DIVISION AND CLASSIFICATION
Sec. 1. Classification, in its widest sense, is a mental grouping of facts
or phenomena according to their resemblances and differences, so as best
to serve some purpose. A "mental grouping": for although in museums we
often see the things themselves arranged in classes, yet such an
arrangement only contains specimens representing a classification. The
classification itself may extend to innumerable objects most of which
have never been seen at all. Extinct animals, for example, are
classified from what we know of their fossils; and some of the fossils
may be seen arranged in a museum; but the animals themselves have
disappeared for many ages.
Again, things are classed according to their resemblances and
differences: that is to say, those that most closely resemble one
another are classed together on that ground; and those that differ from
one another in important ways, are distributed into other classes. The
more the things differ, the wider apart are their classes both in
thought and in the arrangements of a museum. If their differences are
very great, as with animals, vegetables and minerals, the classing of
them falls to different departments of thought or science, and is often
represented in different museums, zoological, botanical, mineralogical.
We must not, however, suppose that there is only one way of classifying
things. The same objects may be classed in various ways according to
the purpose in view. For gardening, we are usually content to classify
plants into trees, shrubs, flowers, grasses and weeds; the ordinary
crops of English agriculture are distinguished, in settling their
rotation, into white and green; the botanist divides the higher plants
into gymnosperms and angiosperms, and the latter into monocotyledons and
dicotyledons. The principle of resemblance and difference is recognised
in all these cases; but what resemblances or differences are important
depends upon the purpose to be served.
Purposes are either ([alpha]) special or practical, as in gardening or
hunting, or ([beta]) general or scientific, as in Botany or Zoology. The
scientific purpose is merely knowledge; it may indeed subserve all
particular or practical ends, but has no other end than knowledge
directly in view. And whilst, even for knowledge, different
classifications may be suitable for different lines of inquiry, in
Botany and Zoology the Morphological Classification is that which gives
the most general and comprehensive knowledge (see Huxley, _On the
Classification of Animals_, ch. 1). Most of what a logician says about
classification is applicable to the practical kind; but the scientific
(often called 'Natural Classification'), as the most thorough and
comprehensive, is what he keeps most constantly before him.
Scientific classification comes late in human history, and at first
works over earlier classifications which have been made by the growth of
intelligence, of language, and of the practical arts. Even in the
distinctions recognised by animals, may be traced the grounds of
classification: a cat does not confound a dog with one of its own
species, nor water with milk, nor cabbage with fish. But it is in the
development of language that the progress of instinctive classification
may best be seen. The use of general names implies the recognition of
classes of things corresponding to them, which form their denotation,
and whose resembling qualities, so far as recognised, form their
connotation; and such names are of many degrees of generality. The use
of abstract names shows that the objects classed have also been
analysed, and that their resembling qualities have been recognised
amidst diverse groups of qualities.
Of the classes marked by popular language it is worth while to
distinguish two sorts (_cf._ chap. xix. Sec. 4): Kinds, and those having
but few points of agreement.
But the popular classifications, made by language and the primitive
arts, are very imperfect. They omit innumerable things which have not
been found useful or noxious, or have been inconspicuous, or have not
happened to occur in the region inhabited by those who speak a
particular language; and even things recognised and named may have been
very superficially examined, and therefore wrongly classed, as when a
whale or porpoise is called a fish, or a slowworm is confounded with
snakes. A scientific classification, on the other hand, aims at the
utmost comprehensiveness, ransacking the whole world from the depths of
the earth to the remotest star for new objects, and scrutinising
everything with the aid of crucible and dissecting knife, microscope and
spectroscope, to find the qualities and constitution of everything, in
order that it may be classed among those things with which it has most
in common and distinguished from those other things from which it
differs. A scientific classification continually grows more
comprehensive, more discriminative, more definitely and systematically
coherent. Hence the uses of classification may be easily perceived.
Sec. 2. The first use of classification is the better understanding of the
facts of Nature (or of any sphere of practice); for understanding
consists in perceiving and comprehending the likeness and difference of
things, in assimilating and distinguishing them; and, in carrying out
this process systematically, new correlations of properties are
continually disclosed. Thus classification is closely analogous to
explanation. Explanation has been shown (chap. xix. Sec. 5) to consist in
the discovery of the laws or causes of changes in Nature; and laws and
causes imply similarity, or like changes under like conditions: in the
same way classification consists in the discovery of resemblances in the
things that undergo change. We may say (subject to subsequent
qualifications) that Explanation deals with Nature in its dynamic,
Classification in its static aspect. In both cases we have a feeling of
relief. When the cause of any event is pointed out, or an object is
assigned its place in a system of classes, the gaping wonder, or
confusion, or perplexity, occasioned by an unintelligible thing, or by a
multitude of such things, is dissipated. Some people are more than
others susceptible of this pleasure and fastidious about its purity.
A second use of classification is to aid the memory. It strengthens
memory, because one of the conditions of our recollecting things is,
that they resemble what we last thought of; so that to be accustomed to
study and think of things in classes must greatly facilitate
recollection. But, besides this, a classification enables us easily to
run over all the contrasted and related things that we want to think of.
Explanation and classification both tend to rationalise the memory, and
to organise the mind in correspondence with Nature.
Every one knows how a poor mind is always repeating itself, going by
rote through the same train of words, ideas, actions; and that such a
mind is neither interesting nor practical. It is not practical, because
the circumstances of life are rarely exactly repeated, so that for a
present purpose it is rarely enough to remember only one former case; we
need several, that by comparing (perhaps automatically) their
resemblances and differences with the one before us, we may select a
course of action, or a principle, or a parallel, suited to our
immediate needs. Greater fertility and flexibility of thought seem
naturally to result from the practice of explanation and classification.
But it must be honestly added, that the result depends upon the spirit
in which such study is carried on; for if we are too fond of finality,
too eager to believe that we have already attained a greater precision
and comprehension than are in fact attainable, nothing can be more
petrific than 'science,' and our last state may be worse than the first.
Of this, students of Logic have often furnished examples.
Sec. 3. Classification may be either Deductive or Inductive; that is to
say, in the formation of classes, as in the proof of propositions, we
may, on the whole, proceed from the more to the less, or from the less
to the more general; not that these two processes are entirely
independent.
If we begin with some large class, such as 'Animal,' and subdivide it
deductively into Vertebrate and Invertebrate, yet the principle of
division (namely, central structure) has first been reached by a
comparison of examples and by generalisation; if, on the other hand,
beginning with individuals, we group them inductively into classes, and
these again into wider ones (as dogs, rats, horses, whales and monkeys
into mammalia) we are guided both in special cases by hypotheses as to
the best grounds of resemblance, and throughout by the general principle
of classification--to associate things that are alike and to separate
things that are unlike. This principle holds implicitly a place in
classification similar to that of causation in explanation; both are
principles of intelligence. Here, then, as in proof, induction is
implied in deduction, and deduction in induction. Still, the two modes
of procedure may be usefully distinguished: in deduction, we proceed
from the idea of a whole to its parts, from general to special; in
induction, from special (or particular) to general, from parts to the
idea of a whole.
Sec. 4. The process of Deductive Classification, or Formal Division, may be
represented thus:
A
|
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A B A b
| |
---------- ----------
| | | |
A B C A B c A b C A b c
Given any class (A) to be divided:
1. Select one important character, attribute, or quality (B), not common
to all the individuals comprehended in the class, as the basis of
division (_fundamentum divisionis_).
2. Proceed by Dichotomy; that is, cut the given class into two, one
having the selected attribute (say, B), the other not having it (b).
This, like all formal processes, assumes the principles of Contradiction
and Excluded Middle, that 'No A is both B and not-B,' and that 'Every A
is either B or not-B' (chap. vi. Sec. 3); and if these principles are not
true, or not applicable, the method fails.
When a class is thus subdivided, it may be called, in relation to its
subclasses, a Genus; and in relation to it, the subclasses may be called
Species: thus--genus A, species AB and Ab, etc.
3. Proceed gradually in the order of the importance of characters; that
is, having divided the given class, subdivide on the same principle the
two classes thence arising; and so again and again, step by step, until
all the characters are exhausted: _Divisio ne fiat per saltum_.
Suppose we were to attempt an exhaustive classification of things by
this method, we must begin with 'All Things,' and divide them (say) into
phenomenal and not-phenomenal, and then subdivide phenomena, and so on,
thus:
All Things
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Phenomenal Not-phenomenal
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Extended Unextended
| (e.g., Pleasure and Pain)
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----------------
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Resistant Not-resistant
(Matter) (Space)
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Gravitating Not-gravitating
|
-----------
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Simple Compound
Having subdivided 'Simple' by all possible characters, we must then go
back and similarly subdivide Not-phenomenal, Unextended, Not-resistant,
Not-gravitating, and Compound. Now, if we knew all possible characters,
and the order of their importance, we might prepare _a priori_ a
classification of all possible things; at least, of all things that come
under the principles of Contradiction and Excluded Middle. Many of our
compartments might contain nothing actual; there may, for example, be
nothing that is not phenomenal to some mind, or nothing that is extended
and not-resistant (no vacuum), and so forth. This would imply a breach
of the rule, that the dividing quality be not common to the whole class;
but, in fact, doubts have been, and are, seriously entertained whether
these compartments are filled or not. If they are not, we have concepts
representing nothing, which have been generated by the mere force of
grammatical negation, or by the habit of thinking according to the
principle of Excluded Middle; and, on the strength of these empty
concepts, we have been misled into dividing by an attribute, which
(being universal) cannot be a _fundamentum divisionis_. But though in
such a classification places might be empty, there would be a place for
everything; for whatever did not come into some positive class (such as
Gravitating) must fall under one of the negative classes (the 'Nots')
that run down the right-hand side of the Table and of its subdivisions.
This is the ideal of classification. Unfortunately we have to learn what
characters or attributes are possible, by experience and comparison; we
are far from knowing them all: and we do not know the order of their
importance; nor are we even clear what 'important' means in this
context, whether 'widely prevalent,' or 'ancient,' or 'causally
influential,' or 'indicative of others.' Hence, in classifying actual
things, we must follow the inductive method of beginning with
particulars, and sorting them according to their likeness and difference
as discovered by investigation. The exceptional cases, in which
deduction is really useful, occur where certain limits to the number and
combination of qualities happen to be known, as they may be in human
institutions, or where there are mathematical conditions. Thus, we might
be able to classify orders of Architecture, or the classical metres and
stanzas of English poetry; though, in fact, these things are too free,
subtle and complex for deductive treatment: for do not the Arts grow
like trees? The only sure cases are mathematical; as we may show that
there are possible only three kinds of plane triangles, four conic
sections, five regular solids.
Sec. 5. The rules for _testing_ a Division are as follows:
1. Each Sub-class, or Species, should comprise less than the Class, or
Genus, to be divided. This provides that the division shall be a real
one, and not based upon an attribute common to the whole class; that,
therefore, the first rule for making a division shall have been adhered
to. But, as in Sec. 4, we are here met by a logical difficulty. Suppose
that the class to be divided is A, and that we attempt to divide upon
the attribute B, into AB and Ab; is this a true division, if we do not
know any A that is not B? As far as our knowledge extends, we have not
divided A at all. On the other hand, our knowledge of concrete things is
never exhaustive; so that, although we know of no A that is not B, it
may yet exist, and we have seen that it is a logical caution not to
assume what we do not know. In a deductive classification, at least, it
seems better to regard every attribute as a possible ground of division.
Hence, in the above division of 'All Things,'--'Not-phenomenal,'
'Extended-Not-resistant,' 'Resistant-Not-gravitating,' appear as
negative classes (that is, classes based on the negation of an
attribute), although their real existence may be doubtful. But, if this
be justifiable, we must either rewrite the first test of a division
thus: 'Each sub-class should _possibly_ comprise less than the class to
be divided'; or else we must confine the test to (a) thoroughly
empirical divisions, as in dividing Colour into Red and Not-red, where
we know that both sub-classes are real; and (b) divisions under
demonstrable conditions--as in dividing the three kinds of triangles by
the quality equilateral, we know that it is only applicable to
acute-angled triangles, and do not attempt to divide the right-angled or
obtuse-angled by it.
2. The Sub-classes taken together should be equal to the Class to be
divided: the sum of the Species constitutes the Genus. This provides
that the division shall be exhaustive; which dichotomy always secures,
according to the principle of Excluded Middle; because whatever is not
in the positive class, must be in the negative: Red and Not-red include
all colours.
3. The Sub-classes must be opposed or mutually exclusive: Species must
not overlap. This again is secured by dichotomy, according to the
principle of Contradiction, provided the division be made upon one
attribute at a time. But, if we attempt to divide simultaneously upon
two attributes, as 'Musicians' upon 'nationality' and 'method,' we get
what is called a Cross-division, thus 'German Musicians.' 'Not-German,'
'Classical,' 'Not-Classical;' for these classes may overlap, the same
men sometimes appearing in two groups--Bach in 'German' and 'Classical,'
Pergolesi in 'Not-German' and 'Classical.' If, however, we divide
Musicians upon these attributes successively, cross division will be
avoided, thus:
Musicians
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Classical Not-classical
| |
------------ ------------
| | | |
German Not-German German Not-German
Here no Musician will be found in two classes, _unless_ he has written
works in two styles, _or unless_ there are works whose style is
undecided. This "unless--or unless" may suggest caution in using
dichotomy as a short cut to the classification of realities.
4. No Sub-class must include anything that is not comprised in the class
to be divided: the Genus comprises all the Species. We must not divide
Dogs into fox-terriers and dog-fish.
Sec. 6. The process of Inductive Classification may be represented thus:
Given any multitude of individuals to be classified:
(1) Place together in groups (or in thought) those things that have in
common the most, the most widely diffused and the most important
qualities.
(2) Connect those groups which have, as groups, the greater resemblance,
and separate those that have the greater difference.
(3) Demarcate, as forming higher or more general classes, those groups
of groups that have important characters in common; and, if possible, on
the same principle, form these higher classes into classes higher still:
that is to say, graduate the classification upwards.
Whilst in Division the terms 'Genus' and 'Species' are entirely relative
to one another and have no fixed positions in a gradation of classes, it
has been usual, in Inductive Classification, to confine the term
'Species' to classes regarded as lowest in the scale, to give the term
'Genera' to classes on the step above, and at each higher step to find
some new term such as 'Tribe,' 'Order,' 'Sub-kingdom,' 'Kingdom'; as may
be seen by turning to any book on Botany or Zoology. If, having fixed
our Species, we find them subdivisible, it is usual to call the
Sub-species 'Varieties.'
Suppose an attempt to classify by this method the objects in a
sitting-room. We see at a glance carpets, mats, curtains, grates,
fire-irons, coal-scuttles, chairs, sofas, tables, books, pictures,
musical instruments, etc. These may be called 'Species.' Carpets and
mats go together; so do chairs and sofas; so do grates, fire-irons, and
coal-scuttles and so on. These greater groups, or higher classes, are
'Genera.' Putting together carpets, mats and curtains as
'warmth-fabrics'; chairs, sofas and tables as 'supports'; books,
pictures and musical instruments as 'means of culture'; these groups we
may call Orders. Sum up the whole as, from the housewife's point of
view, 'furniture.' If we then subdivide some of the species, as books
into poetry, novels, travels, etc., these Sub-species may be
considered 'Varieties.'
A Classification thus made, may be tested by the same rules as those
given for testing a Division; but if it does not stand the test, we must
not infer that the classification is a bad one. If the best possible, it
is good, though formally imperfect: whatever faults are found must then
be charged upon the 'matter,' which is traditionally perverse and
intractable. If, for example, there is a hammock in the room, it must be
classed not with the curtains as a warmth-fabric, but with the sofas as
a support; and books and pictures may be classed as, in a peculiar
sense, means of culture, though all the objects in the room may have
been modified and assorted with a view to gratifying and developing good
taste.
Sec. 7. The difficulty of classifying natural objects is very great. It is
not enough to consider their external appearance: exhaustive knowledge
of their internal structure is necessary, and of the functions of every
part of their structure. This is a matter of immense research, and has
occupied many of the greatest minds for very many years. The following
is a tabular outline of the classification of the
Animal Kingdom
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SUB-KINGDOM, Vertebrates Invertebrates (10 Sub-kingdom 5)
or |
Phylum: |-------------------------------
| | |
| Sauropsida Ichthyopsida
| | |
| --------- -----------
| | | | |
CLASS: Mammalia Birds Reptiles Amphibia Fishes
|
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SUB-CLASS: Placental Implacental
| |
| -------------------
| | |
DIVISION: Monodelphia Didelphia Ornithodelphia
|
---------------------------------------
| | | | |
ORDER: Quadrumana Rodentia Carnivora Ungulata Caetacea, etc.
|
----------------------------------
| | |
SECTION: Pinnigrada Plantigrada Digitigrada
| | |
(Seals, etc.) (Bears, etc.) |
|
--------------------------------------
| | | | |
GENUS: Mustelidae Viverridae Hyaaenidae Canidae Felidae
| | |
(Weasels, etc.) (Civets, etc.) |
|
---------------------------------------
| | | | | |
SPECIES: Lion Tiger Leopard Puma Lynx Cat, etc.
|
---------------------
| | |
VARIETY: African Syrian Cave-lion (extinct)
As there is not space enough to tabulate such a classification in full,
I have developed at each step the most interesting groups: Vertebrates,
Mammalia, Monodelphia Carnivora, Digitigrada, Felidae, Lion. Most of the
other groups in each grade are also subdivisible, though some of them
contain far fewer sub-classes than others.
To see the true character of this classification, we must consider that
it is based chiefly upon knowledge of existing animals. Some extinct
animals, known by their fossils, find places in it; for others new
places have been made. But it represents, on the whole, a cross-section,
or cross-sections of Nature as developing in time; and, in order to give
a just view of the relations of animals, it must be seen in the light of
other considerations. The older systems of classification, and the rules
for making them, seem to have assumed that an actual system of classes,
or of what Mill calls 'Kinds,' exists in nature, and that the relations
of Kinds in this system are determined by quantity of resemblance in
co-inherent qualities, as the ground of their affinity.
Sec. 8. Darwin's doctrine of the origin of species affects the conception
of natural classification in several ways, (1) If all living things are
blood-relations, modified in the course of ages according to their
various conditions of life, 'affinity' must mean 'nearness of common
descent'; and it seems irrational to propose a classification upon any
other basis. We have to consider the Animal (or the Vegetable) Kingdom
as a family tree, exhibiting a long line of ancestors, and (descended
from them) all sorts of cousins, first, second, third, etc., perhaps
once, twice, or oftener 'removed.' Animals in the relation of first
cousins must be classed as nearer than second cousins, and so on.
But, if we accept this principle, and are able to trace relationship, it
may not lead to the same results as would be reached by simply relying
upon the present 'quantity of resemblance,' unless we understand this
in a very particular way. For the most obvious features of an animal may
have been recently acquired; which often happens with those characters
that adapt an animal to its habits of life, as the wings of a bat, or
the fish-like shape of a dolphin; or as in cases of 'mimicry.' Some
butterflies, snakes, etc., have grown to resemble closely, in a
superficial way, other butterflies and snakes, from which a stricter
investigation widely separates them; and this superficial resemblance is
probably a recent acquisition, for the sake of protection; the imitated
butterflies being nauseous, and the imitated snakes poisonous. On the
other hand, ancient and important traits of structure may, in some
species, have dwindled into inconspicuous survivals or be still found
only in the embryo; so that only great knowledge and sagacity can
identify them; yet upon ancient traits, though hidden, classification
depends. The seal seems nearer allied to the porpoise than to the tiger,
the shrew nearer to the mouse than to the hedgehog; and the Tasmanian
wolf looks more like a true wolf, the Tasmanian devil more like a
badger, than like a kangaroo: yet the seal is nearer akin to the tiger,
the shrew to the hedgehog, and the Tasmanian flesh-eaters are marsupial,
like the kangaroo. To overcome this difficulty we must understand the
resemblance upon which classification is based to include resemblance of
Causation, that is, the fact itself of descent from common ancestors.
For organic beings, all other rules of classification are subordinate to
one: trace the genealogy of every form.
By this rule we get a definite meaning for the phrase 'important or
fundamental attribute' as determining organic classes; namely, most
ancient, or 'best serving to indicate community of origin.' Grades of
classification will be determined by such fundamental characters, and
may correspond approximately to the more general types (now extinct)
from which existing animals have descended.
(2) By the hypothesis of development the fixity of species is
discredited. The lowest grade of a classification is made up not of
well-defined types unchanging from age to age, but of temporary species,
often connected by uncertain and indistinct varieties: some of which
may, in turn, if the conditions of their existence alter, undergo such
changes as to produce new species. Hence the notion that Kinds exist in
organic nature must be greatly modified. During a given period of a few
thousand years, Kinds may be recognised, because, under such conditions
as now prevail in the world, that period of time is insufficient to
bring about great changes. But, if it be true that lions, tigers, and
leopards have had a common ancestor, from whose type they have gradually
diverged, it is plain that their present distinctness results only from
the death of intermediate specimens and the destruction of intermediate
varieties. Were it possible to restore, by the evidence of fossils, all
the ranks of the great processions that have descended from the common
ancestor, there would nowhere occur a greater difference than between
offspring and parents; and the appearance of Kinds existing in nature,
which is so striking in a museum or zoological garden, would entirely
vanish.
A classification, then, as formerly observed, represents a cross-section
of nature as developing in time: could we begin at the beginning and
follow this development down the course of time, we should find no
classes, but an ever-moving, changing, spreading, branching continuum.
It may be represented thus: Suppose an animal (or plant) A, extending
over a certain geographical area, subject to different influences and
conditions of climate, food, hill and plain, wood and prairie, enemies
and rivals, and undergoing modifications here and there in adaptation to
the varying conditions of life: then varieties appear. These varieties,
diverging more and more, become distinct species (AB, AC, AD, AX). Some
of these species, the more widely diffused, again produce varieties;
which, in turn become species (ABE, ABF, ADG, ADH). From these, again,
ABE, ABFI, ABFJ, AC, ADHK, ADHL, ADHM, the extant species, descend.
A
/|\\_
/ | \ \__
/ | \ \_____
________/ | \__ \
/ | \ \
AB AC AD AX
/\ | /\
/ \ | / \
/ \ | / \
ABE ABF | ADG ADH
/ / \ | /|\
/ / \ | / | \
/ / \ | / | \
ABE ABFI ABFJ AC ADHK ADHL ADHM
If in this age a classifier appears, he finds seven living species,
which can be grouped into four genera (ABE, ABF, AC, ADH), and these
again into three Families (AB, AC, AD), all forming one Order. But the
animals which were their ancestors are all extinct. If the fossils of
any of them--say AB, ADG and AX--can be found, he has three more
species, one more genus (ADG), and one more family (AX). For AC, which
has persisted unchanged, and AX, which has become extinct, are both of
them Families, each represented by only one species. It seems necessary
to treat such ancient types as species on a level with extant forms; but
the naturalist draws our attention to their archaic characteristics, and
tries to explain their places in the order of evolution and their
relationships.
But now suppose that he could find a fossil specimen of every generation
(hundreds of thousands of generations), from ABFI, etc., up to A;
then, as each generation would only differ from the preceding as
offspring from parents, he would be unable at any point to distinguish a
species; at most, he would observe a slightly marked variety. ABFI and
ABFJ would grow more and more alike, until they became indistinguishable
in ABF; ABF and ABE would merge into AB; AB, AC, AD and AX would merge
into A. Hence, the appearance of species is due to our taking
cross-sections of time, or comparing forms that belong to periods remote
from one another (like AX, ADG, and ADHK, or AD, ADH and ADHK), and this
appearance of species depends upon the destruction of ancestral
intermediate forms.
(3) The hypothesis of development modifies the logical character of
classification: it no longer consists in a direct induction of
co-inherent characters, but is largely a deduction of these from the
characters of earlier forms, together with the conditions of variation;
in other words, the definition of a species must, with the progress of
science, cease to be a mere empirical law of co-inherence and become a
derivative law of Causation. But this was already implied in the
position that causation is the fundamental principle of the explanation
of concrete things; and accordingly, the derivative character of species
or kinds extends beyond organic nature.
Sec. 9. The classification of inorganic bodies also depends on causation.
There is the physical classification into Solids, Liquids, and Gases.
But these states of matter are dependent on temperature; at different
temperatures, the same body may exist in all three states. They cannot
therefore be defined as solid, liquid, or gaseous absolutely, but only
within certain degrees of temperature, and therefore as dependent upon
causation. Similarly, the geological classification of rocks, according
to relative antiquity (primary, secondary, tertiary, with their
subdivisions), and mode of formation (igneous and aqueous), rests upon
causation; and so does the chemical classification of compound bodies
according to the elements that enter into them in definite proportions.
Hence, only the classification of the elements themselves (amongst
concrete things), at present, depends largely upon empirical
Coinherence. If the elements remain irresolvable into anything simpler,
the definitions of the co-inherent characters that distinguish them must
be reckoned amongst the ultimate Uniformities of Nature. But if a
definite theory of their origin both generally and severally, whether
out of ether-vortices, or groups of electric corpuscles, or whatnot,
shall ever gain acceptance, similarity of genesis or causation will
naturally be the leading consideration in classifying the chemical
elements. To find common principles of causation, therefore, constitutes
the verification of every Natural Classification. The ultimate
explanation of nature is always causation; the Law of Causation is the
backbone of the system of Experience.