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 larvae are sometimes
extremely different; and as it has been used in classing varieties which
have undergone a certain, and sometimes a considerable amount of
modification, may not this same element of descent have been unconsciously
used in grouping species under genera, and genera under higher groups,
though in these cases the modification has been greater in degree, and has
taken a longer time to complete? I believe it has thus been unconsciously
used; and only thus can I understand the several rules and guides which
have been followed by our best systematists. We have no written pedigrees;
we have to make out community of descent by resemblances of any kind.
Therefore we choose those characters which, as far as we can judge, are the
least likely to have been modified in relation to the conditions of life to
which each species has been recently exposed. Rudimentary structures on
this view are as good as, or even sometimes better than, other parts of the
organisation. We care 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
Linnaeus, 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 be
tween 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