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 (_Chlöeon_), during its
development, molts, 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 climax 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 larvæ, and these others, which
finally are developed into mature males and females, propagating their
kind in the ordinary manner by eggs.
* * * * *
[Page 387.]
It has been already 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 can not be given than the statement by Von Baer that
“the embryos of mammalia, of birds, lizards, and snakes, probably also
of chelonia, are in their 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 larvæ 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 _Leguminosæ_.
TWO PRINCIPLES THAT EXPLAIN THE FACTS.
[Page 390.]
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 larvæ 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, larvæ,
which have to provide for their own wants, being perfectly adapted to
the surrounding conditions; and, lastly, the fact of certain larvæ
standing higher in the scale of organization 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, can not positively tell, until some time after
birth, what will be the merits or demerits of their young animals.
We see this plainly in our own children; we can not 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 each 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 remains in
its mother’s womb or in the egg, or 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 a 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 first 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.
EMBRYOLOGY AGAINST ABRUPT CHANGES.
[Origin of
Species,
page 203.]
Unless we admit transformations as prodigious as those advocated by
Mr. Mivart, such as the sudden development of the wings of birds or
bats, or the sudden conversion of a Hipparion into a horse, hardly any
light is thrown by the belief in abrupt modifications on the deficiency
of connecting links in our geological formations. But against the
belief in such abrupt changes embryology enters a strong protest. It
is notorious that the wings of birds and bats, and the legs of horses
or other quadrupeds, are undistinguishable at an early embryonic
period, and that they become differentiated by insensibly fine steps.
Embryological resemblances of all kinds can be accounted for, as we
shall hereafter see, by the progenitors of our existing species having
varied after early youth, and having transmitted their newly acquired
characters to their offspring at a corresponding age. The embryo
is thus left almost unaffected, and serves as a record of the past
condition of the species. Hence it is that existing species during
the early stages of their development so often resemble ancient and
extinct forms belonging to the same class. On this view of the meaning
of embryological resemblances, and indeed on any view, it is incredible
that an animal should have undergone such momentous and abrupt
transformations as those above indicated, and yet should not bear even
a trace in its embryonic condition of any sudden modification, every
detail in its structure being developed by insensibly fine steps.
He who believes that some ancient form was transformed suddenly through
an internal force or tendency into, for instance, one furnished with
wings, will be almost compelled to assume, in opposition to all
analogy, that many individuals varied simultaneously. It can not be
denied that such abrupt and great changes of structure are widely
different from those which most species apparently have undergone. He
will further be compelled to believe that many structures beautifully
adapted to all the other parts of the same creature and to the
surrounding conditions, have been suddenly produced; and of such
complex and wonderful coadaptations he will not be able to assign a
shadow of an explanation. He will be forced to admit that these great
and sudden transformations have left no trace of their action on the
embryo. To admit all this is, as it seems to me, to enter into the
realms of miracle, and to leave those of science.
RUDIMENTARY ORGANS ONLY TO BE EXPLAINED ON THE THEORY OF DEVELOPMENT.
[Descent of
Man, page 11.]
Not one of the higher animals can be named which does not bear some
part in a rudimentary condition; and man forms no exception to the
rule. Rudimentary organs must be distinguished from those that are
nascent, though in some cases the distinction is not easy. The former
are either absolutely useless, such as the mammæ of male quadrupeds,
or the incisor teeth of ruminants which never cut through the gums; or
they are of such slight service to their present possessors that we
can hardly suppose that they were developed under the conditions which
now exist. Organs in this latter state are not strictly rudimentary,
but they are tending in this direction. Nascent organs, on the other
hand, though not fully developed, are of high service to their
possessors, and are capable of further development. Rudimentary organs
are eminently variable; and this is partly intelligible, as they are
useless, or nearly useless, and consequently are no longer subjected
to natural selection. They often become wholly suppressed. When this
occurs, they are nevertheless liable to occasional reappearance through
reversion--a circumstance well worthy of attention.
* * * * *
[Page 12.]
Rudiments of various muscles have been observed in many parts of the
human body; and not a few muscles which are regularly present in some
of the lower animals can occasionally be detected in man in a greatly
reduced condition. Every one must have noticed the power which many
animals, especially horses, possess of moving or twitching their skin;
and this is effected by the _panniculus carnosus_. Remnants of this
muscle in an efficient state are found in various parts of our bodies:
for instance, the muscle on the forehead, by which the eyebrows are
raised.
* * * * *
[Page 13.]
Some few persons have the power of contracting the superficial muscles
on their scalps; and these muscles are in a variable and partially
rudimentary condition. M. A. de Candolle has communicated to me a
curious instance of the long-continued persistence or inheritance of
this power, as well as of its unusual development. He knows a family in
which one member, the present head of the family, could, when a youth,
pitch several heavy books from his head by the movement of the scalp
alone; and he won wagers by performing this feat. His father, uncle,
grandfather, and his three children possess the same power to the same
unusual degree. This family became divided eight generations ago into
two branches; so that the head of the above-mentioned branch is cousin
in the seventh degree to the head of the other branch. This distant
cousin resides in another part of France; and, on being asked whether
he possessed the same faculty, immediately exhibited his power. This
case offers a good illustration how persistent may be the transmission
of an absolutely useless faculty, probably derived from our remote
semi-human progenitors, since many monkeys have, and frequently use,
the power of largely moving their scalps up and down.
* * * * *
[Page 23.]
It is well known that in the males of all mammals, including man,
rudimentary mammæ exist. These in several instances have become well
developed, and have yielded a copious supply of milk. Their essential
identity in the two sexes is likewise shown by their occasional
sympathetic enlargement in both during an attack of the measles.
“NO OTHER EXPLANATION HAS EVER BEEN GIVEN.”
[Page 24.]
The homological construction of the whole frame in the members of the
same class is intelligible, if we admit their descent from a common
progenitor, together with their subsequent adaptation to diversified
conditions. On any other view, the similarity of pattern between the
hand of a man or monkey, the foot of a horse, the flipper of a seal,
the wing of a bat, etc., is utterly inexplicable. It is no scientific
explanation to assert that they have all been formed on the same
ideal plan. With respect to development, we can clearly understand,
on the principle of variations supervening at a rather late embryonic
period, and being inherited at a corresponding period, how it is
that the embryos of wonderfully different forms should still retain,
more or less perfectly, the structure of their common progenitor. No
other explanation has ever been given of the marvelous fact that the
embryos of a man, dog, seal, bat, reptile, etc., can at first hardly be
distinguished from each other. In order to understand the existence of
rudimentary organs, we have only to suppose that a former progenitor
possessed the parts in question in a perfect state, and that under
changed habits of life they became greatly reduced, either from simple
disuse or through the natural selection of those individuals which were
least encumbered with a superfluous part, aided by the other means
previously indicated.
Thus we can understand how it has come to pass that man and all other
vertebrate animals have been constructed on the same general model, why
they pass through the same early stages of development, and why they
retain certain rudiments in common. Consequently we ought frankly to
admit their community of descent; to take any other view is to admit
that our own structure, and that of all the animals around us, is a
mere snare laid to entrap our judgment. This conclusion is greatly
strengthened, if we look to the members of the whole animal series, and
consider the evidence derived from their affinities or classification,
their geographical distribution, and geological succession. It is only
our natural prejudice and that arrogance which made our forefathers
declare that they were descended from demi-gods which leads us to demur
to this conclusion. But the time will before long come when it will be
thought wonderful that naturalists, who were well acquainted with the
comparative structure and development of man and other mammals, should
have believed that each was the work of a separate act of creation.
UNITY OF TYPE EXPLAINED BY RELATIONSHIP.
[Origin of
Species,
page 382.]
We have seen that the members of the same class, independently of
their habits of life, resemble each other in the general plan of their
organization. This resemblance is often expressed by the term “unity
of type”; or by saying that the several parts and organs in the
different species of the class are homologous. The whole subject is
included under the general term of Morphology. This is one of the most
interesting departments of natural history, and may almost be said to
be its very soul. What can be more curious than that the hand of a man,
formed for grasping, that of a mole for digging, the leg of the horse,
the paddle of the porpoise, and the wing of the bat, should all be
constructed on the same pattern, and should include similar bones, in
the same relative positions? How curious it is, to give a subordinate
though striking instance, that the hind-feet of the kangaroo, which
are so well fitted for bounding over the open plains, those of the
climbing, leaf-eating koala, equally well fitted for grasping the
branches of trees, those of the ground-dwelling, insect or root eating,
bandicoots, and those of some other Australian marsupials, should all
be constructed on the same extraordinary type, namely, with the bones
of the second and third digits extremely slender and enveloped within
the same skin, so that they appear like a single toe furnished with
two claws! Notwithstanding this similarity of pattern, it is obvious
that the hind-feet of these several animals are used for as widely
different purposes as it is possible to conceive. The case is rendered
all the more striking by the American opossums, which follow nearly
the same habits of life as some of their Australian relatives, having
feet constructed on the ordinary plan. Professor Flower, from whom
these statements are taken, remarks in conclusion, “We may call this
conformity to type, without getting much nearer to an explanation of
the phenomenon”; and he then adds, “but is it not powerfully suggestive
of true relationship, of inheritance from a common ancestor?”
INEXPLICABLE ON THE ORDINARY VIEW OF CREATION.
[Page 384.]
How inexplicable are the cases of serial homologies on the ordinary
view of creation! Why should the brain be inclosed in a box composed
of such numerous and such extraordinarily shaped pieces of bone,
apparently representing vertebræ? 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
specialized forms; therefore the unknown progenitor of the Vertebrata
probably possessed many vertebræ; 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.
DESCENT WITH MODIFICATION THE ONLY EXPLANATION.
[Origin of
Species,
page 400.]
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.
THE HISTORY OF LIFE ON THE THEORY OF DESCENT WITH MODIFICATION.
[Origin of
Species,
page 424.]
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 can not 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.