Lucy Fitch Perkins - "THE JAPANESE TWINS"

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Emile Zola - "The Three Cities Trilogy: Lourdes, Complete"

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Books: On the Origin of Species

C >> Charles Darwin >> On the Origin of Species




As the evidence appears to me conclusive, that the several domestic
breeds of Pigeon have descended from one wild species, I compared
young pigeons of various breeds, within twelve hours after being
hatched; I carefully measured the proportions (but will not here give
details) of the beak, width of mouth, length of nostril and of eyelid,
size of feet and length of leg, in the wild stock, in pouters,
fantails, runts, barbs, dragons, carriers, and tumblers. Now some of
these birds, when mature, differ so extraordinarily in length and form
of beak, that they would, I cannot doubt, be ranked in distinct
genera, had they been natural productions. But when the nestling birds
of these several breeds were placed in a row, though most of them
could be distinguished from each other, yet their proportional
differences in the above specified several 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 all its
proportions, almost exactly as much as in the adult state.

The two principles above given seem to me to explain these facts in
regard to the later embryonic stages of our domestic varieties.
Fanciers select their horses, dogs, and pigeons, for breeding, when
they are nearly grown up: they are indifferent whether the desired
qualities and structures have been acquired earlier or later in life,
if the full-grown animal possesses them. And the cases just given,
more especially that of pigeons, seem to show that the characteristic
differences which give value to each breed, and which have been
accumulated by man's selection, have not generally first appeared at
an early period of life, and have been inherited by the offspring at a
corresponding not early period. But the case of the short-faced
tumbler, which when twelve hours old had acquired its proper
proportions, 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 the corresponding, but at an earlier age.

Now let us apply these facts and the above two principles--which
latter, though not proved true, can be shown to be in some degree
probable--to species in a state of nature. Let us take a genus of
birds, descended on my theory from some one parent-species, and of
which the several new species have become modified through natural
selection in accordance with their diverse habits. Then, from the many
slight successive steps of variation having supervened at a rather
late age, and having been inherited at a corresponding age, the young
of the new species of our supposed genus will manifestly tend to
resemble each other much more closely than do the adults, just as we
have seen in the case of pigeons. We may extend this view to whole
families or even classes. The fore-limbs, for instance, which served
as legs in the parent-species, may become, by a long course of
modification, adapted in one descendant to act as hands, in another as
paddles, in another as wings; and on the above two principles--namely
of each successive modification supervening at a rather late age, and
being inherited at a corresponding late age--the fore-limbs in the
embryos of the several descendants of the parent-species will still
resemble each other closely, for they will not have been modified. But
in each individual new species, the embryonic fore-limbs will differ
greatly from the fore-limbs in the mature animal; the limbs in the
latter having undergone much modification at a rather late period of
life, and having thus been converted into hands, or paddles, or wings.
Whatever influence long-continued exercise or use on the one hand, and
disuse on the other, may have in modifying an organ, such influence
will mainly affect the mature animal, which has come to its full
powers of activity and has to gain its own living; and the effects
thus produced will be inherited at a corresponding mature age. Whereas
the young will remain unmodified, or be modified in a lesser degree,
by the effects of use and disuse.

In certain cases the successive steps of variation might supervene,
from causes of which we are wholly ignorant, at a very early period of
life, or each step might be inherited at an earlier period than that
at which it first appeared. In either case (as with the short-faced
tumbler) the young or embryo would closely resemble the mature
parent-form. We have seen that this is the rule of development in
certain whole groups of animals, as with cuttle-fish and spiders, and
with a few members of the great class of insects, as with Aphis. With
respect to the final cause of the young in these cases not undergoing
any metamorphosis, or closely resembling their parents from their
earliest age, we can see that this would result from the two following
contingencies; firstly, from the young, during a course of
modification carried on for many generations, having to provide for
their own wants at a very early stage of development, and secondly,
from their following exactly the same habits of life with their
parents; for in this case, it would be indispensable for the existence
of the species, that the child should be modified at a very early age
in the same manner with its parents, in accordance with their similar
habits. Some further explanation, however, of the embryo not
undergoing any metamorphosis is perhaps requisite. If, on the other
hand, it profited the young to follow habits of life in any degree
different from those of their parent, and consequently to be
constructed in a slightly different manner, then, on the principle of
inheritance at corresponding ages, the active young or larvae might
easily be rendered by natural selection different to any conceivable
extent from their parents. Such differences might, also, become
correlated with successive stages of development; so that the larvae,
in the first stage, might differ greatly from the larvae in the second
stage, as we have seen to be the case with cirripedes. The adult might
become fitted for sites or habits, in which organs of locomotion or of
the senses, etc., would be useless; and in this case the final
metamorphosis would be said to be retrograde.

As all the organic beings, extinct and recent, which have ever lived
on this earth have to be classed together, and as all have been
connected by the finest gradations, the best, or indeed, if our
collections were nearly perfect, the only possible arrangement, would
be genealogical. Descent being on my view 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. For the embryo is the
animal in its less modified state; and in so far it reveals the
structure of its progenitor. In two groups of animal, however much
they may at present differ from each other in structure and habits, if
they pass through the same or similar embryonic stages, we may feel
assured that they have both descended from the same or nearly similar
parents, and are therefore in that degree closely related. Thus,
community in embryonic structure reveals community of descent. It will
reveal this community of descent, however much the structure of the
adult may have been modified and obscured; we have seen, for instance,
that cirripedes can at once be recognised by their larvae as belonging
to the great class of crustaceans. As the embryonic state of each
species and group of species partially shows us the structure of their
less modified ancient progenitors, we can clearly see why ancient and
extinct forms of life should resemble the embryos of their
descendants,--our existing species. Agassiz believes this to be a law
of nature; but I am bound to confess that I only hope to see the law
hereafter proved true. It can be proved true in those cases alone in
which the ancient state, now supposed to be represented in many
embryos, has not been obliterated, either by the successive variations
in a long course of modification having supervened at a very early
age, or by the variations having been inherited at an earlier period
than that at which they first appeared. It should also be borne in
mind, that the supposed law of resemblance of ancient forms of life to
the embryonic stages of recent forms, 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.

Thus, as it seems to me, the leading facts in embryology, which are
second in importance to none in natural history, are explained on the
principle of slight modifications not appearing, in the many
descendants from some one ancient progenitor, at a very early period
in the life of each, though perhaps caused at the earliest, and being
inherited at a corresponding not early period. Embryology rises
greatly in interest, when we thus look at the embryo as a picture,
more or less obscured, of the common parent-form of each great class
of animals.

RUDIMENTARY, ATROPHIED, OR ABORTED ORGANS.

Organs or parts in this strange condition, bearing the stamp of
inutility, are extremely common throughout nature. For instance,
rudimentary mammae are very general in the males of mammals: I presume
that the "bastard-wing" in birds may be safely considered as a digit
in a rudimentary state: in very many snakes one lobe of the lungs is
rudimentary; in other snakes there are rudiments of the pelvis and
hind limbs. Some of the cases of rudimentary organs are extremely
curious; for instance, the presence of teeth in foetal whales, which
when grown up have not a tooth in their heads; and the presence of
teeth, which never cut through the gums, in the upper jaws of our
unborn calves. It has even been stated on good authority that
rudiments of teeth can be detected in the beaks of certain embryonic
birds. Nothing can be plainer than that wings are formed for flight,
yet in how many insects do we see wings so reduced in size as to be
utterly incapable of flight, and not rarely lying under wing-cases,
firmly soldered together!

The meaning of rudimentary organs is often quite unmistakeable: for
instance there are beetles of the same genus (and even of the same
species) resembling each other most closely in all respects, one of
which will have full-sized wings, and another mere rudiments of
membrane; and here it is impossible to doubt, that the rudiments
represent wings. Rudimentary organs sometimes retain their
potentiality, and are merely not developed: this seems to be the case
with the mammae of male mammals, for many instances are on record of
these organs having become well developed in full-grown males, and
having secreted milk. So again there are normally four developed and
two rudimentary teats in the udders of the genus Bos, but in our
domestic cows the two sometimes become developed and give milk. In
individual plants of the same species the petals sometimes occur as
mere rudiments, and sometimes in a well-developed state. In plants
with separated sexes, the male flowers often have a rudiment of a
pistil; and Kolreuter found that by crossing such male plants with an
hermaphrodite species, the rudiment of the pistil in the hybrid
offspring was much increased in size; and this shows that the rudiment
and the perfect pistil are essentially alike in nature.

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 protected in the ovarium
at its base. 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 pistil, which is in a rudimentary state, for it is
not crowned with a stigma; but the style remains well developed, and
is clothed with hairs as in other compositae, for the purpose of
brushing the pollen out of the surrounding anthers. Again, an organ
may become rudimentary for its proper purpose, and be used for a
distinct object: in certain fish 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. Other similar
instances could be given.

Rudimentary organs in the individuals of the same species are very
liable to vary in degree of development and in other respects.
Moreover, in closely allied species, the degree to which the same
organ has been rendered rudimentary occasionally differs much. This
latter fact is well exemplified in the state of the wings of the
female moths in certain groups. Rudimentary organs may be utterly
aborted; and this implies, that we find in an animal or plant no trace
of an organ, which analogy would lead us to expect to find, and which
is occasionally found in monstrous individuals of the species. Thus in
the snapdragon (antirrhinum) we generally do not find a rudiment of a
fifth stamen; but this may sometimes be seen. In tracing the
homologies of the same part in different members of a class, nothing
is more common, or more necessary, than the use and discovery of
rudiments. This is well shown in the drawings given by Owen of the
bones of the leg 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 or organ is of greater size
relatively to the adjoining parts in the embryo, 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, also, a rudimentary
organ in the adult, is often said to have retained its 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 plainly 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 seems to me no
explanation, merely a restatement of the fact. Would it be thought
sufficient to say that because planets revolve in elliptic courses
round the sun, satellites follow the same course round the planets,
for the sake of symmetry, and to complete the scheme of nature? An
eminent physiologist accounts for the presence of rudimentary organs,
by supposing that they serve to excrete matter in excess, or injurious
to the system; but can we suppose that the minute papilla, which often
represents the pistil in male flowers, and which is formed merely of
cellular tissue, can thus act? Can we suppose that the formation of
rudimentary teeth which are subsequently absorbed, can be of any
service to the rapidly growing embryonic calf by the excretion of
precious phosphate of lime? When a man's fingers have been amputated,
imperfect nails sometimes appear on the stumps: I could as soon
believe that these vestiges of nails have appeared, not from unknown
laws of growth, but in order to excrete horny matter, as that the
rudimentary nails on the fin of the manatee were formed for this
purpose.

On my view of descent with modification, the origin of rudimentary
organs is simple. 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,--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 I doubt whether species
under nature ever undergo abrupt changes. I believe that disuse has
been the main agency; that it has led in successive generations to the
gradual reduction of various organs, until they have become
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 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 would continue slowly to reduce the organ, until it was
rendered harmless and rudimentary.

Any change in function, which can be effected by insensibly small
steps, is within the power of natural selection; so that an organ
rendered, during changed habits of life, useless or injurious for one
purpose, might easily be modified and used for another purpose. Or an
organ might be retained for one alone of its former functions. An
organ, when rendered useless, may well be variable, for its variations
cannot be checked by natural selection. At whatever period of life
disuse or selection reduces an organ, and this will generally be when
the being has come to maturity and to its full powers of action, the
principle of inheritance at corresponding ages will reproduce the
organ in its reduced state at the same age, and consequently will
seldom affect or reduce it in the embryo. Thus we can understand the
greater relative size of rudimentary organs in the embryo, and their
lesser relative size in the adult. But if each step of the process of
reduction were to be inherited, not at the corresponding age, but at
an extremely early period of life (as we have good reason to believe
to be possible) the rudimentary part would tend to be wholly lost, and
we should have a case of complete abortion. The principle, also, of
economy, explained in a former chapter, by which the materials forming
any part or structure, if not useful to the possessor, will be saved
as far as is possible, will probably often come into play; and this
will tend to cause the entire obliteration of a rudimentary organ.

As the presence of rudimentary organs is thus due to the tendency in
every part of the organisation, which has long existed, to be
inherited--we can understand, on the genealogical view of
classification, how it is that systematists have 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 in seeking 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 ordinary doctrine of creation, might even
have been anticipated, and can be accounted for by the laws of
inheritance.

SUMMARY.

In this chapter I have attempted to show, that the subordination of
group to group in all organisms throughout all time; that the nature
of the relationship, by which all living and extinct beings are united
by complex, radiating, and circuitous lines of affinities into one
grand system; the rules followed and the difficulties encountered by
naturalists in their classifications; the value set upon characters,
if constant and prevalent, whether of high vital importance, or of the
most trifling importance, or, as in 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 on the view of the common parentage of
those forms which are considered by naturalists as allied, together
with their modification through natural selection, with its
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, and acknowledged varieties of the same
species, however different they may be in structure. If we extend the
use of this element of descent,--the only 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, all the great facts in
Morphology become intelligible,--whether we look to the same pattern
displayed in the homologous organs, to whatever purpose applied, of
the different species of a class; or to the homologous parts
constructed on the same pattern 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 great
leading facts in Embryology; namely, the resemblance in an individual
embryo of the homologous parts, which when matured will become widely
different from each other in structure and function; and the
resemblance in different species of a class of the homologous parts or
organs, though fitted in the adult members for purposes as different
as possible. Larvae are active embryos, which have become specially
modified in relation to their habits of life, through the principle of
modifications being inherited at corresponding ages. On this same
principle--and bearing in mind, that when organs are reduced in size,
either from disuse or 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 principle of inheritance--the occurrence of
rudimentary organs and their final abortion, present to us no
inexplicable difficulties; on the contrary, their presence might have
been even anticipated. The importance of embryological characters and
of rudimentary organs in classification is intelligible, on the view
that an arrangement is only so far natural as it is 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 of organic beings, with which this world
is peopled, have 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 14. RECAPITULATION AND CONCLUSION.

Recapitulation of the difficulties on 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 grave objections may be advanced against the theory of
descent with modification through 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 should 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 gradations in the perfection of any organ
or instinct, which we may consider, either do now exist or could have
existed, each good of its kind,--that all organs and instincts are, in
ever so slight a degree, variable,--and, lastly, that there is a
struggle for existence leading to the preservation of each profitable
deviation of structure or instinct. 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
amongst broken and failing groups of organic beings; but we see so
many strange gradations in nature, as is proclaimed by the canon,
"Natura non facit saltum," that we ought to be extremely cautious in
saying that any organ or instinct, or any whole being, could not have
arrived at its present state by many graduated steps. There are, it
must be admitted, cases of special difficulty on the theory of natural
selection; and one of the most curious of these is the existence of
two or three defined castes of workers or sterile females in the same
community of ants; but I have attempted to show how this difficulty
can be mastered.