§ 98. That orderly arrangement of objects called Classification, has two purposes; which, though not absolutely distinct, are distinct in great part. It may be employed to facilitate identification; or it may be employed to organize our knowledge. If a librarian places his books in the alphabetical succession of the author's names, he places them in such way that any particular book may easily be found; but not in such way that books of a given nature stand together.
When, conversely, he makes a distribution of books according to their subjects, he neglects various superficial similarities and distinctions, and groups them according to certain primary and secondary and tertiary attributes, which severally imply many other attributes — groups them so that any one volume being inspected, the general characters of all the neighbouring volumes may be inferred. He puts together in one great division, all works on History; in another all Biographical works; in another all works that treat of Science; in another Voyages and Travels; and so on. Each of his great groups he separates into sub-groups; as when he puts difierent kinds of pure Literature, under the heads of Fiction, Poetry, and the Drama. In some cases he makes sub- sub- groups; as when, having divided his Scientific treatises into abstract and concrete, putting in the one Logic and Mathematics, and in the other Physics, Astronomy, Geology, Chemistry, Physiology, &c.; ho goes on to sub-divide his books on Physics, into those which treat of Mechanical Motion, those which treat of Heat, those which treat of Licrht, of Electricity, of Magnetism.
Between these two modes of classification, note the essential distinctions. Arrangement according to any single conspicuous attribute is comparatively easy, and is the first that suggests itself: a child may place books in the order of thnir sizes, or according to the styles of their bindings. But arrangement according to combinations of attributes, which, though fundamental, are not conspicuous, requires analysis; and does not suggest itself till analysis has made some progress. Even when aided by the information which the author gives on his title page, it requires considerable knowledge to classify rightly an essay on Polarization; and in the absence of a title page, it requires much more knowledge. Again, classification by a single attribute, which the objects possess in difierent degrees, may be more or less serial, or linear. Books may be put in the order of their dates, in single file; or if they are grouped as works in one volume, works in two volumes, works in three volumes, &c., the groups maj^ be placed in an ascending succession. But groups severally formed of things distinguished by some common attribute which implies many other attributes, do not admit of serial arrangement. You cannot rationally say, either that Historical Works should come before Scientific Works, or Scientific Works before Historical Works; nor of the sub- divisions of creative Literature, into Fiction, Poetry, and the Drama, can you give a good reason why any one should take precedence of the others.
Hence this grouping of the like and separation of the unlike, which constitutes Classification, can reach its complete form only by slow steps. We saw (First Principles, § 36) that, other things equal, the relations among phenomena are recognized in the order of their conspicuousness; and that, other things equal, they are recognized in the order of their I simplicity. The first classifications are sure, therefore, to be groupings of objects that resemble each other in external or easily-perceived attributes, and attributes that are not of complex characters. Those likenesses among things which are due to their possession in common of simple obvious properties, may or may not coexist with further likenesses among them.
When geometrical figures are classed as curvilinear "and rectilinear, or when the rectilinear are divided into trilateral, quadrilateral, &c., the distinctions made, connote various other distinctions, with which they are necessarily bound up; but if liquids be classed according to their visible characters— if water, alcohol, sulphuret of carbon, &c., be grouped as colourless and transparent, we have things placed together which are unlike in their essential natures. Thus, where the objects classed have numerous attributes, the probabilities are, that the early classifications, based on simple and manifest attributes, unite under the same head many objects that have no resemblances in the majority of their attributes. As the knowledge of objects increases, it becomes possible to make groups of which the members have more numerous properties in common; and to ascertain what property, or combination of properties, is most characteristic of each group. And the classification eventually arrived at, is one in which the segregation has been carried so far, that the objects integrated in each group have more attributes in common with one another, than they have in common with any excluded objects; one in which the groups of such groups are integrated on ^the same principle; and one in which the degrees of differentiation and integration are proportioned to the degrees of intrinsic unlikeness and likeness. And the ultimate classification, while it serves most completely to identify the things, serves also to express the greatest amount of knowledge concerning the things — enables us to predicate the greatest number of facts concerning each thing; and by so doing proves that it expresses the most precise correspondence between our conceptions and the realities.
§ 99. Biological classifications illustrate well these phases, througli wliicli classifications in general necessarily pass. In early attempts to arrange organic beings' in some systematic manner, we see at first, a guidance by conspicuous and simple characters, and a tendency towards arrangement in linear order. In successively later attempts, we see more regard paid to combinations of characters which are essential but often inconspicuous; and a gradual abandonment of a linear arrangement for an arrangement in divergent groups and re-divergent sub-groups.
In the popular mind, plants are still classed under the heads of Trees, Shrubs, and Herbs; and this serial classing according to the single attribute of magnitude, swayed the earliest observers. They would have thought it absurd to call a bamboo, thirty feet high, a kind of grass; and would have been incredulous if told that the 'Hart's-tongue should be placed in the same great division with the Tree-ferns. The zoological classifications that were current before Natural History became a science, had divisions similarly superficial and simple. Beasts, Birds, Fishes, and Creeping-things, are names of groups marked off from one another by conspicuous differences of appearance and^ modes of life — creatures that walk and run, creatures that fly, creatures that live in the water, creatures that crawl. And these groups were thought of in the order of their importance.
The first arrangements made by naturalists were based either on single characters, or on very simple combinacions of characters. Describing plant-classifications, Lindley says: — " Hivinus invented, in 1690, a system depending upon the formation of the corolla; Kamel, in 1693, upon the fruit alone; Magnol, in 1720, on the calyx and corolla; and finally, Linnceus, in 1731, on variations in the stamens and pistil." In this last system, which has been for so long current as a means of identification, simple external attributes are still depended on; and an arrangement, in great measure serial, is based on the degrees in which these attributes are possessed. In 1703, some thirty years before the time of Linnaeus, our countryman Ray had sketched the outlines of a more advanced system. He said that — Plants are either riowerless, or Flowering; and these are Dicotyledones, or Monocotyledones. Among the minor groups which he placed under these general heads, " were Fungi, Mosses, Ferns, Composites, Cichoraceoe Umbellifers, Papilionaceous plants. Conifers, Labiates, &c., under other names, but with limits not very different from those now assigned to them." Being much in advance of his age, Ray's ideas remained dormant until the time of Jussieu; by whom they were developed into what has become known as the Natural System. Passing through various modifications in the hands of successive botanists, the JN^atural System has now taken the following form; which I copy (adding the alliances to the classes) from Prof. Lindley's Vegetable Kingdom* * From this table I have omitted the class Rhizogens, which other botanists do not agree with Lindley in regarding as a separate class. The plants respecting which there has arisen this difference of opinion, are certain flowering plants, which grow parasitically on the roots of trees. The reasons assigned by Endlicher and Lindley, for erecting them into a separate group of Pheenogams, are, that in place of true leaves they have only cellular scales; that the stem is an amorphous fungous mass, imperfectly supplied with spiral vessels; and that they are without chlorophyll. Mr Griffith and Dr Hooker, however, have given preponderating reasons why they should be restored to the class Exogens. It seems here worth remarking, that certain zoological facts suggest an explanation of these anomalous botanical facts; and confirm the conclusion reached by Dr Hooker and Mr Griffith. It very commonly happens that animal-parasites are aberrant forms of the types to which they belong; and, by analogy, we may not unreasonably expect to find among.parasitic plants, the most aberrant forms of vegetal types. More than this is^true. The kind of aberration which we see in the one case, we see in the other; and in both cases, the meaning of the aberration is manifest. In such Epizoa as the Lernece, the Crustacean type is disguised by the almost entire loss of the limbs and organs of sense, by the simplification of the digestive apparatus, and by the great development of the reproductive system: Asexual, or Flower less Plants Stems and loaves uiitlistinffuisliublo Stems and leaves distinguishable I. Thallooens < Funjfalcs vljichonalcs rMuscales < li.vcopodalos II. ACUOOENS ^Filicalcs Sexual, or Flowering Plants.
Wood of stem youngest in centre; cotyledon single.
Leaves |)ara]l(^l-voincd, permanent; wood coiilused Leaves net-veined, deciduous; wood, when perennial, arran2;ed in a circlc^with a central pith Wood of stem youngest at circumference, always concentric; cotyledons two or more.
Seeds quite naked fGlumales AraJes Palmal(>s Ilydrale.s NarciMsales III. EnDOGENS i Anionialcs Oi'ehidales Xyridales Juneales Lilialcs LAlismales IV. DiCTYOGEKB.
V. Gymnogens.
r Diclinous Seeds enclosed in seed- vessels VI. EXOGENS ■■ Hypogynous Perigynous Epigynous ^Amentalcs J Urticalos I Eupliorbiales r Viol ales i Cistales 1 Malvales {Ficoidalcs Daphnales Rosales rCampanalcs j Myrtales I Cactales Here, linear arrangement has disappeared: there is a breaking up into groups and sub-groups and sub-sub-groups, whicb do not admit of being placed in serial order, but only in divergent and re- divergent order. Were there space to exhibit the way in which the Alliances are subdivided into Orders, and these into Genera, and these into Species; the the parts no longer needed, abort, and those parts develop which favour the preservation of the race. Similarly in the Ehizogens, the abortive development of the leaves, the absence of chlorophyll, and the imperfect supply of spiral vessels, are changes towards a structure fit for a plant which lives on the juices absorbed from another plant; while the rapid and great development of the fructifying organs, are correlative changes advantageous to a plant, the seeds of which have but small chances of rooting themselves. And just the same reason that exists for the production of immensely numerous but extremely small eggs by Utitozoa, exists for the production by Mhizof/ens, of seeds that are great in number and almost spore-like in size.
298 THE INDUCTIONS OF lUOLOGY.
same principle of co-ordination would be still further manifested. On studying the definitions of these primary, secondary, and tertiary classes, it will be found that the largest arc marked off from each other by some attribute which connotes sundry other attributes; that each of the smaller classes comprehended in one of these largest classes, is marked off in a similar way from the smaller classes bound up with it; and that so, each successively smaller class, has an increased number of co-existing attributes.
§ 100. Zoological classification has had a parallel history. The first attempt which we need notice, to arrange animals in such a way as to display their affinities, is that of Linnseus. He grouped them thus:* — Cl. 1. Mammalia. Ord, Primates, Bruta, Eerse, Glires, Pecora, Belluse, Cete.
Cl. 2. Ayes. Ord. Accipitres, Picas, Ansercs, Grallfe, Galliusc, Passcrcs.
Cl. 3. Amphibia. Ord. Reptiles, Serpentes, Nantes.
Cl. 4. Pisces. Ord. Apodes, Jugulares, Thoracici, xibdominales.
Cl. 5. Insecta. Or^. Coleoptera, Hemiptera, Lepidoptera, Neuroptera, Diptera, Aptera.
Cl. 6. Veemes. Ord. lutestina, Mollusca, Testacea, Litliophyta, Zoopliyta.
This arrangement of classes, is obviously based on apparent gradations of rank; and the placing of the orders similarly betrays an endeavour to make successions, beginning with the most superior forms and ending with the most inferior forms. While the general and vague idea of perfection, determines the leading character of the classification, its detailed groupings are determined by the most conspicuous external attributes. JN^ot only Linnaeus, but his opponents, who proposed other systems, were ** under the impression that animals were to be arranged together into classes, orders, genera, and species, according to their more or less close external resemblance." This conception survived till the time of Cuvier. *' Naturalists," * This classification, and the three which follow it, I quote (abridging some of them) from Prof. Agassiz's "Essay on Classificatiou."
I says Agassiz, " were bent upon establisliing one continual uniform series to embrace all animals, between the links of wliicli it was supposed there were no unequal intervals. The watchword of their school was: Natara non facit saltum. They called their system la chainc den etrea!''
The classification of Cuvier, based on internal organization instead of external appearance, was a great advance, lie asserted that there are four principal forms, or four general plans, on which animals arc constructed; and in pursuance of this assertion, he drew out the following scheme.
First Branch. Animalia Yertebrata.
Cl. 1. Mammalia.
Cl. 2. Birds.
Cl. 3. Reptilia.
Cl. 4, Fishes.
Second Branch. Animalia Mollusca.
Cl. 1. Cephalapoda. Cl. 2. Pteropoda. Cl. 3. Gasteropoda.
Cl. 4. ACEPHALA.
Cl. 5. Brachiopoda.
Cl. 6, CiRRHOPODA.
Third Branch. Animalia Articulata.
Cl. 1. Annelides.
Cl. 2. Crustacea.
Cl. 3. Arachnides.
Cl. 4. Insects.
Fourth Branch. Animalia Radiata. Cl. 1. Echinoderms. Cl. 2. Intestinal Worms. Cl. 3. Acaleph^. Cl. 4. Polypi. Cl. 5. Infusoria.
But though Cuvier emancipated himself from the conception of a serial progression throughout the Animal- Kingdom; sundry of his contemporaries and successors remained fettered by the old error. Less regardful of the differentlyco-ordinated sets of attributes displayed by the different subkingdoms; and swayed by the belief in a progressive development, which was erroneously supposed to imply the possibility of arranging animals in a linear series; they persisted in thrusting organic forms into a quite unnatural order. The following classification of Lamarck illustrates this.
I.
II.
INYEETEBEATA. Apathetic Animals.
Cl.
1.
Infusoria.
Cl.
2.
Polypi.
Cl.
3.
Radiaria.
Cl.
4.
TUNICATA.
Cl.
5.
Vermes.
Sen siTiVE Animals.
Cl.
6.
Insects.
Cl.
7.
Arachnids.
Cl.
8.
Crustacea.
Cl.
9.
Annelids.
Cl.
CiRRIPEDS.
Cl.
Conchifera Cl.
MOLLUSKS.
III. Intelligent Animals.
Cl. 13. Fishes. Cl. 14. Reptiles. Cl. 15. Birds. Cl. 16. Mammalia.
Do not feel, and move only by their excited irritability. No brain, ► not elongated medullary mass; no senses; forms varied; rarely articulations.
Feel, but obtain from their sensations only perceptions of objects, a sort of simple ideas, vrhich they are unable to combine to obtain complex > ones. No vertebral column; a brain and mostly an elongated medullary mass; some distinct senses; muscles attached under the skin; form symmetrical, the parts being in pairs.
YEETEBEATA.
Feel; acquire preservable ideas; perform with them operations by which they obtain others; are intelligent in different degrees. A ver- ► tebral column; a brain and a spinal marrow; distinct senses; the muscles attached to the internal skelej ton; form symmetrical, the parts being in pairs.
Fussing over sundry classifications in which the serial arrangement dictated by the notion of ascending complexity, is variously modified by the recognition of conspicuous anatomical facts, we come to the classifications which recognize another order of facts — those of development. The embryological inquiries of Von Baer, led him to arrange animals a^ follows: — I. Peripheric Type. (Radiata.) Evolutio radiata. The development proceeds from a centre, producing identical parts in a radiating order. II. Massive Type. (Mollusca.) Evolutio contorta. The development produces identical parts curved around a conical or other space.
III. Longitudinal Type. (Articulata.) Evolutio gemma. The development produces identical parts arising on both sides of an axis, and closing up along a line opposite the axis.
lY. Doubly Symmetrical type. (Yertebrata.) Evolutio higemina. The development produces identical parts arising on both sides of an axis, growing upwards and downwards, and shutting up along two lines, so that the inner layer of the germ is inclosed below, and the upper layer above. The embryos of these animals have a dorsal cord, dorsal plates, and ventral plates, a nervous tube and branchial fissures.
Recognizing these fundamental differences in the modes of evolution, as answering to fundamental divisions in the animal kingdom. Yon Baer shows (among the Verteh rata at least) how the minor differences that arise at successively later stages of evolution, correspond with the minor divisions.
Like the modern classification of plants, the classification of animals that has now been arrived at, is one in which the linear order is completely broken up. In his lectures at the Royal Institution, in 1857, Prof. Huxley expressed the relations existing among the several great groups of the animal kingdom, by placing these groups at the ends of four or five radii, diverging from a centre. The diagram I cannot obtain; but in the published reports of his lectures at the School of Mines the groups were arranged thus: — Yertebrata {Abranchiata) Mammalia Aves Eeptilia [Branchiaia) Amphibia Pisces MOLLUSCA Cephalopoda Hcteropoda Gasteropodadioecia I Pulraonata Gasteropoda- ( Ptevopoda monoccia Laraellibranchiata Annulosa Articulata Insocta Arachuida Myriapoda Crustacea Annuloida Anncllata Scoleida3 Echinodermata Treniatoda Eotifera Toeniada3 Tiirbellaria Nematoidea CffiLENTERATA Ilvdrozoa Actinozoa.
Protozoa Tiifusoria Noctilucidoi Spon^^iadfo Foramiuifera Crefravinidpe Thallassicollidce What remnant there may scorn to be of linear succession in some of these sub-groups, is merely an accident of typographical convenience. Each of them is to be regarded simply as a cluster. Were Prof. Pluxley now to revise this scheme, he would probably separate more completely some of the great sub-groups, in conformity with the views expressed in his Ilunterian Lectures deli veered at the College of Surgeons in 1863. And if he were further to develop the arrangement, by dispersing the sub-groups and sub-subgroups on the same principle, there would result an arrange- CLASSIFICATION. o03 mcnt pcrliaps not very much unlike that shown in the annexed didgram.
\ V \ \ j lAft/ria/ioda annu'losa ^. f^colecldcL • ^ \ A/TLTLiiloida m.c7icvci-a, • • \ \ / / / / / MOLL US C A ^\ • / Slionqlda Jvfusona Hydrczca 1% •» J CCEL E N TERATA • In this diagram, the dots represent orders, the names of which it is impracticable to insert. If it be supposed that when magnified, each of these dots resolves itself into a cluster of clusters, representing genera and species, an approximate idea wall be formed of the relations among the successively-subordinate groups constituting the animal kingdom. Besides the subordination of groups and their general distribution, some other facts are indicated. By the distances of the great divisions from the general centre, are rudely symbolized their respective degrees of divergence from the form of simple, undifferentiated organic matter; which we may regard as their common source. Within each group, the remoteness from the local centre represents, in a rough way, the degree of departure from the general plan of the group. And the distribution of the sub-groups within each group, is in most cases such, that those which come nearest to neighbouring groups, are those which show the nearest resemblances to them — in their analogies though not in their homologies. No diagram, however, can give a correct conception. Even supposing the above diagram expressed the relations of animals to one another as truly as they can be expressed on a plane surface, (which of course it does not,) it would still be inadequate. Such relations cannot be represented in space of two dimensions; but only in space of three dimensions.
§ 101. While the classifications of botanists and zoologists have become more and more natural in their arrangements, there has grown up a certain artificiality in their abstract nomenclature. When aggregating the smallest groups into larger groups, and these into groups still larger, naturalists adopted certain general terms expressive of the successively more comprehensive divisions; and the habitual use of these terms, needful for purposes of convenience, has led to the tacit assumption that they answer to actualities in Nature. It has been taken for granted that species, genera, orders, and classes, are assemblages of definite values — that every genus is the equivalent of every other genus, in respect of its degree of distinctness; and that orders are separated by lines of demarcation that are as broad in one place as another. Though this conviction is not a formulated one, yet the disputes continually arising among naturalists on the I I questions, wlicthcr such and such organisms arc specifically or generically distinct, and whether this or that peculiarity is or is not of ordinal importance, imply that the conviction is entertained even where it is not avowed. Yet that differences of opinion like these continually arise, and remain unsettled, except when they end in the establishment of subspecies, sub-genera, sub-orders, and sub-classes, sufficiently shows that no such conviction is justifiable. And this is equally shown by the impossibility of obtaining any definition of the degree of difference, which warrants each further elevation in the hierarchy of classes.
It is, indeed, a wholly gratuitous assumption that organisms admit of being placed in groups of equivalent values; and that these may be united into larger groups that are also of equivalent values; and so on. There is no a priori reason for expecting this; and there is no a posteriori evidence implying it, save that which begs the question — that which asserts one distinction to be generic and another to be ordinal, because it is assumed that such distinctions must be either generic or ordinal. The endeavour to thrust plants and animals into these definite partitions, is of the same nature as the endeavour to thrust them into a linear series. Not that it does violence to the facts in anything like the same degree; but still, it does violence to the facts. Doubtless the making of divisions and sub-divisions, is extremely useful; or rather, it is absolutely necessary. Doubtless, too, in reducing the facts to something like order, they must be partially distorted. So long as the distorted form is not mistaken for the actual form, no harm results. But it is needful for us to remember, that while our successively subordinate groups have a certain general correspondence with the realities, they inevitably give to the realities a regularity which does not exist.
§ 102. A general truth of much significance is exhibited ol in these classifications. On observing the natures of the attributes wliich are common to the members of any group of the first, second, third, or fourth rank, we see that groups of the widest generality are based on characteristics of the greatest importance, physiologically considered; and that the characteristics of the successively-subordinate groups, are characteristics of successively-subordinate importance. The structural peculiarity in which all members of one subkingdom difier from all members of another sub-kingdom, is a peculiarity that affects the vital actions more profoundly, than does the structural peculiarity which distinguishes all members of one class from all members of another class. Let us look at a few cases.
We saw ( § 56) f that the broadest division among the functions is the division into " the accumulation of force (latent in food); the expenditure of force (latent in the tissues and certain matters absorbed by them); and the transfer of force (latent in the prepared nutriment or blood) from the parts which accumulate to the parts which expend." I^ow the lowest animals, united under the general name Protozoa, are those in which there is either no separation of the parts performing these functions or very indistinct separation: in the Hhizopoda, all parts are alike accumulators of force, expenders of force, and transferrers of force; and though in the most differentiated members of the group, the Infusoria, there are something like specializations corresponding to these functions, yet there are no distinct tissues appropriated to them. The animals known as Cmlenterata are characterized in common by the possession of a part which accumulates force more or less marked off from the part which does not accumulate force, but only expends it; and the Ilydrozoa and Actinozoa, which are sub-divisions of the Cielentcrata, are contrasted in this, that in the one these parts are very indefinitely distinguished, but in the other definitely separated, as well as more complicated. Besides a completer differentiation of the organs respectively devoted to the accumulation of force and the expenditure of force, CLASSIFICATION. 'JOT the animals classed as Molhi^coida^ possess rude appliances for the transfer of force: the peri-visceral sac, or closed cavity between tlie intestine and the walls of the body, serves as a reservoir of absorbed nutriment, from which the surrounding tissues take up the materials they need. The more highly-organized animals, belonging to whichever subkingdom, all of them possess definitely-constructed channels for the transfer of force; and in all of them, the function of expenditure is divided between a directive apparatus and an executive apparatus — a nervous system and a muscular system. But these higher sub-kingdoms are clearly separated from each other by differences in the relative positions of their component sets of organs. Prof. Huxley defines the type of the Vertehrata^ as one in which the ganglionic nervous system lies on the dorsal side of the alimentary canal, while the central vascular system lies on its ventral side; and one which is yet further characterized by the possession of a second, and more conspicuous, nervous system, placed on the dorsal side of the vertebral axis — an extra endowment which is perhaps the most essentially distinctive. The types of the Annulosa and Mollusca, are together marked off from the vertebrate type, by the singleness of the nervous system, and by its occupation of the ventral side of the body: the habitual attitudes of annulose and molluscous creatures, is such that the neural centres are below the alimentary canal and the hoemal centres above. And while by these traits the annulose and molluscous types are separated from the vertebrate, they are separated from each other by this, that in the one the body is " composed of successive segments, usually pro\dded with limbs,'' but the other, the body is not segmented, " and no true articulated limbs are ever developed.'^