ON THE NATURE OF THE AFFINITIES AND CLASSIFICATION OF ORGANIC
BEINGS{425}
{425} Ch. XIII of the _Origin_, Ed. i., Ch. XIV Ed. vi. begins with
a similar statement. In the present Essay the author adds a
note:--"The obviousness of the fact (_i.e._ the natural grouping of
organisms) alone prevents it being remarkable. It is scarcely
explicable by creationist: groups of aquatic, of vegetable feeders
and carnivorous, &c., might resemble each other; but why as it is.
So with plants,--analogical resemblance thus accounted for. Must
not here enter into details." This argument is incorporated with
the text in the _Origin_, Ed. i.
_Gradual appearance and disappearance of groups._
It has been observed from the earliest times that organic beings fall
into groups{426}, and these groups into others of several values, such
as species into genera, and then into sub-families, into families,
orders, &c. The same fact holds with those beings which no longer exist.
Groups of species seem to follow the same laws in their appearance and
extinction{427}, as do the individuals of any one species: we have
reason to believe that, first, a few species appear, that their numbers
increase; and that, when tending to extinction, the numbers of the
species decrease, till finally the group becomes extinct, in the same
way as a species becomes extinct, by the individuals becoming rarer and
rarer. Moreover, groups, like the individuals of a species, appear to
become extinct at different times in different countries. The
Palaeotherium was extinct much sooner in Europe than in India: the
Trigonia{428} was extinct in early ages in Europe, but now lives in the
seas of Australia. As it happens that one species of a family will
endure for a much longer period than another species, so we find that
some whole groups, such as Mollusca, tend to retain their forms, or to
remain persistent, for longer periods than other groups, for instance
than the Mammalia. Groups therefore, in their appearance, extinction,
and rate of change or succession, seem to follow nearly the same laws
with the individuals of a species{429}.
{426} _Origin_, Ed. i. p. 411, vi. p. 566.
{427} _Origin_, Ed. i. p. 316, vi. p. 457.
{428} _Origin_, Ed. i. p. 321, vi. p. 463.
{429} In the _Origin_, Ed. i. this preliminary matter is replaced
(pp. 411, 412, vi. pp. 566, 567) by a discussion in which
extinction is also treated, but chiefly from the point of view of
the theory of divergence.
_What is the Natural System?_
The proper arrangement of species into groups, according to the natural
system, is the object of all naturalists; but scarcely two naturalists
will give the same answer to the question, What is the natural system
and how are we to recognise it? The most important characters{430} it
might be thought (as it was by the earliest classifiers) ought to be
drawn from those parts of the structure which determine its habits and
place in the economy of nature, which we may call the final end of its
existence. But nothing is further from the truth than this; how much
external resemblance there is between the little otter (Chironectes) of
Guiana and the common otter; or again between the common swallow and the
swift; and who can doubt that the means and ends of their existence are
closely similar, yet how grossly wrong would be the classification,
which put close to each other a Marsupial and Placental animal, and two
birds with widely different skeletons. Relations, such as in the two
latter cases, or as that between the whale and fishes, are denominated
"analogical{431}," or are sometimes described as "relations of
adaption." They are infinitely numerous and often very singular; but are
of no use in the classification of the higher groups. How it comes, that
certain parts of the structure, by which the habits and functions of the
species are settled, are of no use in classification, whilst other
parts, formed at the same time, are of the greatest, it would be
difficult to say, on the theory of separate creations.
{430} _Origin_, Ed. i. p. 414, vi. p. 570.
{431} _Origin_, Ed. i. p. 414, vi. p. 570.
Some authors as Lamarck, Whewell &c., believe that the degree of
affinity on the natural system depends on the degrees of resemblance in
organs more or less physiologically important for the preservation of
life. This scale of importance in the organs is admitted to be of
difficult discovery. But quite independent of this, the proposition, as
a general rule, must be rejected as false; though it may be partially
true. For it is universally admitted that the same part or organ, which
is of the highest service in classification in one group, is of very
little use in another group, though in both groups, as far as we can
see, the part or organ is of equal physiological importance: moreover,
characters quite unimportant physiologically, such as whether the
covering of the body consists of hair or feathers, whether the nostrils
communicated with the mouth{432} &c., &c., are of the highest generality
in classification; even colour, which is so inconstant in many species,
will sometimes well characterise even a whole group of species. Lastly,
the fact, that no one character is of so much importance in determining
to what great group an organism belongs, as the forms through which the
embryo{433} passes from the germ upwards to maturity, cannot be
reconciled with the idea that natural classification follows according
to the degrees of resemblance in the parts of most physiological
importance. The affinity of the common rock-barnacle with the
Crustaceans can hardly be perceived in more than a single character in
its mature state, but whilst young, locomotive, and furnished with eyes,
its affinity cannot be mistaken{434}. The cause of the greater value of
characters, drawn from the early stages of life, can, as we shall in a
succeeding chapter see, be in a considerable degree explained, on the
theory of descent, although inexplicable on the views of the
creationist.
{432} These instances occur with others in the _Origin_, Ed. i. p.
416, vi. p. 572.
{433} _Origin_, Ed. i. p. 418, vi. p. 574.
{434} _Origin_, Ed. i. pp. 419, 440, vi. pp. 575, 606.
Practically, naturalists seem to classify according to the resemblance
of those parts or organs which in related groups are most uniform, or
vary least{435}: thus the aestivation, or manner in which the petals etc.
are folded over each other, is found to afford an unvarying character in
most families of plants, and accordingly any difference in this respect
would be sufficient to cause the rejection of a species from many
families; but in the Rubiaceae the aestivation is a varying character, and
a botanist would not lay much stress on it, in deciding whether or not
to class a new species in this family. But this rule is obviously so
arbitrary a formula, that most naturalists seem to be convinced that
something ulterior is represented by the natural system; they appear to
think that we only discover by such similarities what the arrangement of
the system is, not that such similarities make the system. We can only
thus understand Linnaeus'{436} well-known saying, that the characters do
not make the genus; but that the genus gives the characters: for a
classification, independent of characters, is here presupposed. Hence
many naturalists have said that the natural system reveals the plan of
the Creator: but without it be specified whether order in time or place,
or what else is meant by the plan of the Creator, such expressions
appear to me to leave the question exactly where it was.
{435} _Origin_, Ed. i. pp. 418, 425, vi. pp. 574, 581.
{436} _Origin_, Ed. i. p. 413, vi. p. 569.
Some naturalists consider that the geographical position{437} of a
species may enter into the consideration of the group into which it
should be placed; and most naturalists (either tacitly or openly) give
value to the different groups, not solely by their relative differences
in structure, but by the number of forms included in them. Thus a genus
containing a few species might be, and has often been, raised into a
family on the discovery of several other species. Many natural families
are retained, although most closely related to other families, from
including a great number of closely similar species. The more logical
naturalist would perhaps, if he could, reject these two contingents in
classification. From these circumstances, and especially from the
undefined objects and criterions of the natural system, the number of
divisions, such as genera, sub-families, families, &c., &c., has been
quite arbitrary{438}; without the clearest definition, how can it be
possible to decide whether two groups of species are of equal value, and
of what value? whether they should both be called genera or families; or
whether one should be a genus, and the other a family{439}?
{437} _Origin_, Ed. i. pp. 419, 427, vi. pp. 575, 582.
{438} This is discussed from the point of view of divergence in the
_Origin_, Ed. i. pp. 420, 421, vi. pp. 576, 577.
{439} <Footnote by the author.> I discuss this because if Quinarism
true, I false. <The Quinary System is set forth in W. S. Macleay's
_Horae Entomologicae_, 1821.>
_On the kind of relation between distinct groups._
I have only one other remark on the affinities of organic beings; that
is, when two quite distinct groups approach each other, the approach is
_generally_ generic{440} and not special; I can explain this most easily
by an example: of all Rodents the Bizcacha, by certain peculiarities in
its reproductive system, approaches nearest to the Marsupials; of all
Marsupials the Phascolomys, on the other hand, appears to approach in
the form of its teeth and intestines nearest to the Rodents; but there
is no special relation between these two genera{441}; the Bizcacha is no
nearer related to the Phascolomys than to any other Marsupial in the
points in which it approaches this division; nor again is the
Phascolomys, in the points of structure in which it approaches the
Rodents, any nearer related to the Bizcacha than to any other Rodent.
Other examples might have been chosen, but I have given (from
Waterhouse) this example as it illustrates another point, namely, the
difficulty of determining what are analogical or adaptive and what real
affinities; it seems that the teeth of the Phascolomys though _appearing
closely_ to resemble those of a Rodent are found to be built on the
Marsupial type; and it is thought that these teeth and consequently the
intestines may have been adapted to the peculiar life of this animal and
therefore may not show any real relation. The structure in the Bizcacha
that connects it with the Marsupials does not seem a peculiarity related
to its manner of life, and I imagine that no one would doubt that this
shows a real affinity, though not more with any one Marsupial species
than with another. The difficulty of determining what relations are real
and what analogical is far from surprising when no one pretends to
define the meaning of the term relation or the ulterior object of all
classification. We shall immediately see on the theory of descent how it
comes that there should be "real" and "analogical" affinities; and why
the former alone should be of value in classification--difficulties
which it would be I believe impossible to explain on the ordinary theory
of separate creations.
{440} In the corresponding passage in the _Origin_, Ed. i. p. 430,
vi. p. 591, the term _general_ is used in place of _generic_, and
seems a better expression. In the margin the author gives
Waterhouse as his authority.
{441} _Origin_, Ed. i. p. 430, vi. p. 591.
_Classification of Races or Varieties._
Let us now for a few moments turn to the classification of the generally
acknowledged varieties and subdivisions of our domestic beings{442}; we
shall find them systematically arranged in groups of higher and higher
value. De Candolle has treated the varieties of the cabbage exactly as
he would have done a natural family with various divisions and
subdivisions. In dogs again we have one main division which may be
called the _family_ of hounds; of these, there are several (we will call
them) _genera_, such as blood-hounds, fox-hounds, and harriers; and of
each of these we have different _species_, as the blood-hound of Cuba
and that of England; and of the latter again we have breeds truly
producing their own kind, which may be called races or varieties. Here
we see a classification practically used which typifies on a lesser
scale that which holds good in nature. But amongst true species in the
natural system and amongst domestic races the number of divisions or
groups, instituted between those most alike and those most unlike, seems
to be quite arbitrary. The number of the forms in both cases seems
practically, whether or not it ought theoretically, to influence the
denomination of groups including them. In both, geographical
distribution has sometimes been used as an aid to classification{443};
amongst varieties, I may instance, the cattle of India or the sheep of
Siberia, which from possessing some characters in common permit a
classification of Indian and European cattle, or Siberian and European
sheep. Amongst domestic varieties we have even something very like the
relations of "analogy" or "adaptation{444}"; thus the common and Swedish
turnip are both artificial varieties which strikingly resemble each
other, and they fill nearly the same end in the economy of the
farm-yard; but although the swede so much more resembles a turnip than
its presumed parent the field cabbage, no one thinks of putting it out
of the cabbages into the turnips. Thus the greyhound and racehorse,
having been selected and trained for extreme fleetness for short
distances, present an analogical resemblance of the same kind, but less
striking as that between the little otter (Marsupial) of Guiana and the
common otter; though these two otters are really less related than <are>
the horse and dog. We are even cautioned by authors treating on
varieties, to follow the _natural_ in contradistinction of an artificial
system and not, for instance, to class two varieties of the
pine-apple{445} near each other, because their fruits accidentally
resemble each other closely (though the fruit may be called _the final
end_ of this plant in the economy of its world, the hothouse), but to
judge from the general resemblance of the entire plants. Lastly,
varieties often become extinct; sometimes from unexplained causes,
sometimes from accident, but more often from the production of more
useful varieties, and the less useful ones being destroyed or bred out.
{442} In a corresponding passage in the _Origin_, Ed. i. p. 423,
vi. p. 579, the author makes use of his knowledge of pigeons. The
pseudo-genera among dogs are discussed in _Var. under Dom._, Ed.
ii. vol. I. p. 38.
{443} _Origin_, Ed. i. pp. 419, 427, vi. pp. 575, 582.
{444} _Origin_, Ed. i. pp. 423, 427, vi. pp. 579, 583.
{445} _Origin_, Ed. i. p. 423, vi. p. 579.
I think it cannot be doubted that the main cause of all the varieties
which have descended from the aboriginal dog or dogs, or from the
aboriginal wild cabbage, not being equally like or unlike--but on the
contrary, obviously falling into groups and sub-groups--must in chief
part be attributed to different degrees of true relationship; for
instance, that the different kinds of blood-hound have descended from
one stock, whilst the harriers have descended from another stock, and
that both these have descended from a different stock from that which
has been the parent of the several kinds of greyhound. We often hear of
a florist having some choice variety and breeding from it a whole group
of sub-varieties more or less characterised by the peculiarities of the
parent. The case of the peach and nectarine, each with their many
varieties, might have been introduced. No doubt the relationship of our
different domestic breeds has been obscured in an extreme degree by
their crossing; and likewise from the slight difference between many
breeds it has probably often happened that a "sport" from one breed has
less closely resembled its parent breed than some other breed, and has
therefore been classed with the latter. Moreover the effects of a
similar climate{446} may in some cases have more than counterbalanced
the similarity, consequent on a common descent, though I should think
the similarity of the breeds of cattle of India or sheep of Siberia was
far more probably due to the community of their descent than to the
effects of climate on animals descended from different stocks.
{446} A general statement of the influence of conditions on
variation occurs in the _Origin_, Ed. i. pp. 131-3, vi. pp. 164-5.
Notwithstanding these great sources of difficulty, I apprehend every
one would admit, that if it were possible, a genealogical classification
of our domestic varieties would be the most satisfactory one; and as far
as varieties were concerned would be the natural system: in some cases
it has been followed. In attempting to follow out this object a person
would have to class a variety, whose parentage he did not know, by its
external characters; but he would have a distinct ulterior object in
view, namely, its descent in the same manner as a regular systematist
seems also to have an ulterior but undefined end in all his
classifications. Like the regular systematist he would not care whether
his characters were drawn from more or less important organs as long as
he found in the tribe which he was examining that the characters from
such parts were persistent; thus amongst cattle he does value a
character drawn from the form of the horns more than from the
proportions of the limbs and whole body, for he finds that the shape of
the horns is to a considerable degree persistent amongst cattle{447},
whilst the bones of the limbs and body vary. No doubt as a frequent rule
the more important the organ, as being less related to external
influences, the less liable it is to variation; but he would expect that
according to the object for which the races had been selected, parts
more or less important might differ; so that characters drawn from parts
generally most liable to vary, as colour, might in some instances be
highly serviceable--as is the case. He would admit that general
resemblances scarcely definable by language might sometimes serve to
allocate a species by its nearest relation. He would be able to assign a
clear reason why the close similarity of the fruit in two varieties of
pine-apple, and of the so-called root in the common and Swedish turnips,
and why the similar gracefulness of form in the greyhound and
racehorse, are characters of little value in classification; namely,
because they are the result, not of community of descent, but either of
selection for a common end, or of the effects of similar external
conditions.
{447} _Origin_, Ed. i. p. 423, vi. p. 579. In the margin Marshall
is given as the authority.
_Classification of "races" and species similar._
Thus seeing that both the classifiers of species and of varieties{448}
work by the same means, make similar distinctions in the value of the
characters, and meet with similar difficulties, and that both seem to
have in their classification an ulterior object in view; I cannot avoid
strongly suspecting that the same cause, which has made amongst our
domestic varieties groups and sub-groups, has made similar groups (but
of higher values) amongst species; and that this cause is the greater or
less propinquity of actual descent. The simple fact of species, both
those long since extinct and those now living, being divisible into
genera, families, orders &c.--divisions analogous to those into which
varieties are divisible--is otherwise an inexplicable fact, and only not
remarkable from its familiarity.
{448} _Origin_, Ed. i. p. 423, vi. p. 579.
_Origin of genera and families._
Let us suppose{449} for example that a species spreads and arrives at
six or more different regions, or being already diffused over one wide
area, let this area be divided into six distinct regions, exposed to
different conditions, and with stations slightly different, not fully
occupied with other species, so that six different races or species
were formed by selection, each best fitted to its new habits and
station. I must remark that in every case, if a species becomes modified
in any one sub-region, it is probable that it will become modified in
some other of the sub-regions over which it is diffused, for its
organization is shown to be capable of being rendered plastic; its
diffusion proves that it is able to struggle with the other inhabitants
of the several sub-regions; and as the organic beings of every great
region are in some degree allied, and as even the physical conditions
are often in some respects alike, we might expect that a modification in
structure, which gave our species some advantage over antagonist species
in one sub-region, would be followed by other modifications in other of
the sub-regions. The races or new species supposed to be formed would be
closely related to each other; and would either form a new genus or
sub-genus, or would rank (probably forming a slightly different section)
in the genus to which the parent species belonged. In the course of
ages, and during the contingent physical changes, it is probable that
some of the six new species would be destroyed; but the same advantage,
whatever it may have been (whether mere tendency to vary, or some
peculiarity of organization, power of mind, or means of distribution),
which in the parent-species and in its six selected and changed
species-offspring, caused them to prevail over other antagonist species,
would generally tend to preserve some or many of them for a long period.
If then, two or three of the six species were preserved, they in their
turn would, during continued changes, give rise to as many small groups
of species: if the parents of these small groups were closely similar,
the new species would form one great genus, barely perhaps divisible
into two or three sections: but if the parents were considerably
unlike, their species-offspring would, from inheriting most of the
peculiarities of their parent-stocks, form either two or more sub-genera
or (if the course of selection tended in different ways) genera. And
lastly species descending from different species of the newly formed
genera would form new genera, and such genera collectively would form a
family.
{449} The discussion here following corresponds more or less to the
_Origin_, Ed. i. pp. 411, 412, vi. pp. 566, 567; although the
doctrine of divergence is not mentioned in this Essay (as it is in
the _Origin_) yet the present section seems to me a distinct
approximation to it.
The extermination of species follows from changes in the external
conditions, and from the increase or immigration of more favoured
species: and as those species which are undergoing modification in any
one great region (or indeed over the world) will very often be allied
ones from (as just explained) partaking of many characters, and
therefore advantages in common, so the species, whose place the new or
more favoured ones are seizing, from partaking of a common inferiority
(whether in any particular point of structure, or of general powers of
mind, of means of distribution, of capacity for variation, &c., &c.),
will be apt to be allied. Consequently species of the same genus will
slowly, one after the other, _tend_ to become rarer and rarer in
numbers, and finally extinct; and as each last species of several allied
genera fails, even the family will become extinct. There may of course
be occasional exceptions to the entire destruction of any genus or
family. From what has gone before, we have seen that the slow and
successive formation of several new species from the same stock will
make a new genus, and the slow and successive formation of several other
new species from another stock will make another genus; and if these two
stocks were allied, such genera will make a new family. Now, as far as
our knowledge serves, it is in this slow and gradual manner that groups
of species appear on, and disappear from, the face of the earth.
The manner in which, according to our theory, the arrangement of species
in groups is due to partial extinction, will perhaps be rendered clearer
in the following way. Let us suppose in any one great class, for
instance in the Mammalia, that every species and every variety, during
each successive age, had sent down one unaltered descendant (either
fossil or living) to the present time; we should then have had one
enormous series, including by small gradations every known mammiferous
form; and consequently the existence of groups{450}, or chasms in the
series, which in some parts are in greater width, and in some of less,
is solely due to former species, and whole groups of species, not having
thus sent down descendants to the present time.
{450} The author probably intended to write "groups separated by
chasms."
With respect to the "analogical" or "adaptive" resemblances between
organic beings which are not really related{451}, I will only add, that
probably the isolation of different groups of species is an important
element in the production of such characters: thus we can easily see, in
a large increasing island, or even a continent like Australia, stocked
with only certain orders of the main classes, that the conditions would
be highly favourable for species from these orders to become adapted to
play parts in the economy of nature, which in other countries were
performed by tribes especially adapted to such parts. We can understand
how it might happen that an otter-like animal might have been formed in
Australia by slow selection from the more carnivorous Marsupial types;
thus we can understand that curious case in the southern hemisphere,
where there are no auks (but many petrels), of a petrel{452} having been
modified into the external general form so as to play the same office
in nature with the auks of the northern hemisphere; although the habits
and form of the petrels and auks are normally so wholly different. It
follows, from our theory, that two orders must have descended from one
common stock at an immensely remote epoch; and we can perceive when a
species in either order, or in both, shows some affinity to the other
order, why the affinity is usually generic and not particular--that is
why the Bizcacha amongst Rodents, in the points in which it is related
to the Marsupial, is related to the whole group{453}, and not
particularly to the Phascolomys, which of all Marsupialia is related
most to the Rodents. For the Bizcacha is related to the present
Marsupialia, only from being related to their common parent-stock; and
not to any one species in particular. And generally, it may be observed
in the writings of most naturalists, that when an organism is described
as intermediate between two _great_ groups, its relations are not to
particular species of either group, but to both groups, as wholes. A
little reflection will show how exceptions (as that of the Lepidosiren,
a fish closely related to _particular_ reptiles) might occur, namely
from a few descendants of those species, which at a very early period
branched out from a common parent-stock and so formed the two orders or
groups, having survived, in nearly their original state, to the present
time.
{451} A similar discussion occurs in the _Origin_, Ed. i. p. 427,
vi. p. 582.
{452} _Puffinuria berardi_, see _Origin_, Ed. i. p. 184, vi. p.
221.
{453} _Origin_, Ed. i. p. 430, vi. p. 591.
Finally, then, we see that all the leading facts in the affinities and
classification of organic beings can be explained on the theory of the
natural system being simply a genealogical one. The similarity of the
principles in classifying domestic varieties and true species, both
those living and extinct, is at once explained; the rules followed and
difficulties met with being the same. The existence of genera, families,
orders, &c., and their mutual relations, naturally ensues from
extinction going on at all periods amongst the diverging descendants of
a common stock. These terms of affinity, relations, families, adaptive
characters, &c., which naturalists cannot avoid using, though
metaphorically, cease being so, and are full of plain signification.