John Lothrop Motley - "The Rise of the Dutch Republic, 1573 74"

Roy Benson, Jr. - "The Biography of a Rabbit"

Samuel Butler - "The Note Books of Samuel Butler"

Rudyard Kipling - "The Light That Failed"

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FlatSigned Press Alleges Don Imus Remarks Damage Legacy of President Gerald R. Ford
NEW YORK, N.Y. -- Nathan Yungerberg, an accomplished model scout and professional child photographer is launching a nation-wide casting call to find the cover model for his highly anticipated book release, 'The Model Child: A Parents Guide to the Child Modeling Industry' (ISBN: 978-0-9817018-0-6).

Books: On the Origin of Species

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




Extinction, as we have seen in the fourth chapter, has played an
important part in defining and widening the intervals between the
several groups in each class. We may thus account even for the
distinctness of whole classes from each other--for instance, of birds
from all other vertebrate animals--by the belief that many ancient
forms of life have been utterly lost, through which the early
progenitors of birds were formerly connected with the early
progenitors of the other vertebrate classes. There has been less
entire extinction of the forms of life which once connected fishes
with batrachians. There has been still less in some other classes, as
in that of the Crustacea, for here the most wonderfully diverse forms
are still tied together by a long, but broken, chain of affinities.
Extinction has only separated groups: it has by no means made them;
for if every form which has ever lived on this earth were suddenly to
reappear, though it would be quite impossible to give definitions by
which each group could be distinguished from other groups, as all
would blend together by steps as fine as those between the finest
existing varieties, nevertheless a natural classification, or at least
a natural arrangement, would be possible. We shall see this by turning
to the diagram: the letters, A to L, may represent eleven Silurian
genera, some of which have produced large groups of modified
descendants. Every intermediate link between these eleven genera and
their primordial parent, and every intermediate link in each branch
and sub-branch of their descendants, may be supposed to be still
alive; and the links to be as fine as those between the finest
varieties. In this case it would be quite impossible to give any
definition by which the several members of the several groups could be
distinguished from their more immediate parents; or these parents from
their ancient and unknown progenitor. Yet the natural arrangement in
the diagram would still hold good; and, on the principle of
inheritance, all the forms descended from A, or from I, would have
something in common. In a tree we can specify this or that branch,
though at the actual fork the two unite and blend together. We could
not, as I have said, define the several groups; but we could pick out
types, or forms, representing most of the characters of each group,
whether large or small, and thus give a general idea of the value of
the differences between them. This is what we should be driven to, if
we were ever to succeed in collecting all the forms in any class which
have lived throughout all time and space. We shall certainly never
succeed in making so perfect a collection: nevertheless, in certain
classes, we are tending in this direction; and Milne Edwards has
lately insisted, in an able paper, on the high importance of looking
to types, whether or not we can separate and define the groups to
which such types belong.

Finally, we have seen that natural selection, which results from the
struggle for existence, and which almost inevitably induces extinction
and divergence of character in the many descendants from one dominant
parent-species, explains that great and universal feature in the
affinities of all organic beings, namely, their subordination in group
under group. We use the element of descent in classing the individuals
of both sexes and of all ages, although having few characters in
common, under one species; we use descent in classing acknowledged
varieties, however different they may be from their parent; and I
believe this element of descent is the hidden bond of connexion which
naturalists have sought under the term of the Natural System. On this
idea of the natural system being, in so far as it has been perfected,
genealogical in its arrangement, with the grades of difference between
the descendants from a common parent, expressed by the terms genera,
families, orders, etc., we can understand the rules which we are
compelled to follow in our classification. We can understand why we
value certain resemblances far more than others; why we are permitted
to use rudimentary and useless organs, or others of trifling
physiological importance; why, in comparing one group with a distinct
group, we summarily reject analogical or adaptive characters, and yet
use these same characters within the limits of the same group. We can
clearly see how it is that all living and extinct forms can be grouped
together in one great system; and how the several members of each
class are connected together by the most complex and radiating lines
of affinities. We shall never, probably, disentangle the inextricable
web of affinities between the members of any one class; but when we
have a distinct object in view, and do not look to some unknown plan
of creation, we may hope to make sure but slow progress.

MORPHOLOGY.

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
organisation. 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 name of Morphology. This is the most
interesting department of natural history, and may 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 the same bones, in
the same relative positions? Geoffroy St. Hilaire has insisted
strongly on the high importance of relative connexion in homologous
organs: the parts may change to almost any extent in form and size,
and yet they always remain connected together in the same order. We
never find, for instance, the bones of the arm and forearm, or of the
thigh and leg, transposed. Hence the same names can be given to the
homologous bones in widely different animals. We see the same great
law in the construction of the mouths of insects: what can be more
different than the immensely long spiral proboscis of a 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 different purposes,
are formed by infinitely numerous modifications of an upper lip,
mandibles, and two pairs of maxillae. Analogous laws govern 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 so pleased the
Creator to construct each animal and plant.

The explanation is manifest on the theory of the natural selection of
successive slight modifications,--each modification being profitable
in some way to the modified form, but often affecting by correlation
of growth other parts of the organisation. In changes of this nature,
there will be little or no tendency to modify the original pattern, or
to transpose parts. The bones of a limb might be shortened and widened
to any extent, and become gradually enveloped in thick membrane, so as
to serve as a fin; or a webbed foot might have all its bones, or
certain bones, lengthened to any extent, and the membrane connecting
them increased to any extent, so as to serve as a wing: yet in all
this great amount of modification there will be no tendency to alter
the framework of bones or the relative connexion of the several parts.
If we suppose that the ancient progenitor, the archetype as it may be
called, of all mammals, 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 whole class. So with the mouths of insects, we
have only to suppose that their common progenitor had an upper lip,
mandibles, and two pair 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 atrophy
and ultimately by the complete abortion of certain parts, by the
soldering together 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 extinct gigantic
sea-lizards, and in the mouths of certain suctorial crustaceans, the
general pattern seems to have been thus to a certain extent obscured.

There is another and equally curious branch of the present subject;
namely, the comparison not of the same part in different members of a
class, but of the different parts or organs in the same individual.
Most physiologists believe that the bones of the skull are homologous
with--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 each member of the vertebrate and articulate
classes are plainly homologous. We see the same law in comparing the
wonderfully complex jaws and legs in 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 in embryonic
crustaceans and in many other animals, and in flowers, that organs,
which when mature become extremely different, are at an early stage of
growth exactly alike.

How inexplicable are these facts 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? As Owen has remarked, the
benefit derived from the yielding of the separate pieces in the act of
parturition of mammals, will by no means explain the same construction
in the skulls of birds. Why should similar bones have been created in
the formation of the wing and leg of a bat, used as they are for such
totally different purposes? 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 any individual
flower, though fitted for such widely different purposes, be all
constructed on the same pattern?

On the theory of natural selection, we can satisfactorily answer these
questions. In the vertebrata, we see a series of internal vertebrae
bearing certain processes and appendages; in the articulata, we see
the body divided into a series of segments, bearing external
appendages; and in flowering plants, we see a series of successive
spiral whorls of leaves. An indefinite repetition of the same part or
organ is the common characteristic (as Owen has observed) of all low
or little-modified forms; therefore we may readily believe that the
unknown progenitor of the vertebrata possessed many vertebrae; the
unknown progenitor of the articulata, many segments; and the unknown
progenitor of flowering plants, many spiral whorls of leaves. We have
formerly seen that parts many times repeated are eminently liable to
vary in number and structure; consequently it is quite probable that
natural selection, during a long-continued course of modification,
should have seized on a certain number of the primordially similar
elements, many times repeated, and have adapted them to the most
diverse purposes. And as the whole amount of modification will have
been effected by slight successive steps, we need not wonder at
discovering in such parts or organs, a certain degree of fundamental
resemblance, retained by the strong principle of inheritance.

In the great class of molluscs, though we can homologise the parts of
one species with those of another and distinct species, we can
indicate but few serial homologies; that is, we are seldom enabled to
say that one part or organ is homologous with another 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.

Naturalists frequently speak of the skull as formed of metamorphosed
vertebrae: the jaws of crabs as metamorphosed legs; the stamens and
pistils of flowers as metamorphosed leaves; but it would in these
cases probably be more correct, as Professor Huxley has remarked, to
speak of both skull and vertebrae, both jaws and legs, etc.,--as
having been metamorphosed, not one from the other, but from some
common element. 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 modified into skulls or
jaws. Yet so strong is the appearance of a modification of this nature
having occurred, that naturalists can hardly avoid employing language
having this plain signification. On my view these terms may be used
literally; and the wonderful fact of the jaws, for instance, of a crab
retaining numerous characters, which they would probably have retained
through inheritance, if they had really been metamorphosed during a
long course of descent from true legs, or from some simple appendage,
is explained.

EMBRYOLOGY.

It has already been casually remarked that certain organs in the
individual, which when mature become widely different and serve for
different purposes, are in the embryo exactly alike. The embryos,
also, of distinct animals within the same class are often strikingly
similar: a better proof of this cannot be given, than a circumstance
mentioned by Agassiz, namely, that having forgotten to ticket the
embryo of some vertebrate animal, he cannot now tell whether it be
that of a mammal, bird, or reptile. The vermiform larvae of moths,
flies, beetles, etc., resemble each other much more closely than do
the mature insects; but in the case of larvae, the embryos are active,
and have been adapted for special lines of life. A trace of the law of
embryonic resemblance, sometimes lasts till a rather late age: thus
birds of the same genus, and of closely allied genera, often resemble
each other in their first and second plumage; as we see in the spotted
feathers in the thrush group. In the cat tribe, most of the species
are striped or spotted in lines; and stripes can be plainly
distinguished in the whelp of the lion. We occasionally though rarely
see something of this kind in plants: thus the embryonic leaves of the
ulex or furze, and the first leaves of the phyllodineous acaceas, are
pinnate or divided like the ordinary leaves of the leguminosae.

The points of structure, in which the embryos of widely different
animals of 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
course 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 same 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 will suppose that the stripes on the whelp
of a lion, or the spots on the young blackbird, are of any use to
these animals, or are related to the conditions to which they are
exposed.

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. From such
special adaptations, the similarity of the larvae or active embryos of
allied animals is sometimes much obscured; and cases could be given of
the larvae of two species, or of two groups of species, differing
quite as much, or even more, from each other than do their adult
parents. 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, as it certainly is, a
crustacean; but a glance at the larva shows this to be the case in an
unmistakeable manner. So again the two main divisions of cirripedes,
the pedunculated and sessile, which differ widely in external
appearance, have larvae in all their several 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 the 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 is generally 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 legs, a
very 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 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, and very 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 either 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, or other organ of importance, excepting
for reproduction.

We are so much accustomed to see differences in structure between the
embryo and the adult, and likewise a close similarity in the embryos
of widely different animals within the same class, that we might be
led to look at these facts as necessarily contingent in some manner on
growth. But there is no obvious reason why, for instance, the wing of
a bat, or the fin of a porpoise, should not have been sketched out
with all the parts in proper proportion, as soon as any structure
became visible in the embryo. And in some whole groups of animals and
in certain members of other groups, 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;" and
again in spiders, "there is nothing worthy to be called a
metamorphosis." The larvae of insects, whether adapted to the most
diverse and active habits, or quite inactive, being fed by their
parents or placed in the midst of proper nutriment, yet nearly all
pass through a similar worm-like stage of development; but in some few
cases, as in that of Aphis, if we look to the admirable drawings by
Professor Huxley of the development of this insect, we see no trace of
the vermiform stage.

How, then, can we explain these several facts in embryology,--namely
the very general, but not universal difference in structure between
the embryo and the adult;--of parts in the same individual embryo,
which ultimately become very unlike and serve for diverse purposes,
being at this early period of growth alike;--of embryos of different
species within the same class, generally, but not universally,
resembling each other;--of the structure of the embryo not being
closely related to its conditions of existence, except when the embryo
becomes at any period of life active and has to provide for
itself;--of the embryo apparently having sometimes a higher
organisation than the mature animal, into which it is developed. I
believe that all these facts can be explained, as follows, on the view
of descent with modification.

It is commonly assumed, perhaps from monstrosities often affecting the
embryo at a very early period, that slight variations necessarily
appear at an equally early period. But we have little evidence on this
head--indeed the evidence rather points the other way; for it is
notorious that breeders of cattle, horses, and various fancy animals,
cannot positively tell, until some time after the animal has been
born, what its merits or form will ultimately turn out. We see this
plainly in our own children; we cannot always tell whether the child
will be tall or short, or what its precise features will be. The
question is not, at what period of life any variation has been caused,
but at what period it is fully displayed. The cause may have acted,
and I believe generally has acted, even before the embryo is formed;
and the variation may be due to the male and female sexual elements
having been affected by the conditions to which either parent, or
their ancestors, have been exposed. Nevertheless an effect thus caused
at a very early period, even before the formation of the embryo, may
appear late in life; as when an hereditary disease, which appears in
old age alone, has been communicated to the offspring from the
reproductive element of one parent. Or again, as when the horns of
cross-bred cattle have been affected by the shape of the horns of
either parent. For the welfare of 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, it must be quite unimportant
whether most of its characters are fully acquired a little earlier or
later in life. It would not signify, for instance, to a bird which
obtained its food best by having a long beak, whether or not it
assumed a beak of this particular length, as long as it was fed by its
parents. Hence, I conclude, that it is quite possible, that each of
the many successive modifications, by which each species has acquired
its present structure, may have supervened at a not very early period
of life; and some direct evidence from our domestic animals supports
this view. But in other cases it is quite possible that each
successive modification, or most of them, may have appeared at an
extremely early period.

I have stated in the first chapter, that there is some evidence to
render it probable, 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 horns of almost full-grown
cattle. But further than this, variations which, for all that we can
see, might have appeared earlier or later in life, tend to appear at a
corresponding age in the offspring and parent. I am far from meaning
that this is invariably the case; and I could give a good many 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, if their truth be admitted, will, I believe,
explain all the above specified leading facts in embryology. But first
let us look at a few analogous cases in domestic varieties. Some
authors who have written on Dogs, maintain that the greyhound and
bulldog, though appearing so different, are really varieties most
closely allied, and have probably 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 nearly acquired their full amount of
proportional difference. So, again, I was told that the foals of cart
and race-horses differed as much as the full-grown animals; and this
surprised me greatly, as I think it probable that the difference
between these two breeds has been wholly caused by selection under
domestication; but having had careful measurements made of the dam and
of a three-days old colt of a race and heavy cart-horse, I find that
the colts have by no means acquired their full amount of proportional
difference.