Charles Darwin.

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other, must be strictly genealogical in order to be natural; but that the
amount of difference in the several branches or groups, though allied in the
same degree in blood to their common progenitor, may differ greatly, being
due to the different degrees of modification which they have undergone;
and this is expressed by the forms being ranked under different genera,
families, sections, or orders. The reader will best understand what is meant
if he will take the trouble to refer to the diagram in the fourth chapter. We
will suppose the letters A and L to represent allied genera existing during
the Silurian epoch, and descended from some still earlier form. In three of
these genera (A, F, and I) a species has transmitted modified descendants
to the present day, represented by the fifteen genera {a^^ to z^^) on the up-
permost horizontal line. Now, all these modified descendants from a single
species are related in blood or descent in 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 /^* 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 organisms belong to Silurian
genera. So that the comparative value of the differences between these
organic beings, which are all related to each other in the same degree in
blood, has come to be widely diff'erent. 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 parents, as will all the
descendants from I; so will it be with each subordinate branch of de-
scendants at each successive stage. If, however, we suppose any descendant
of A or of I to have become so much modified as to have lost all traces of
its parentage in this case, its place in the natural system will be lost, as seems
to have occurred with some few 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 form a single genus. But this genus, though much
isolated, will still occupy its proper intermediate position. The representation
of the groups, as here given in the diagram on a flat surface, is much too
simple. The branches ought to have diverged in all directions. If the names
of the groups had been simply written down in a linear series, the repre-
sentation would have been still less natural ; and it is notoriously not possible
to represent in a series, on a flat surface, the affinities which we discover in
nature among the beings of the same group. Thus, the natural system is
genealogical in its arrangement, like a pedigree. But the amount of modifica-
tion which the different groups have undergone has to be expressed by
ranking them under different so-called genera, sub-families, families, sec-
tions, 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 classifica-
tion of the various languages now spoken throughout the world; and if all
extinct languages, and all intermediate and slowly changing dialects, were
to be included, such an arrangement would be the only possible one. Yet it
might be that some ancient languages had altered very little and had given
rise to few new languages, while others had altered much, owing to the
spreading, isolation, and state of civilization of the several codescended races,
and had thus given rise to many new dialects and languages. The various
degrees of difference between the languages of the same stock would have to
be expressed by groups subordinate to groups; but the proper or even the
only possible arrangement would still be genealogical; and this would be
strictly natural, as it would connect together all languages, extinct and
recent, by the closest affinities, and would give the filiation and origin of each

In confirmation of this view, let us glance at the classification of varieties,
which are known or believed to be descended from a single species. These
are grouped under the species, with the sub-varieties under the varieties;
and in some cases, as with the domestic pigeon, with several other grades
of difference. Nearly the same rules are followed as in classifying species.
Authors have insisted on the necessity of arranging varieties on a natural
instead of an artificial system; we are cautioned, for instance, not to class
two varieties of the pineapple together, merely because their fruit, though
the most important part, happens to be nearly identical; no one puts the
Swedish and common turnip 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 color of the body, etc. ; whereas with sheep the horns are much
less serviceable, because less constant. In classing varieties, I apprehend
that if we had a real pedigree, a genealogical classification would be uni-
versally preferred; and it has been attempted in some cases. For we might
feel sure, whether there had been more or less modification, that the principle
of inheritance would keep the forms together which were alUed in the greatest
number of points. In tumbler pigeons, though some of the sub-varieties differ
in the important character of the length of the beak, yet all are kept to-
gether from having the common habit of tumbling; but the short-faced
breed has nearly or quite lost this habit; nevertheless, without any thought
on the subject, these tumblers are kept in the same group, because allied in
blood and alike in some other respects.

With species in a state of nature, every naturalist has in fact brought
descent into his classification; for he includes in his lowest grade, that of
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 adult males and hermaphrodites
of certain cirripedes, and yet no one dreams of separating them. As soon as
the three Orchidean forms, Monachanthus, Myanthus, and Catasetum,
which had previously been ranked as three, distinct genera, were known to
be sometimes produced on the same plant, they were immediately considered
as varieties; and now I have been able to show that they are the male,
female, and hermaphrodite forms of the same species. The naturalist includes
as one species the various larval stages of the same individual, however much
they may differ from each other and from the adult, as well as the so-
called alternate generations of Steenstrup, which can only in a technical
sense be considered as the same individual. He includes monsters and
varieties, not from their partial resemblance to the parent-form, but because
they are descended from it.

As descent has universally been used in classing together the individuals
of the same species, though the males and females and larvas 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, all under
the so-called natural system? I believe it has been unconsciously used; and
thus only can I understand the several rules and guides which have been
followed by our best systematists. As ^ve have no written pedigrees, we are
forced to trace community of descent by resemblances of any kind. There-
fore, we choose those characters which are the least likely to have been
modified, in relation to the conditions of life to which each species has been
recently exposed. Rudimentar)^ structures on this \'iew are as good, or even
sometimes better than other parts of the organization. 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 dif-
ferent habits, only by 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, concur 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 aggregated characters have especial value in classification.

We can understand why a species or a group of species may depart from
its allies, in several of its most important characteristics, 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, betray 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 presently see why embryological characters are of
such high classificatory importance. Geographical distribution may some-
times be brought usefully into play in classing large genera, because all the
species of the same genus, inhabiting any distinct and isolated region, are in
all probability descended from the same parents.


We can understand, on the above views, the very important distinction
between real affinities and analogical or adaptive resemblances. Lamarck
first called attention to this subject, and he has been ably followed by
Macleay and others. The resemblances in the shape of the body and in the
fin-like anterior limbs between dugongs and whales, and between these two
orders of mammals and fishes, are analogical. So is the resemblance between
a mouse and a shrew-mouse (Sorex), which belong to different orders; and
the still closer resemblance, insisted on by Mr. Mivart, between the mouse
and a small marsupial animal (Antechinus) of Australia. These latter
resemblances may be accounted for, as it seems to me, by adaptation for
similarly active movements through thickets and herbage, together with con-
cealment from enemies.

Among insects there are innumerable similar instances; thus Linnaeus,
misled by external appearances, actually classed an homopterous insect as a
moth. We see something of the same kind even with our domestic varieties,


as in the strikingly similar shape of the body in the improved breeds of the
Chinese and common pig, which are descended from distinct species; and
in the similarly thickened stems of the common and specifically distinct
Swedish turnip. The resemblance between the greyhound and the race-
horse is hardly more fanciful than the analogies which have been drawn by
some authors between widely different animals.

On the 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 characters, 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 have become' adapted to similar
conditions, and thus have assumed a close external resemblance; but such
resemblances will not reveal — will rather tend to conceal their blood-
relationship. We can thus also understand the apparent paradox, that the
very same characters are analogical when one group is compared with
another, but give true affinities when the members of the same group are
compared together: 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 between the several mem-
bers of the whale family, the shape of the body and the fin-like limbs offer
characters exhibiting true affinity; for as these parts are so nearly similar
throughout the whole family, we cannot doubt that they have been inherited
from a common ancestor. So it is with fishes.

Numerous cases could be given of striking resemblances in quite distinct
beings between single parts of organs, which have been adapted for the same
functions. A good instance is afforded by the close resemblance of the jaws
of the dog and Tasmanian wolf or Thylacinus — animals which are widely
sundered in the natural system. But this resemblance is confined to general
appearance, as in the prominence of the canines, and in the cutting shape
of the molar teeth. For the teeth really differ much: thus the dog has on
each side of the upper jaw four pre-molars and only two molars; while the
Thylacinus has three pre-molars and four molars. The molars also differ
much in the two animals in relative size and structure. The adult dentition is
preceded by a widely different milk dentition. Any one may, of course, deny
that the teeth in either case have been adapted for tearing flesh, through
the natural selection of successive variations; but if this be admitted in the
one case, it is unintelligible to me that it should be denied in the other. I
am glad to find that so high an authority as Professor Flower has come to
this same conclusion.

The extraordinary cases given in a former chapter, of widely different
fishes possessing electric organs — of widely different insects possessing
luminous organs — and of orchids and asclepiads having pollen-masses with
viscid disks, come under this same head of analogical resemblances. But
these cases are so wonderful that they were introduced as difficulties or
objections to our theory. In all such cases some fundamental difference in
the growth or development of the parts, and generally in their matured


structure, can be detected. The end gained is the same, but the means,
though appearing superficially to be the same, are essentially different. The
principle formerly alluded to under the term of analogical variation has
probably in these cases often come into play; that is, the members of the
same class, although only distantly alHed, have inherited so much in common
in their constitution, that they are apt to vary under siroilar exciting causes in.
a similar manner; and this would obviously aid in the acquirement through
natural selection of parts or organs, strikingly Hke each other, independently
of their direct inheritance from a common progenitor.

As species belonging to distinct classes have often been adapted by suc-
cessive 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 of distinct classes. A naturalist,
struck with a parallelism of this nature, by arbitrarily raising or sinking the
value of the groups in several classes (and all our experience shows that their
valuation is as yet arbitrary), could easily extend the parallelism over a
wide range; and thus the septenary, quinary, quaternary, and ternary
classifications have probably arisen.

There is another and curious class of cases in which close external re-
semblance does not depend on adaptation to similar habits of life, but has
been gained for the sake of protection. I allude to the wonderful manner in
which certain butterflies imitate, as first described by Mr. Bates, other and
quite distinct species. This excellent observer has shown that in some districts
of South America, where, for instance, an Ithomia abounds in gaudy swarms,
another butterfly, namely, a Leptalis, is often found mingled in the same
flock; and the latter so closely resembles the Ithomia in every shade and
stripe of color, and even in the shape of its wings, that Mr. Bates, with his eyes
sharpened by collecting during eleven years, was, though always on his guard,
continually deceived. When the mockers and the mocked are caught and
compared, they are found to be very different in essential structure, and to
belong not only to distinct genera, but often to distinct families. Had this
mimicry occurred in only one or two instances, it might have been passed
over as a strange coincidence. But, if we proceed from a district where one
Leptalis imitates an Ithomia, another mocking and mocked species, be-
longing to the same two genera, equally close in their resemblance, may be
found. Altogether no less than ten genera are enumerated, which include
species that imitate other butterflies. The mockers and mocked always
inhabit the same region; we never find an imitator living remote from the
form which it imitates. The mockers are almost invariably rare insects; the
mocked in almost every case abounds in swarms. In the same district in
which a species of Leptalis closely imitates an Ithomia, there are sometimes
other Lepidoptera mimicking the same Ithomia: so that in the same place,
species of three genera of butterflies and even a moth are found all closely
resembling a butterfly belonging to a fourth genus. It deserves especial notice,
that many of the mimicking forms of the Leptalis, as well as of the mimicked.


forms, can be shown by a graduated series to be merely varieties of the same
species; while others are undoubtedly distinct species. But why, it may be
asked, are certain forms treated as the mimicked and others as the
mimickers? Mr. Bates satisfactorily answers this question by showing that
the form which is imitated keeps the usual dress of the group to which it
belongs, while the counterfeiters have changed their dress and do not re-
semble their nearest allies.

We are next led to inquire what reason can be assigned for certain butter-
flies and moths so often assuming the dress of another and quite distinct
form; why, to the perplexity of naturalists, has nature condescended to the
tricks of the stage? Mr. Bates has, no doubt, hit on the true explanation.
The mocked forms, which always abound in numbers, must habitually
escape destruction to a large extent, otherwise they could not exist in such
swarms; and a large amount of evidence has now been collected, showing
that they are distasteful to birds and other insect-devouring animals. The
mocking forms, on the other hand, that inhabit the same district, are com-
paratively rare, and belong to rare groups ; hence, they must suffer habitually
from some danger, for otherwise, from the number of eggs laid by all butter-
flies, they would in three or four generations swarm over the whole country.
Now if a member of one of these persecuted and rare groups were to assume
a dress so like that of a well-protected species that it continually deceived
the practised eyes of an entomologist, it would often deceive predaceous
birds and insects, and thus often escape destruction. Mr. Bates may almost
be said to have actually v>^itnessed the process by which the mimickers have
come so closely to resemble the mimicked; for he found that some of the
forms of Leptalis which mimic so many other butterflies, varied in an
extreme degree. In one district several varieties occurred, and of these one
alone resembled, to a certain extent, the common Ithomia of the same dis-
trict. In another district there were two or three varieties, one of which was
much commoner than the others, and this closely mocked another form of
Ithomia. From facts of this nature, Mr. Bates concludes that the Leptalis
first varies; and when a variety happens to resemble in some degree any
common butterfly inhabiting the same district, this variety, from its re-
semblance to a flourishing and little persecuted kind, has a better chance of
escaping destruction from predaceous birds and insects, and is consequently
oftener preserved; "the less perfect degrees of resemblance being generation
after generation eliminated, and only the others left to propagate their kind."
So that here we have an excellent illustration of natural selection.

Messrs. Wallace and Trimen have likewise described several equally
striking cases of imitation in the Lepidoptera of the Malay Archipelago and
Africa, and with some other insects. Mr. Wallace has also detected one such
case with birds, but we have none with the larger quadrupeds. The much
greater frequency of imitation with insects than with other animals, is
probably the consequence of their small size; insects cannot defend them-
selves, excepting indeed the kinds furnished with a sting, and I have never
heard of an instance of such kinds mocking other insects, though they are


mocked; insects cannot easily escape by flight from the larger animals which
prey on them; therefore, speaking metaphorically, they are reduced, like
most weak creatures, to trickery and dissimulation.

It should be observed that the process of imitation probably never com-
menced between forms widely dissimilar in color. But, starting with species
already somewhat like each other, the closest resemblance, if beneficial, could
readily be gained by the above means, and if the imitated form was sub-
sequently and gradually modified through any agency, the imitating form
would be led along the same track, and thus be altered to almost any extent,
so that it might ultimately assume an appearance or coloring wholly unlike
that of the other members of the family to which it belonged. There is, how-
ever, some difficulty on this head, for it is necessary to suppose in some cases
that ancient members belonging to several distinct groups, before they had
diverged to their present extent, accidentally resembled a member of another
and protected group in a sufficient degree to afford some slight protection,
this having given the basis for the subsequent acquisition of the most perfect


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 within each class 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 and under still fewer classes.
As showing how few the higher groups are in number, and how widely they
are spread throughout the world, the fact is striking that the discovery of
Australia has not added an insect belonging to a new class, and that in the
vegetable kingdom, as I learn from Dr. Hooker, it has added only two or
three families of small size.

In the chapter on geological succession I attempted to show, on the prin-
ciple 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 degree intermediate between
existing groups. As some few of the old and intermediate forms have

Online LibraryCharles DarwinThe origin of species → online text (page 42 of 50)