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CHAPTER
XIII. MUTUAL
AFFINITIES OF ORGANIC BEINGS: 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 dominant species, tend to go on
increasing indefinitely in size. I further attempted to show that from the
varying descendants of each species trying to occupy as many and as different
places as possible in the economy of nature, there is a constant tendency in
their characters to diverge. This conclusion was supported by looking at the
great diversity of the forms of life which, in any small area, come into the
closest competition, and by looking to certain facts in naturalisation. 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 judgment fully explained. Naturalists
try to arrange the species, genera, and families in each class, on what is
called the Natural System. But what is meant by this system? Some authors look
at it merely as a scheme for arranging together those living objects which are
most alike, and for separating those which are most unlike; or as an artificial
means for enunciating, as briefly as possible, general propositions,—that is,
by one sentence to give the characters common, for instance, to all mammals, by
another those common to all carnivora, by another those common to the
dog-genus, and then by adding a single sentence, a full description is given of
each kind of dog. The ingenuity and utility of this system are indisputable.
But many naturalists think that something more is meant by the Natural System;
they believe that it reveals the plan of the Creator; but unless it be
specified whether order in time or space, or what else is meant by the plan of
the Creator, it seems to me that nothing is thus added to our knowledge. Such
expressions as that famous one of Linnæus, and which we often meet with in a
more or less concealed form, that the characters do not make the genus, but
that the genus gives the characters, seem to imply that something more is
included in our classification, than mere resemblance. I believe that something
more is included; and that propinquity of descent,—the only known cause of the
similarity of organic beings,—is the bond, hidden as it is by various degrees
of modification, which is partially revealed to us by our classifications. 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 the less any part of the organisation is
concerned with special habits, the more important it becomes for
classification. As an instance: Owen, in speaking of the dugong, says, “The
generative organs being those which are most remotely related to the habits and
food of an animal, I have always regarded as affording very clear indications
of its true affinities. We are least likely in the modifications of these
organs to mistake a merely adaptive for an essential character.” So with
plants, how remarkable it is that the organs of vegetation, on which their
whole life depends, are of little signification, excepting in the first main divisions;
whereas the organs of reproduction, with their product the seed, are of
paramount importance! 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 its 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 Proteaceæ, 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 Connaraceæ “differ in having one or more ovaria, in the
existence or absence of albumen, in the imbricate or valvular æstivation. Any
one of these characters singly is frequently of more than generic importance,
though here even when all taken together they appear insufficient to separate
Cnestis from Connarus.” To give an example amongst insects, in one great
division of the Hymenoptera, the antennæ, as Westwood has remarked, are most
constant in structure; in another division they differ much, and the
differences are of quite subordinate value in classification; yet no one
probably will say that the antennæ in these two divisions of the same order are
of unequal physiological importance. Any number of instances could be given of
the varying importance for classification of the same important organ within
the same group of beings. 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 Algæ—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 depends on their
being correlated with several other characters of more or less importance. The
value indeed of an aggregate of characters is very evident in natural history.
Hence, as has often been remarked, a species may depart from its allies in
several characters, both of high physiological importance and of almost
universal prevalence, and yet leave us in no doubt where it should be ranked.
Hence, also, it has been found, that a classification founded on any single character,
however important that may be, has always failed; for no part of the
organisation is universally constant. The importance of an aggregate of
characters, even when none are important, alone explains, I think, that saying
of Linnæus, that the characters do not give the genus, but the genus gives the
characters; for this saying seems founded on an appreciation of many trifling
points of resemblance, too slight to be defined. Certain plants, belonging to
the Malpighiaceæ, bear perfect and degraded flowers; in the latter, as A. de
Jussieu has remarked, “the greater number of the characters proper to the
species, to the genus, to the family, to the class, disappear, and thus laugh
at our classification.” But when Aspicarpa produced in France, during several
years, only degraded flowers, departing so wonderfully in a number of the most
important points of structure from the proper type of the order, yet M. Richard
sagaciously saw, as Jussieu observes, that this genus should still be retained
amongst the Malpighiaceæ. This case seems to me well to illustrate the spirit
with which our classifications are sometimes necessarily founded. 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 animals, important organs, such as those for
propelling the blood, or for aërating it, or those for propagating the race,
are found nearly uniform, they are considered as highly serviceable in
classification; but in some groups of animals all these, the most important
vital organs, are found to offer characters of quite subordinate value. 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, sub-families, 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 of 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 descendants to the present day, represented by the fifteen
genera (a14 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 natural arrangement; and it is notoriously not possible
to represent in a series, on a flat surface, the affinities which we discover
in nature amongst the beings of the same group. Thus, on the view which I hold,
the natural system is genealogical in its arrangement, like a pedigree; but the
degrees of modification which the different groups have undergone, have to be
expressed by ranking them under different so-called genera, sub-families,
families, sections, orders, and classes. 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 class two varieties of the
pine-apple together, merely because their fruit, though the most important
part, happens to be nearly identical; no one puts the swedish and common
turnips together, though the esculent and thickened stems are so similar.
Whatever part is found to be most constant, is used in classing varieties: thus
the great agriculturist Marshall says the horns are very useful for this
purpose with cattle, because they are less variable than the shape or colour of
the body, &c.; whereas with sheep the horns are much less serviceable,
because less constant. In classing varieties, I apprehend if we had a real
pedigree, a genealogical classification would be universally preferred; and it
has been attempted by some authors. For we might feel sure, whether there had
been more or less modification, the principle of inheritance would keep the
forms together which were allied in the greatest number of points. In tumbler
pigeons, though some sub-varieties differ from the others in the important
character of having a longer beak, yet all are kept together from having the
common habit of tumbling; but the short-faced breed has nearly or quite lost
this habit; nevertheless, without any reasoning or thinking on the subject,
these tumblers are kept in the same group, because allied in blood and alike in
some other respects. If it could be proved that the Hottentot had descended
from the Negro, I think he would be classed under the Negro group, however much
he might differ in colour and other important characters from negroes. 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 closely resemble the parent-form, but
because they are descended from it. He who believes that the cowslip is
descended from the primrose, or conversely, ranks them together as a single
species, and gives a single definition. As soon as three Orchidean forms
(Monochanthus, Myanthus, and Catasetum), which had previously been ranked as
three distinct genera, were known to be sometimes produced on the same spike,
they were immediately included as a single species. But it may
be asked, what ought we to do, if it could be proved that one species of
kangaroo had been produced, by a long course of modification, from a bear?
Ought we to rank this one species with bears, and what should we do with the
other species? The supposition is of course preposterous; and I might answer by
the argumentum ad hominem, and ask what should be done if a perfect
kangaroo were seen to come out of the womb of a bear? According to all analogy,
it would be ranked with bears; but then assuredly all the other species of the
kangaroo family would have to be classed under the bear genus. The whole case
is preposterous; for where there has been close descent in common, there will
certainly be close resemblance or affinity. As descent
has universally been used in classing together the individuals of the same
species, though the males and females and larvæ 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 not how trifling a character may be—let it be the mere
inflection of the angle of the jaw, the manner in which an insect’s wing is
folded, whether the skin be covered by hair or feathers—if it prevail
throughout many and different species, especially those having very different
habits of life, it assumes high value; for we can account for its presence in
so many forms with such different habits, only by its inheritance from a common
parent. We may err in this respect in regard to single points of structure, but
when several characters, let them be ever so trifling, occur together
throughout a large group of beings having different habits, we may feel almost
sure, on the theory of descent, that these characters have been inherited from
a common ancestor. And we know that such correlated or aggregated characters
have especial value in classification. 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 he has been ably followed by Macleay and
others. The resemblance, in the shape of the body and in the fin-like anterior
limbs, between the dugong, which is a pachydermatous animal, and the whale, and
between both these mammals and fishes, is analogical. Amongst insects there are
innumerable instances: thus Linnæus, misled by external appearances, actually
classed an homopterous insect as a moth. We see something of the same kind even
in our domestic varieties, as in the thickened stems of the common and swedish
turnip. The resemblance of the greyhound and racehorse is hardly more fanciful
than the analogies which have been drawn by some authors between very distinct
animals. On my view of characters being of real importance for classification,
only in so far as they reveal descent, we can clearly understand why analogical
or adaptive character, although of the utmost importance to the welfare of the
being, are almost valueless to the systematist. For animals, belonging to two
most distinct lines of descent, may readily become adapted to similar
conditions, and thus assume a close external resemblance; but such resemblances
will not reveal—will rather tend to conceal their blood-relationship to their
proper lines of descent. We can also understand the apparent paradox, that the
very same characters are analogical when one class or order is compared with
another, but give true affinities when the members of the same class or order
are compared one with another: thus the shape of the body and fin-like limbs
are only analogical when whales are compared with fishes, being adaptations in
both classes for swimming through the water; but the shape of the body and
fin-like limbs serve as characters exhibiting true affinity between the several
members of the whale family; for these cetaceans agree in so many characters,
great and small, that we cannot doubt that they have inherited their general
shape of body and structure of limbs from a common ancestor. So it is with
fishes. 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 of this nature in any
one class, by arbitrarily raising or sinking the value of the groups in other
classes (and all our experience shows that this valuation has hitherto been
arbitrary), could easily extend the parallelism over a wide range; and thus the
septenary, quinary, quaternary, and ternary classifications have probably
arisen. 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; but such richness in
species, as I find after some investigation, does not commonly fall to the lot
of aberrant genera. We can, I think, account for this fact only by looking at
aberrant forms as failing groups conquered by more successful competitors, with
a few members preserved by some unusual coincidence of favourable
circumstances. 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, together with their retention by
inheritance of some characters in common, we can understand the excessively
complex and radiating affinities by which all the members of the same family or
higher group are connected together. For the common parent of a whole family of
species, now broken up by extinction into distinct groups and sub-groups, will
have transmitted some of its characters, modified in various ways and degrees,
to all; and the several species will consequently be related to each other by
circuitous lines of affinity of various lengths (as may be seen in the diagram
so often referred to), mounting up through many predecessors. As it is
difficult to show the blood-relationship between the numerous kindred of any
ancient and noble family, even by the aid of a genealogical tree, and almost
impossible to do this without this aid, we can understand the extraordinary
difficulty which naturalists have experienced in describing, without the aid of
a diagram, the various affinities which they perceive between the many living
and extinct members of the same great natural class. 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, &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 maxillæ. 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 maxillæ, 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
vertebræ. 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 vertebræ 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 vertebræ; 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 vertebræ: 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 vertebræ, 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—vertebræ 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 larvæ of moths, flies, beetles,
&c., resemble each other much more closely than do the mature insects; but
in the case of larvæ, 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 leguminosæ. 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 larvæ or active
embryos of allied animals is sometimes much obscured; and cases could be given
of the larvæ 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 larvæ, 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
larvæ 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 larvæ 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 antennæ; 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 antennæ, 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
larvæ 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 larvæ 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. As the
evidence appears to me conclusive, that the several domestic breeds of Pigeon
have descended from one wild species, I compared young pigeons of various
breeds, within twelve hours after being hatched; I carefully measured the
proportions (but will not here give details) of the beak, width of mouth,
length of nostril and of eyelid, size of feet and length of leg, in the wild
stock, in pouters, fantails, runts, barbs, dragons, carriers, and tumblers. Now
some of these birds, when mature, differ so extraordinarily in length and form
of beak, that they would, I cannot doubt, be ranked in distinct genera, had
they been natural productions. But when the nestling birds of these several
breeds were placed in a row, though most of them could be distinguished from
each other, yet their proportional differences in the above specified several
points were incomparably less than in the full-grown birds. Some characteristic
points of difference—for instance, that of the width of mouth—could hardly be
detected in the young. But there was one remarkable exception to this rule, for
the young of the short-faced tumbler differed from the young of the wild
rock-pigeon and of the other breeds, in all its proportions, almost exactly as
much as in the adult state. The two
principles above given seem to me to explain these facts in regard to the later
embryonic stages of our domestic varieties. Fanciers select their horses, dogs,
and pigeons, for breeding, when they are nearly grown up: they are indifferent
whether the desired qualities and structures have been acquired earlier or
later in life, if the full-grown animal possesses them. And the cases just
given, more especially that of pigeons, seem to show that the characteristic
differences which give value to each breed, and which have been accumulated by
man’s selection, have not generally first appeared at an early period of life,
and have been inherited by the offspring at a corresponding not early period.
But the case of the short-faced tumbler, which when twelve hours old had
acquired its proper proportions, proves that this is not the universal rule;
for here the characteristic differences must either have appeared at an earlier
period than usual, or, if not so, the differences must have been inherited, not
at the corresponding, but at an earlier age. Now let us
apply these facts and the above two principles—which latter, though not proved
true, can be shown to be in some degree probable—to species in a state of
nature. Let us take a genus of birds, descended on my theory from some one
parent-species, and of which the several new species have become modified
through natural selection in accordance with their diverse habits. Then, from
the many slight successive steps of variation having supervened at a rather
late age, and having been inherited at a corresponding age, the young of the
new species of our supposed genus will manifestly tend to resemble each other
much more closely than do the adults, just as we have seen in the case of
pigeons. We may extend this view to whole families or even classes. The
fore-limbs, for instance, which served as legs in the parent-species, may
become, by a long course of modification, adapted in one descendant to act as
hands, in another as paddles, in another as wings; and on the above two
principles—namely of each successive modification supervening at a rather late
age, and being inherited at a corresponding late age—the fore-limbs in the
embryos of the several descendants of the parent-species will still resemble
each other closely, for they will not have been modified. But in each
individual new species, the embryonic fore-limbs will differ greatly from the
fore-limbs in the mature animal; the limbs in the latter having undergone much
modification at a rather late period of life, and having thus been converted
into hands, or paddles, or wings. Whatever influence long-continued exercise or
use on the one hand, and disuse on the other, may have in modifying an organ,
such influence will mainly affect the mature animal, which has come to its full
powers of activity and has to gain its own living; and the effects thus
produced will be inherited at a corresponding mature age. Whereas the young
will remain unmodified, or be modified in a lesser degree, by the effects of
use and disuse. In certain
cases the successive steps of variation might supervene, from causes of which
we are wholly ignorant, at a very early period of life, or each step might be
inherited at an earlier period than that at which it first appeared. In either
case (as with the short-faced tumbler) the young or embryo would closely
resemble the mature parent-form. We have seen that this is the rule of
development in certain whole groups of animals, as with cuttle-fish and
spiders, and with a few members of the great class of insects, as with Aphis.
With respect to the final cause of the young in these cases not undergoing any
metamorphosis, or closely resembling their parents from their earliest age, we
can see that this would result from the two following contingencies; firstly,
from the young, during a course of modification carried on for many
generations, having to provide for their own wants at a very early stage of
development, and secondly, from their following exactly the same habits of life
with their parents; for in this case, it would be indispensable for the
existence of the species, that the child should be modified at a very early age
in the same manner with its parents, in accordance with their similar habits.
Some further explanation, however, of the embryo not undergoing any
metamorphosis is perhaps requisite. If, on the other hand, it profited the
young to follow habits of life in any degree different from those of their
parent, and consequently to be constructed in a slightly different manner,
then, on the principle of inheritance at corresponding ages, the active young
or larvæ might easily be rendered by natural selection different to any
conceivable extent from their parents. Such differences might, also, become
correlated with successive stages of development; so that the larvæ, in the
first stage, might differ greatly from the larvæ in the second stage, as we
have seen to be the case with cirripedes. The adult might become fitted for
sites or habits, in which organs of locomotion or of the senses, &c., would
be useless; and in this case the final metamorphosis would be said to be
retrograde. As all the
organic beings, extinct and recent, which have ever lived on this earth have to
be classed together, and as all have been connected by the finest gradations,
the best, or indeed, if our collections were nearly perfect, the only possible
arrangement, would be genealogical. Descent being on my view the hidden bond of
connexion which naturalists have been seeking under the term of the natural
system. On this view we can understand how it is that, in the eyes of most
naturalists, the structure of the embryo is even more important for
classification than that of the adult. For the embryo is the animal in its less
modified state; and in so far it reveals the structure of its progenitor. In
two groups of animal, however much they may at present differ from each other
in structure and habits, if they pass through the same or similar embryonic
stages, we may feel assured that they have both descended from the same or
nearly similar parents, and are therefore in that degree closely related. Thus,
community in embryonic structure reveals community of descent. It will reveal
this community of descent, however much the structure of the adult may have
been modified and obscured; we have seen, for instance, that cirripedes can at
once be recognised by their larvæ as belonging to the great class of
crustaceans. As the embryonic state of each species and group of species
partially shows us the structure of their less modified ancient progenitors, we
can clearly see why ancient and extinct forms of life should resemble the
embryos of their descendants,—our existing species. Agassiz believes this to be
a law of nature; but I am bound to confess that I only hope to see the law
hereafter proved true. It can be proved true in those cases alone in which the
ancient state, now supposed to be represented in many embryos, has not been
obliterated, either by the successive variations in a long course of
modification having supervened at a very early age, or by the variations having
been inherited at an earlier period than that at which they first appeared. It
should also be borne in mind, that the supposed law of resemblance of ancient
forms of life to the embryonic stages of recent forms, may be true, but yet,
owing to the geological record not extending far enough back in time, may
remain for a long period, or for ever, incapable of demonstration. Thus, as
it seems to me, the leading facts in embryology, which are second in importance
to none in natural history, are explained on the principle of slight
modifications not appearing, in the many descendants from some one ancient
progenitor, at a very early period in the life of each, though perhaps caused
at the earliest, and being inherited at a corresponding not early period.
Embryology rises greatly in interest, when we thus look at the embryo as a
picture, more or less obscured, of the common parent-form of each great class
of animals. Rudimentary,
atrophied, or aborted organs.—Organs or parts in this strange
condition, bearing the stamp of inutility, are extremely common throughout
nature. For instance, rudimentary mammæ are very general in the males of
mammals: I presume that the “bastard-wing” in birds may be safely considered as
a digit in a rudimentary state: in very many snakes one lobe of the lungs is
rudimentary; in other snakes there are rudiments of the pelvis and hind limbs.
Some of the cases of rudimentary organs are extremely curious; for instance,
the presence of teeth in fœtal whales, which when grown up have not a tooth in
their heads; and the presence of teeth, which never cut through the gums, in
the upper jaws of our unborn calves. It has even been stated on good authority
that rudiments of teeth can be detected in the beaks of certain embryonic
birds. Nothing can be plainer than that wings are formed for flight, yet in how
many insects do we see wings so reduced in size as to be utterly incapable of
flight, and not rarely lying under wing-cases, firmly soldered together! The
meaning of rudimentary organs is often quite unmistakeable: for instance there
are beetles of the same genus (and even of the same species) resembling each
other most closely in all respects, one of which will have full-sized wings,
and another mere rudiments of membrane; and here it is impossible to doubt,
that the rudiments represent wings. Rudimentary organs sometimes retain their
potentiality, and are merely not developed: this seems to be the case with the
mammæ 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
Kölreuter 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 Compositæ, 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 compositæ, for the purpose of brushing the
pollen out of the surrounding anthers. Again, an organ may become rudimentary
for its proper purpose, and be used for a distinct object: in certain fish the
swim-bladder seems to be rudimentary for its proper function of giving
buoyancy, but has become converted into a nascent breathing organ or lung.
Other similar instances could be given. Rudimentary
organs in the individuals of the same species are very liable to vary in degree
of development and in other respects. Moreover, in closely allied species, the
degree to which the same organ has been rendered rudimentary occasionally
differs much. This latter fact is well exemplified in the state of the wings of
the female moths in certain groups. Rudimentary organs may be utterly aborted;
and this implies, that we find in an animal or plant no trace of an organ,
which analogy would lead us to expect to find, and which is occasionally found
in monstrous individuals of the species. Thus in the snapdragon (antirrhinum)
we generally do not find a rudiment of a fifth stamen; but this may sometimes
be seen. In tracing the homologies of the same part in different members of a
class, nothing is more common, or more necessary, than the use and discovery of
rudiments. This is well shown in the drawings given by Owen of the bones of the
leg of the horse, ox, and rhinoceros. It is an
important fact that rudimentary organs, such as teeth in the upper jaws of
whales and ruminants, can often be detected in the embryo, but afterwards
wholly disappear. It is also, I believe, a universal rule, that a rudimentary
part or organ is of greater size relatively to the adjoining parts in the
embryo, than in the adult; so that the organ at this early age is less
rudimentary, or even cannot be said to be in any degree rudimentary. Hence,
also, a rudimentary organ in the adult, is often said to have retained its
embryonic condition. I have now
given the leading facts with respect to rudimentary organs. In reflecting on
them, every one must be struck with astonishment: for the same reasoning power
which tells us plainly that most parts and organs are exquisitely adapted for
certain purposes, tells us with equal plainness that these rudimentary or
atrophied organs, are imperfect and useless. In works on natural history
rudimentary organs are generally said to have been created “for the sake of
symmetry,” or in order “to complete the scheme of nature;” but this seems to me
no explanation, merely a restatement of the fact. Would it be thought
sufficient to say that because planets revolve in elliptic courses round the
sun, satellites follow the same course round the planets, for the sake of
symmetry, and to complete the scheme of nature? An eminent physiologist
accounts for the presence of rudimentary organs, by supposing that they serve
to excrete matter in excess, or injurious to the system; but can we suppose
that the minute papilla, which often represents the pistil in male flowers, and
which is formed merely of cellular tissue, can thus act? Can we suppose that
the formation of rudimentary teeth which are subsequently absorbed, can be of
any service to the rapidly growing embryonic calf by the excretion of precious
phosphate of lime? When a man’s fingers have been amputated, imperfect nails
sometimes appear on the stumps: I could as soon believe that these vestiges of
nails have appeared, not from unknown laws of growth, but in order to excrete
horny matter, as that the rudimentary nails on the fin of the manatee were
formed for this purpose. On my view
of descent with modification, the origin of rudimentary organs is simple. We
have plenty of cases of rudimentary organs in our domestic productions,—as the
stump of a tail in tailless breeds,—the vestige of an ear in earless breeds,—the
reappearance of minute dangling horns in hornless breeds of cattle, more
especially, according to Youatt, in young animals,—and the state of the whole
flower in the cauliflower. We often see rudiments of various parts in monsters.
But I doubt whether any of these cases throw light on the origin of rudimentary
organs in a state of nature, further than by showing that rudiments can be
produced; for I doubt whether species under nature ever undergo abrupt changes.
I believe that disuse has been the main agency; that it has led in successive
generations to the gradual reduction of various organs, until they have become
rudimentary,—as in the case of the eyes of animals inhabiting dark caverns, and
of the wings of birds inhabiting oceanic islands, which have seldom been forced
to take flight, and have ultimately lost the power of flying. Again, an organ
useful under certain conditions, might become injurious under others, as with
the wings of beetles living on small and exposed islands; and in this case
natural selection would continue slowly to reduce the organ, until it was
rendered harmless and rudimentary. Any change
in function, which can be effected by insensibly small steps, is within the
power of natural selection; so that an organ rendered, during changed habits of
life, useless or injurious for one purpose, might easily be modified and used
for another purpose. Or an organ might be retained for one alone of its former
functions. An organ, when rendered useless, may well be variable, for its
variations cannot be checked by natural selection. At whatever period of life
disuse or selection reduces an organ, and this will generally be when the being
has come to maturity and to its full powers of action, the principle of
inheritance at corresponding ages will reproduce the organ in its reduced state
at the same age, and consequently will seldom affect or reduce it in the
embryo. Thus we can understand the greater relative size of rudimentary organs
in the embryo, and their lesser relative size in the adult. But if each step of
the process of reduction were to be inherited, not at the corresponding age,
but at an extremely early period of life (as we have good reason to believe to
be possible) the rudimentary part would tend to be wholly lost, and we should
have a case of complete abortion. The principle, also, of economy, explained in
a former chapter, by which the materials forming any part or structure, if not
useful to the possessor, will be saved as far as is possible, will probably
often come into play; and this will tend to cause the entire obliteration of a
rudimentary organ. As the
presence of rudimentary organs is thus due to the tendency in every part of the
organisation, which has long existed, to be inherited—we can understand, on the
genealogical view of classification, how it is that systematists have found
rudimentary parts as useful as, or even sometimes more useful than, parts of
high physiological importance. Rudimentary organs may be compared with the
letters in a word, still retained in the spelling, but become useless in the
pronunciation, but which serve as a clue in seeking for its derivation. On the
view of descent with modification, we may conclude that the existence of organs
in a rudimentary, imperfect, and useless condition, or quite aborted, far from
presenting a strange difficulty, as they assuredly do on the ordinary doctrine
of creation, might even have been anticipated, and can be accounted for by the
laws of inheritance. Summary.—In this
chapter I have attempted to show, that the subordination of group to group in
all organisms throughout all time; that the nature of the relationship, by
which all living and extinct beings are united by complex, radiating, and
circuitous lines of affinities into one grand system; the rules followed and
the difficulties encountered by naturalists in their classifications; the value
set upon characters, if constant and prevalent, whether of high vital
importance, or of the most trifling importance, or, as in rudimentary organs,
of no importance; the wide opposition in value between analogical or adaptive
characters, and characters of true affinity; and other such rules;—all
naturally follow on the view of the common parentage of those forms which are
considered by naturalists as allied, together with their modification through
natural selection, with its contingencies of extinction and divergence of
character. In considering this view of classification, it should be borne in
mind that the element of descent has been universally used in ranking together
the sexes, ages, and acknowledged varieties of the same species, however
different they may be in structure. If we extend the use of this element of
descent,—the only certainly known cause of similarity in organic beings,—we
shall understand what is meant by the natural system: it is genealogical in its
attempted arrangement, with the grades of acquired difference marked by the
terms varieties, species, genera, families, orders, and classes. On this
same view of descent with modification, all the great facts in Morphology
become intelligible,—whether we look to the same pattern displayed in the
homologous organs, to whatever purpose applied, of the different species of a
class; or to the homologous parts constructed on the same pattern in each
individual animal and plant. On the
principle of successive slight variations, not necessarily or generally
supervening at a very early period of life, and being inherited at a
corresponding period, we can understand the great leading facts in Embryology;
namely, the resemblance in an individual embryo of the homologous parts, which
when matured will become widely different from each other in structure and
function; and the resemblance in different species of a class of the homologous
parts or organs, though fitted in the adult members for purposes as different
as possible. Larvæ are active embryos, which have become specially modified in
relation to their habits of life, through the principle of modifications being
inherited at corresponding ages. On this same principle—and bearing in mind,
that when organs are reduced in size, either from disuse or selection, it will
generally be at that period of life when the being has to provide for its own
wants, and bearing in mind how strong is the principle of inheritance—the occurrence
of rudimentary organs and their final abortion, present to us no inexplicable
difficulties; on the contrary, their presence might have been even anticipated.
The importance of embryological characters and of rudimentary organs in
classification is intelligible, on the view that an arrangement is only so far
natural as it is genealogical. Finally,
the several classes of facts which have been considered in this chapter, seem
to me to proclaim so plainly, that the innumerable species, genera, and
families of organic beings, with which this world is peopled, have all
descended, each within its own class or group, from common parents, and have
all been modified in the course of descent, that I should without hesitation
adopt this view, even if it were unsupported by other facts or arguments. |
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