CLASSIFICATION, groups subordinate to groups -- Natural system -- Rules and difficulties in classification, explained on the theory of descent with modification -- Classification of varieties -- Descent always used in classification -- Analogical or adaptive characters -- Affinities, general, complex and radiating -- Extinction separates and defines groups -- MORPHOLOGY, between members of the same class, between parts of the same individual -- EMBRYOLOGY, laws of, explained by variations not supervening at an early age, and being inherited at a corresponding age -- RUDIMENTARY ORGANS; their origin explained -- Summary
From the first dawn of
life, all organic beings are found to resemble each other in
descending degrees, so that they can be classed in groups under
groups. This classification is evidently not arbitrary like the
grouping of the stars in constellations. The existence of groups would
have been of simple signification, if one group had been exclusively
fitted to inhabit the land, and another the water; one to feed on
flesh, another on vegetable matter, and so on; but the case is widely
different in nature; for it is notorious how commonly members of even
the same subgroup have different habits. In our second and fourth
chapters, on Variation and on Natural Selection, I have attempted to
show that it is the widely ranging, the much diffused and common, that
is the dominant species belonging to the larger genera, which vary
most. The varieties, or incipient species, thus produced ultimately
become converted, as I believe, into new and distinct species; and
these, on the principle of inheritance, tend to produce other new and
dominant species. Consequently the groups which are now large, and
which generally include many
I attempted also to show that there is a constant tendency in the forms which are increasing in number and diverging in character, to supplant and exterminate the less divergent, the less improved, and preceding forms. I request the reader to turn to the diagram illustrating the action, as formerly explained, of these several principles; and he will see that the inevitable result is that the modified descendants proceeding from one progenitor become broken up into groups subordinate to groups. In the diagram each letter on the uppermost line may represent a genus including several species; and all the genera on this line form together one class, for all have descended from one ancient but unseen parent, and, consequently, have inherited something in common. But the three genera on the left hand have, on this same principle, much in common, and form a sub-family, distinct from that including the next two genera on the right hand, which diverged from a common parent at the fifth stage of descent. These five genera have also much, though less, in common; and they form a family distinct from that including the three genera still further to the right hand, which diverged at a still earlier period. And all these genera, descended from (A), form an order distinct from the genera descended from (I). So that we here have many species descended from a single progenitor grouped into genera; and the genera are included in, or subordinate to, sub-families, families, and orders, all united into one class. Thus, the grand fact in natural history of the subordination of group under group, which, from its familiarity, does not always sufficiently strike us, is in my judgement fully explained.
Naturalists try to arrange the species, genera, and families in
each class, on what is called the Natural System. But what is
Let us now consider the rules followed in classification, and the
difficulties which are encountered on the view that classification
either gives some unknown plan of creation, or is simply a scheme for
enunciating general propositions and of placing together the forms
most like each other. It might have been thought (and was in ancient
times thought) that those parts of the structure which determined the
habits of life, and the general place of each being in the economy of
nature, would be of very high importance in classification. Nothing
can be more false. No one regards the external similarity of a mouse
to a shrew, of a dugong to a whale, of a whale to a fish, as of any
importance. These resemblances, though so intimately connected with
the whole life of the being, are ranked as merely `adaptive or
analogical characters;' but to the consideration of these resemblances
we shall have to recur. It may even be given as a general rule, that
We must not, therefore, in classifying, trust to resemblances in
parts of the organisation, however important they may be for the
welfare of the being in relation to the outer world. Perhaps from this
cause it has partly arisen, that almost all naturalists lay the
greatest stress on resemblances in organs of high vital or
physiological importance. No doubt this view of the classificatory
importance of organs which are important is generally, but by no means
always, true. But their importance for classification, I believe,
depends on their greater constancy throughout large groups of species;
and this constancy depends on such organs having generally been
subjected to less change in the adaptation of the species to their
conditions of life. That the mere physiological importance of an organ
does not determine the classificatory value, is almost shown by the
one fact, that in allied groups, in which the same organ, as we have
every reason to suppose, has nearly the same physiological value, its
classificatory value is widely different. No naturalist can have
worked at any group without being struck with this fact; and it has
been most fully acknowledged in the writings of almost every author.
It will suffice to quote the highest authority, Robert Brown, who in
speaking of certain organs in the Proteaceae, says their generic
importance, `like that of all their parts, not only in this but, as I
apprehend, in every natural family, is very unequal, and in some cases
seems to be entirely lost.' Again in another work he says, the genera
of the Connaraceae `differ in having one or more ovaria, in the
existence or absence of albumen, in the
Again, no one will say that rudimentary or atrophied organs are of high physiological or vital importance; yet, undoubtedly, organs in this condition are often of high value in classification. No one will dispute that the rudimentary teeth in the upper jaws of young ruminants, and certain rudimentary bones of the leg, are highly serviceable in exhibiting the close affinity between Ruminants and pachyderms. Robert Brown has strongly insisted on the fact that the rudimentary florets are of the highest importance in the classification of the Grasses.
Numerous instances could be given of characters derived from parts which must be considered of very trifling physiological importance, but which are universally admitted as highly serviceable in the definition of whole groups. For instance, whether or not there is an open passage from the nostrils to the mouth, the only character, according to Owen, which absolutely distinguishes fishes and reptiles -- the inflection of the angle of the jaws in Marsupials -- the manner in which the wings of insects are folded -- mere colour in certain Algae -- mere pubescence on parts of the flower in grasses -- the nature of the dermal covering, as hair or feathers, in the Vertebrata. If the Ornithorhynchus had been covered with feathers instead of hair, this external and trifling character would, I think, have been considered by naturalists as important an aid in determining the degree of affinity of this strange creature to birds and reptiles, as an approach in structure in any one internal and important organ.
The importance, for classification, of trifling characters, mainly
Practically when naturalists are at work, they do not trouble
themselves about the physiological value of the characters which they
use in defining a group, or in allocating any particular species. If
they find a character nearly uniform, and common to a great number of
forms, and not common to others, they use it as one of high value; if
common to some lesser number, they use it as of subordinate value.
This principle has been broadly confessed by some naturalists to be
the true one; and by none more clearly than by that excellent
botanist, Aug. St Hilaire. If certain characters are always found
correlated with others, though no apparent bond of connexion can be
discovered between them, especial value is set on them. As in most
groups of
We can see why characters derived from the embryo should be of equal importance with those derived from the adult, for our classifications of course include all ages of each species. But it is by no means obvious, on the ordinary view, why the structure of the embryo should be more important for this purpose than that of the adult, which alone plays its full part in the economy Of nature. Yet it has been strongly urged by those great naturalists, Milne Edwards and Agassiz, that embryonic characters are the most important of any in the classification of animals; and this doctrine has very generally been admitted as true. The same fact holds good with flowering plants, of which the two main divisions have been founded on characters derived from the embryo, -- on the number and position of the embryonic leaves or cotyledons, and on the mode of development of the plumule and radicle. In our discussion on embryology, we shall see why such characters are so valuable, on the view of classification tacitly including the idea of descent.
Our classifications are often plainly influenced by chains of affinities. Nothing can be easier than to define a number of characters common to all birds; but in the case of crustaceans, such definition has hitherto been found impossible. There are crustaceans at the opposite ends of the series, which have hardly a character in common; yet the species at both ends, from being plainly allied to others, and these to others, and so onwards, can be recognised as unequivocally belonging to this, and to no other class of the Articulata.
Geographical distribution has often been used, though perhaps not
quite logically, in classification, more especially in very large
groups of closely allied forms. Temminck insists on the utility or
even necessity of this practice in certain groups of birds; and it has
been followed by several entomologists and botanists.
Finally, with respect to the comparative value of the various groups of species, such as orders, sub-orders, families, subfamilies, and genera, they seem to be, at least at present, almost arbitrary. Several of the best botanists, such as Mr Bentham and others, have strongly insisted on their arbitrary value. Instances could be given amongst plants and insects, of a group of forms, first ranked by practised naturalists as only a genus, and then raised to the rank of a sub-family or family; and this has been done, not because further research has detected important structural differences, at first overlooked, but because numerous allied species, with slightly different grades of difference, have been subsequently discovered.
All the foregoing rules and aids and difficulties in classification are explained, if I do not greatly deceive myself, on the view that the natural system is founded on descent with modification;, that the characters which naturalists consider as showing true affinity between any two or more species, are those which have been inherited from a common parent, and, in so far, all true classification is genealogical; that community of descent is the hidden bond which naturalists have been unconsciously seeking, and not some unknown plan of creation, or the enunciation of general propositions, and the mere putting together and separating objects more or less alike.
But I must explain my meaning more fully. I believe that the
arrangement of the groups within each class, in due subordination and
relation to the other groups, must be strictly genealogical in order
to be natural; but that the amount of difference in the several
branches or groups, though allied in the same degree in blood to their
common progenitor, may differ greatly, being due to the different
degrees of modification which they have undergone; and this is
expressed by the forms being ranked under different genera, families,
sections, or orders. The reader will best understand what is meant, if
he will take the trouble to referring to the diagram in the fourth
chapter. We will suppose the letters A to L to represent allied
genera, which lived during the Silurian epoch, and these have
descended from a species which existed at an unknown anterior period.
Species of three of these genera (A, F, and I) have transmitted
modified 14 to z14)
on the uppermost horizontal line. Now all these modified descendants
from a single species, are represented as related in blood or descent
to the same degree; they may metaphorically be called cousins to the
same millionth degree; yet they differ widely and in different degrees
from each other. The forms descended from A, now broken up into two or
three families, constitute a distinct order from those descended from
I, also broken up into two families. Nor can the existing species,
descended from A, be ranked in the same genus with the parent A; or
those from I, with the parent I. But the existing genus F14 may be supposed to have been but slightly modified;
and it will then rank with the parent-genus F; just as some few still
living organic beings belong to Silurian genera. So that the amount or
value of the differences between organic beings all related to each
other in the same degree in blood, has come to be widely different.
Nevertheless their genealogical arrangement remains strictly true, not
only at the present time, but at each successive period of descent.
All the modified descendants from A will have inherited something in
common from their common parent, as will all the descendants from I;
so will it be with each subordinate branch of descendants, at each
successive period. If, however, we choose to suppose that any of the
descendants of A or of I have been so much modified as to have more or
less completely lost traces of their parentage, in this case, their
places in a natural classification will have been more or less
completely lost, -- as sometimes seems to have occurred with
existing organisms. All the descendants of the genus F, along its
whole line of descent, are supposed to have been but little modified,
and they yet form a single genus. But this genus, though much
isolated, will still occupy its proper intermediate position; for F
originally was intermediate in character between A and I, and the
several genera descended from these two genera will have inherited to
a certain extent their characters. This natural arrangement is shown,
as far as is possible on paper, in the diagram, but in much too simple
a manner. If a branching diagram had not been used, and only the names
of the groups had been written in a linear series, it would have been
still less possible to have given a
It may be worth while to illustrate this view of classification, by taking the case of languages. If we possessed a perfect pedigree of mankind, a genealogical arrangement of the races of man would afford the best classification of the various languages now spoken throughout the world; and if all extinct languages, and all intermediate and slowly changing dialects, had to be included, such an arrangement would, I think, be the only possible one. Yet it might be that some very ancient language had altered little, and had given rise to few new languages, whilst others (owing to the spreading and subsequent isolation and states of civilisation of the several races, descended from a common race) had altered much, and had given rise to many new languages and dialects. The various degrees of difference in the languages from the same stock, would have to be expressed by groups subordinate to groups; but the proper or even only possible arrangement would still be genealogical; and this would be strictly natural, as it would connect together all languages, extinct and modern, by the closest affinities, and would give the filiation and origin of each tongue.
In confirmation of this view, let us glance at the classification
of varieties, which are believed or known to have descended from one
species. These are grouped under species, with sub-varieties under
varieties; and with our domestic productions, several other grades of
difference are requisite, as we have seen with pigeons. The origin of
the existence of groups subordinate to groups, is the same with
varieties as with species, namely, closeness of descent with various
degrees of modification. Nearly the same rules are followed in
classifying varieties, as with species. Authors have insisted on the
necessity of classing varieties on a natural instead of an artificial
system; we are cautioned, for instance, not to
With species in a state of nature, every naturalist has in fact
brought descent into his classification; for he includes in his lowest
grade, or that of a species, the two sexes; and how enormously these
sometimes differ in the most important characters, is known to every
naturalist: scarcely a single fact can be predicated in common of the
males and hermaphrodites of certain cirripedes, when adult, and yet no
one dreams of separating them. The naturalist includes as one species
the several larval stages of the same individual, however much they
may differ from each other and from the adult; as he likewise includes
the so-called alternate generations of Steenstrup, which can only in a
technical sense be considered as the same individual. He includes
monsters; he includes varieties, not solely because they
As descent has universally been used in classing together the
individuals of the same species, though the males and females and
larvae are sometimes extremely different; and as it has been used in
classing varieties which have undergone a certain, and sometimes a
considerable amount of modification, may not this same element of
descent have been unconsciously used in grouping species under genera,
and genera under higher groups, though in these cases the modification
has been greater in degree, and has taken a longer time to complete? I
believe it has thus been unconsciously used; and only thus can I
understand the several rules and guides which have been followed by
our best systematists. We have no written pedigrees; we have to make
out community of descent by resemblances of any kind. Therefore we
choose those characters which, as far as we can judge, are the least
likely to have been modified in relation to the conditions of life to
which each species has been recently exposed. Rudimentary structures
on this view are as good as, or even sometimes better than, other
parts of the organisation. We care
We can understand why a species or a group of species may depart, in several of its most important characteristics, from its allies, and yet be safely classed with them. This may be safely done, and is often done, as long as a sufficient number of characters, let them be ever so unimportant, betrays the hidden bond of community of descent. Let two forms have not a single character in common, yet if these extreme forms are connected together by a chain of intermediate groups, we may at once infer their community of descent, and we put them all into the same class. As we find organs of high physiological importance -- those which serve to preserve life under the most diverse conditions of existence -- are generally the most constant, we attach especial value to them; but if these same organs, in another group or section of a group, are found to differ much, we at once value them less in our classification. We shall hereafter, I think, clearly see why embryological characters are of such high classificatory importance. Geographical distribution may sometimes be brought usefully into play in classing large and widely-distributed genera, because all the species of the same genus, inhabiting any distinct and isolated region, have in all probability descended from the same parents.
We can understand, on these views, the very important distinction
between real affinities and analogical or adaptive resemblances.
Lamarck first called attention to this distinction, and
As members of distinct classes have often been adapted by
successive slight modifications to live under nearly similar
circumstances, -- to inhabit for instance the three elements of
land, air, and water, -- we can perhaps understand how it is that
a numerical parallelism has sometimes been observed between the
sub-groups in distinct classes. A naturalist, struck by a parallelism
As the modified descendants of dominant species, belonging to the larger genera, tend to inherit the advantages, which made the groups to which they belong large and their parents dominant, they are almost sure to spread widely, and to seize on more and more places in the economy of nature. The larger and more dominant groups thus tend to go on increasing in size; and they consequently supplant many smaller and feebler groups. Thus we can account for the fact that all organisms, recent and extinct, are included under a few great orders, under still fewer classes, and all in one great natural system. As showing how few the higher groups are in number, and how widely spread they are throughout the world, the fact is striking, that the discovery of Australia has not added a single insect belonging to a new order; and that in the vegetable kingdom, as I learn from Dr Hooker, it has added only two or three orders of small size.
In the chapter on geological succession I attempted to show, on the
principle of each group having generally diverged much in character
during the long-continued process of modification, how it is that the
more ancient forms of life often present characters in some slight
degree intermediate between existing groups. A few old and
intermediate parent-forms having occasionally transmitted to the
present day descendants but little modified, will give to us our
so-called osculant or aberrant groups. The more aberrant any form is,
the greater must be the number of connecting forms which on my theory
have been exterminated and utterly lost. And we have some evidence of
aberrant forms having suffered severely from extinction, for they are
generally represented by extremely few species; and such species as do
occur are generally very distinct from each other, which again implies
extinction. The genera Ornithorhynchus and Lepidosiren, for example,
would not have been less aberrant had each been represented by a dozen
species instead of by a single one;
Mr Waterhouse has remarked that, when a member belonging to one group of animals exhibits an affinity to a quite distinct group, this affinity in most cases is general and not special: thus, according to Mr Waterhouse, of all Rodents, the bizcacha is most nearly related to Marsupials; but in the points in which it approaches this order, its relations are general, and not to any one marsupial species more than to another. As the points of affinity of the bizcacha to Marsupials are believed to be real and not merely adaptive, they are due on my theory to inheritance in common. Therefore we must suppose either that all Rodents, including the bizcacha, branched off from some very ancient Marsupial, which will have had a character in some degree intermediate with respect to all existing Marsupials; or that both Rodents and Marsupials branched off from a common progenitor, and that both groups have since undergone much modification in divergent directions. On either view we may suppose that the bizcacha has retained, by inheritance, more of the character of its ancient progenitor than have other Rodents; and therefore it will not be specially related to any one existing Marsupial, but indirectly to all or nearly all Marsupials, from having partially retained the character of their common progenitor, or of an early member of the group. On the other hand, of all Marsupials, as Mr Waterhouse has remarked, the phascolomys resembles most nearly, not any one species, but the general order of Rodents. In this case, however, it may be strongly suspected that the resemblance is only analogical, owing to the phascolomys having become adapted to habits like those of a Rodent. The elder De Candolle has made nearly similar observations on the general nature of the affinities of distinct orders of plants.
On the principle of the multiplication and gradual divergence in
character of the species descended from a common parent,
Extinction, as we have seen in the fourth chapter, has playnl 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
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.
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
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 in 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
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
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
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
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
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 carthorse, I find that the colts have by no means acquired their full amount of proportional difference.
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
hitched; 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
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
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
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
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.
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 Klreuter 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
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
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
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.
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 all 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
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.
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