stone generally embedded in clays or
gravels.
BRACHIOPODA.--A class of marine Mollusca, or soft-bodied animals, furnished
with a bivalve shell, attached to submarine objects by a stalk which passes
through an aperture in one of the valves, and furnished with fringed arms,
by the action of which food is carried to the mouth.
BRANCHIAE.--Gills or organs for respiration in water.
BRANCHIAL.--Pertaining to gills or branchiae.
CAMBRIAN SYSTEM.--A series of very ancient Palaeozoic rocks, between the
Laurentian and the Silurian. Until recently these were regarded as the
oldest fossiliferous rocks.
CANIDAE.--The Dog-family, including the Dog, Wolf, Fox, Jackal, etc.
CARAPACE.--The shell enveloping the anterior part of the body in
Crustaceans generally; applied also to the hard shelly pieces of the
Cirripedes.
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