THE ARGUMENTS FROM CLASSIFICATION.
§ 122. In § 103, we saw that the relations which exist among the species,
genera, orders, and classes of organisms, are not interpretable as results
of any such causes as have usually been assigned. We will here consider
whether they are interpretable as the results of evolution. Let us first
contemplate some familiar facts.
The Norwegians, Swedes, Danes, Germans, Dutch, and Anglo-Saxons, form
together a group of Scandinavian races, which are but slightly divergent in
their characters. Welsh, Irish, and Highlanders, though they have
differences, have not such differences as hide a decided community of
nature: they are classed together as Celts. Between the Scandinavian race
as a whole and the Celtic race as a whole, there is a distinction greater
than that between the sub-divisions which make up the one or the other.
Similarly, the several peoples inhabiting Southern Europe are more nearly
allied to one another, than the aggregate they form is allied to the
aggregates of Northern peoples. If, again, we compare these European
varieties of Man, taken as a group, with that group of Eastern varieties
which had a common origin with it, we see a stronger contrast than between
the groups of European varieties themselves. And once more, ethnologists
find differences of still higher importance between the Aryan stock as a
whole and the Mongolian stock as a whole, or the Negro stock as a whole.
Though these contrasts are partially obscured by intermixtures, they are
not so much obscured as to hide the truths that the most-nearly-allied
varieties of Man are those which diverged from one another at
comparatively-recent periods; that each group of nearly-allied varieties is
more strongly contrasted with other such groups that had a common origin
with it at a remoter period; and so on until we come to the largest groups,
which are the most strongly contrasted, and of whose divergence no trace is
extant.
The relations existing among the classes and sub-classes of languages, have
been briefly referred to by Mr. Darwin in illustration of his argument. We
know that languages have arisen by evolution. Let us then see what grouping
of them evolution has produced. On comparing the dialects of adjacent
counties in England, we find that their differences are so small as
scarcely to distinguish them. Between the dialects of the Northern counties
taken together, and those of the Southern counties taken together, the
contrast is stronger. These clusters of dialects, together with those of
Scotland and Ireland, are nevertheless so similar that we regard them as
one language. The several languages of Scandinavian Europe, including
English, are much more unlike one another than are the several dialects
which each of them includes; in correspondence with the fact that they
diverged from one another at earlier periods than did their respective
dialects. The Scandinavian languages have nevertheless a certain community
of character, distinguishing them as a group from the languages of Southern
Europe; between which there are general and special affinities that
similarly unite them into a group formed of sub-groups containing
sub-sub-groups. And this wider divergence between the order of languages
spoken in Northern Europe and the order of languages spoken in Southern
Europe, answers to the longer time that has elapsed since their
differentiation commenced. Further, these two orders of modern European
languages, as well as Latin and Greek and certain extinct and spoken
languages of the East, are shown to have traits in common which unite them
into one great class known as Aryan languages; radically distinguished from
the classes of languages spoken by the other main divisions of the human
race.
§ 123. Now this kind of subordination of groups which we see arises in the
course of continuous descent, multiplication, and divergence, is just the
kind of subordination of groups which plants and animals exhibit: it is
just the kind of subordination which has thrust itself on the attention of
naturalists in spite of pre-conceptions.
The original idea was that of arrangement in linear order. We saw that even
after a considerable acquaintance with the structures of organisms had been
acquired, naturalists continued their efforts to reconcile the facts with
the notion of a uni-serial succession. The accumulation of evidence
necessitated the breaking up of the imagined chain into groups and
sub-groups. Gradually there arose the conviction that these groups do not
admit of being placed in a line. And the conception finally arrived at, is
that of certain great sub-kingdoms, very widely divergent, each made up of
classes much less divergent, severally containing orders still less
divergent; and so on with genera and species.
Hence this "grand fact in natural history of the subordination of group
under group, which from its familiarity does not always sufficiently strike
us," is perfectly in harmony with the hypothesis of evolution. The extreme
significance of this kind of relation among organic forms is dwelt on by
Mr. Darwin, who shows how an ordinary genealogical tree represents, on a
small scale, a system of grouping analogous to that which exists among
organisms in general, and which is explained on the supposition of a
genealogical tree by which all organisms are affiliated. If, wherever we
can trace direct descent, multiplication, and divergence, this formation of
groups within groups takes place; there results a strong presumption that
the groups within groups which constitute the animal and vegetal kingdoms,
have arisen by direct descent, multiplication, and divergence--that is, by
evolution.
§ 124. Strong confirmation of this inference is yielded by the fact, that
the more marked differences which divide groups are, in both cases,
distinguished from the less marked differences which divide sub-groups, by
this, that they are not simply greater in _degree_, but they are more
radical in _kind_. Objects, as the stars, may present themselves in small
clusters, which are again more or less aggravated into clusters of
clusters, in such manner that the individuals of each simple cluster are
much closer together than are the simple clusters gathered into a compound
cluster: in which case, the trait that unites groups of groups differs from
the trait that unites groups, not in _nature_ but only in _amount_. But
this is not so either with the groups and sub-groups which we know have
resulted from evolution, or with those which we here infer have resulted
from evolution. In both cases the highest or most general classes, are
marked off from one another by fundamental differences that have no common
measure with the differences that mark off small classes. Observe the
parallelism.
We saw that each sub-kingdom of animals is distinguished from other
sub-kingdoms, by some unlikeness in its main plan of organization; such as
the presence or absence of a peri-visceral cavity. Contrariwise, the
members of the smallest groups are united together, and separated from the
members of other small groups, by modifications which do not affect the
relations of essential parts. That this is just the kind of arrangement
which results from evolution, the case of languages will show.
On comparing the dialects spoken in different parts of England, we find
scarcely any difference but those of pronunciation: the structures of the
sentences are almost uniform. Between English and the allied modern
languages there are divergences of structure: there are some unlikenesses
of idiom; some unlikenesses in the ways of modifying the meanings of verbs;
and considerable unlikenesses in the uses of genders. But these
unlikenesses are not sufficient to hide a general community of
organization. A greater contrast of structure exists between these modern
languages of Western Europe, and the classic languages. Differentiation
into abstract and concrete elements, which is shown by the substitution of
auxiliary words for inflections, has produced a higher specialization,
distinguishing these languages as a group from the older languages.
Nevertheless, both the ancient and modern languages of Europe, together
with some Eastern languages derived from the same original, have, under all
their differences of organization, a fundamental likeness; since in all of
them words are formed by such a coalescence and integration of roots as
destroys the independent meanings of the roots. These Aryan languages, and
others which have the _amalgamate_ character, are united by it into a class
distinguished from the _aptotic_ and _agglutinate_ languages; in which the
roots are either not united at all, or so incompletely united that one of
them still retains its independent meaning. And philologists find that
these radical traits which severally determine the grammatical forms, or
modes of combining ideas, characterize the primary divisions among
languages.
So that among languages, where we know that evolution has been going on,
the greatest groups are marked off from one another by the strongest
structural contrasts; and as the like holds among groups of organisms,
there results a further reason for inferring that these have been evolved.
§ 125. There is yet another parallelism of like meaning. We saw (§ 101)
that the successively-subordinate groups--classes, orders, genera, and
species--into which zoologists and botanists segregate animals and plants,
have not, in reality, those definite values conventionally given to them.
There are well-marked species, and species so imperfectly marked that some
systematists regard them as varieties. Between genera strong contrasts
exist in many cases, and in other cases contrasts so much less decided as
to leave it doubtful whether they imply generic distinctions. So, too, is
it with orders and classes: in some of which there have been introduced
sub-divisions, having no equivalents in others. Even of the sub-kingdoms
the same truth holds. The contrast between the _Coelenterata_ and the
_Mollusca_, is far less than that between the _Coelenterata_ and the
_Vertebrata_.
Now just this same indefiniteness of value, or incompleteness of
equivalence, is observable in those simple and compound and re-compound
groups which we see arising by evolution. In every case the endeavour to
arrange the divergent products of evolution, is met by a difficulty like
that which would meet the endeavour to classify the branches of a tree,
into branches of the first, second, third, fourth, &c., orders--the
difficulty, namely, that branches of intermediate degrees of composition
exist. The illustration furnished by languages will serve us once more.
Some dialects of English are but little contrasted; others are strongly
contrasted. The alliances of the several Scandinavian tongues with one
another are different in degree. Dutch is much less distinct from German
than Swedish is; while between Danish and Swedish there is so close a
kinship that they might almost be regarded as widely-divergent dialects.
Similarly on comparing the larger divisions, we see that the various
languages of the Aryan stock have deviated from their original to very
unlike distances. The general conclusion is manifest. While the kinds of
human speech fall into groups, and sub-groups, and sub-sub-groups; yet the
groups are not equal to one another in value, nor have the sub-groups equal
values, nor the sub-sub-groups.
If, then, when classified, organisms fall into assemblages such that those
of the same grade are but indefinitely equivalent; and if, where evolution
is known to have taken place, there have arisen assemblages between which
the equivalence is similarly indefinite; there is additional reason for
inferring that organisms are products of evolution.
§ 126. A fact of much significance remains. If groups of organic forms have
arisen by divergence and re-divergence; and if, while the groups have been
developing from simple groups into compound groups, each group and
sub-group has been giving origin to more complex forms of its own type;
then it is inferable that there once existed greater structural likenesses
between the members of allied groups than exists now. This, speaking
generally, proves to be so.
Between the sub-kingdoms the gaps are extremely wide; but such distant
kinships as may be discerned, bear out anticipation. Thus in the formation
of the germinal layers there is a general agreement among them; and there
is a further agreement among sundry of them in the formation of a gastrula.
This simplest and earliest likeness, significant of primitive kinship, is
in most cases soon obscured by divergent modes of development; but sundry
sub-kingdoms continue to show relationships by the likenesses of their
larval forms; as we see in the trochophores of the _Polyzoa_, _Annelida_,
and _Mollusca_--sub-kingdoms the members of which by their later structural
changes are rendered widely unlike.
More decided approximations exist between the lower members of classes. In
tracing down the _Crustacea_ and the _Arachnida_ from their more complex to
their simpler forms, zoologists meet with difficulties: respecting some of
these simpler forms, it becomes a question which class they belong to. The
_Lepidosiren_, about which there have been disputes whether it is a fish or
an amphibian, is inferior, in the organization of its skeleton, to the
great majority of both fishes and amphibia. Widely as they differ from
them, the lower mammals have some characters in common with birds, which
the higher mammals do not possess.
Now since this kind of relationship of groups is not accounted for by any
other hypothesis, while the hypothesis of evolution gives us a clue to it;
we must include it among the supports of this hypothesis which the facts of
classification furnish.
§ 127. What shall we say of these leading truths when taken together? That
naturalists have been gradually compelled to arrange organisms in groups
within groups, and that this is the arrangement which we see arises by
descent, alike in individual families and among races of men, is a striking
circumstance. That while the smallest groups are the most nearly related,
there exist between the great sub-kingdoms, structural contrasts of the
profoundest kind, cannot but impress us as remarkable, when we see that
where it is known to take place evolution actually produces these
feebly-distinguished small groups, and these strongly-distinguished great
groups. The impression made by these two parallelisms, which add meaning to
each other, is deepened by the third parallelism, which enforces the
meaning of both--the parallelism, namely, that as, between the species,
genera, orders, classes, &c., which naturalists have formed, there are
transitional types; so between the groups, sub-groups, and sub-sub-groups,
which we know to have been evolved, types of intermediate values exist. And
these three correspondences between the known results of evolution and the
results here ascribed to evolution, have further weight given to them by
the fact, that the kinship of groups through their lowest members is just
the kinship which the hypothesis of evolution implies.
Even in the absence of these specific agreements, the broad fact of unity
amid multiformity, which organisms so strikingly display, is strongly
suggestive of evolution. Freeing ourselves from pre-conceptions, we shall
see good reason to think with Mr. Darwin, "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 partly revealed to us
by our classifications." When we consider that this only known cause of
similarity, joined with the only known cause of divergence (the influence
of conditions), gives us a key to these likenesses obscured by
unlikenesses; we shall see that were there none of those remarkable
harmonies above pointed out, the truths of classification would still yield
strong support to our conclusion.