difference between the descendants from a common parent, expressed by the
terms genera, families, orders, &c., 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,
st
amens, 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, &c.,--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, &c., 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: t
he 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