The question next arises, whether, as all Kinds must have a place among the cla.s.ses, so all the cla.s.ses in a natural arrangement must be Kinds?
And to this I answer, certainly not. The distinctions of Kinds are not numerous enough to make up the whole of a cla.s.sification. Very few of the genera of plants, or even of the families, can be p.r.o.nounced with certainty to be Kinds. The great distinctions of Vascular and Cellular, Dicotyledonous or Exogenous and Monocotyledonous or Endogenous plants, are perhaps differences of Kind; the lines of demarcation which divide those cla.s.ses seem (though even on this I would not p.r.o.nounce positively) to go through the whole nature of the plants. But the different species of a genus, or genera of a family, usually have in common only a limited number of characters. A Rose does not seem to differ from a Rubus, or the Umbellifer from the Ranunculace, in much else than the characters botanically a.s.signed to those genera or those families. Unenumerated differences certainly do exist in some cases; there are families of plants which have peculiarities of chemical composition, or yield products having peculiar effects on the animal economy. The Crucifer and Fungi contain an unusual proportion of nitrogen; the l.a.b.i.at are the chief sources of essential oils, the Solane are very commonly narcotic, &c. In these and similar cases there are possibly distinctions of Kind; but it is by no means indispensable that there should be. Genera and Families may be eminently natural, though marked out from one another by properties limited in number; provided those properties are important, and the objects contained in each genus or family resemble each other more than they resemble anything which is excluded from the genus or family.
After the recognition and definition, then, of the _infim species_, the next step is to arrange those _infim species_ into larger groups: making these groups correspond to Kinds wherever it is possible, but in most cases without any such guidance. And in doing this it is true that we are naturally and properly guided, in most cases at least, by resemblance to a type. We form our groups round certain selected Kinds, each of which serves as a sort of exemplar of its group. But though the groups are suggested by types, I cannot think that a group when formed is _determined_ by the type; that in deciding whether a species belongs to the group, a reference is made to the type, and not to the characters; that the characters "cannot be expressed in words." This a.s.sertion is inconsistent with Dr. Whewell"s own statement of the fundamental principle of cla.s.sification, namely, that "general a.s.sertions shall be possible." If the cla.s.s did not possess any characters in common, what general a.s.sertions would be possible respecting it? Except that they all resemble each other more than they resemble anything else, nothing whatever could be predicated of the cla.s.s.
The truth is, on the contrary, that every genus or family is framed with distinct reference to certain characters, and is composed, first and princ.i.p.ally, of species which agree in possessing all those characters.
To these are added, as a sort of appendix, such other species, generally in small number, as possess _nearly_ all the properties selected; wanting some of them one property, some another, and which, while they agree with the rest _almost_ as much as these agree with one another, do not resemble in an equal degree any other group. Our conception of the cla.s.s continues to be grounded on the characters; and the cla.s.s might be defined, those things which _either_ possess that set of characters, _or_ resemble the things that do so, more than they resemble anything else.
And this resemblance itself is not, like resemblance between simple sensations, an ultimate fact, unsusceptible of a.n.a.lysis. Even the inferior degree of resemblance is created by the possession of common characters. Whatever resembles the genus Rose more than it resembles any other genus, does so because it possesses a greater number of the characters of that genus, than of the characters of any other genus. Nor can there be any real difficulty in representing, by an enumeration of characters, the nature and degree of the resemblance which is strictly sufficient to include any object in the cla.s.s. There are always some properties common to all things which are included. Others there often are, to which some things, which are nevertheless included, are exceptions. But the objects which are exceptions to one character are not exceptions to another: the resemblance which fails in some particulars must be made up for in others. The cla.s.s, therefore, is const.i.tuted by the possession of _all_ the characters which are universal, and _most_ of those which admit of exceptions. If a plant had the ovules erect, the stigmata divided, possessed the alb.u.men, and was without stipules, it possibly would not be cla.s.sed among the Rosace.
But it may want any one, or more than one of those characters, and not be excluded. The ends of a scientific cla.s.sification are better answered by including it. Since it agrees so nearly, in its known properties, with the sum of the characters of the cla.s.s, it is likely to resemble that cla.s.s more than any other in those of its properties which are still undiscovered.
Not only, therefore, are natural groups, no less than any artificial cla.s.ses, determined by characters; they are const.i.tuted in contemplation of, and by reason of, characters. But it is in contemplation not of those characters only which are rigorously common to all the objects included in the group, but of the entire body of characters, all of which are found in most of those objects, and most of them in all. And hence our conception of the cla.s.s, the image in our minds which is representative of it, is that of a specimen complete in all the characters; most naturally a specimen which, by possessing them all in the greatest degree in which they are ever found, is the best fitted to exhibit clearly, and in a marked manner, what they are. It is by a mental reference to this standard, not instead of, but in ill.u.s.tration of, the definition of the cla.s.s, that we usually and advantageously determine whether any individual or species belongs to the cla.s.s or not.
And this, as it seems to me, is the amount of truth contained in the doctrine of Types.
We shall see presently that where the cla.s.sification is made for the express purpose of a special inductive inquiry, it is not optional, but necessary for fulfilling the conditions of a correct Inductive Method, that we should establish a type-species or genus, namely, the one which exhibits in the most eminent degree the particular phenomenon under investigation. But of this hereafter. It remains, for completing the theory of natural groups, that a few words should be said on the principles of the nomenclature adapted to them.
5. A Nomenclature in science, is, as we have said, a system of the names of Kinds. These names, like other cla.s.s-names, are defined by the enumeration of the characters distinctive of the cla.s.s. The only merit which a set of names can have beyond this, is to convey, by the mode of their construction, as much information as possible: so that a person who knows the thing, may receive all the a.s.sistance which the name can give in remembering what he knows, while he who knows it not, may receive as much knowledge respecting it as the case admits of, by merely being told its name.
There are two modes of giving to the name of a Kind this sort of significance. The best, but which unfortunately is seldom practicable, is when the word can be made to indicate, by its formation, the very properties which it is designed to connote. The name of a Kind does not, of course, connote all the properties of the Kind, since these are inexhaustible, but such of them as are sufficient to distinguish it; such as are sure marks of all the rest. Now, it is very rarely that one property, or even any two or three properties, can answer this purpose.
To distinguish the common daisy from all other species of plants would require the specification of many characters. And a name cannot, without being too c.u.mbrous for use, give indication, by its etymology or mode of construction, of more than a very small number of these. The possibility, therefore, of an ideally perfect Nomenclature, is probably confined to the one case in which we are happily in possession of something nearly approaching to it; the Nomenclature of elementary Chemistry. The substances, whether simple or compound, with which chemistry is conversant, are Kinds, and, as such, the properties which distinguish each of them from the rest are innumerable; but in the case of compound substances (the simple ones are not numerous enough to require a systematic nomenclature), there is one property, the chemical composition, which is of itself sufficient to distinguish the Kind; and is (with certain reservations not yet thoroughly understood) a sure mark of all the other properties of the compound. All that was needful, therefore, was to make the name of every compound express, on the first hearing, its chemical composition; that is, to form the name of the compound, in some uniform manner, from the names of the simple substances which enter into it as elements. This was done, most skilfully and successfully, by the French chemists. The only thing left unexpressed by them was the exact proportion in which the elements were combined; and even this, since the establishment of the atomic theory, it has been found possible to express by a simple adaptation of their phraseology.
But where the characters which must be taken into consideration in order sufficiently to designate the Kind, are too numerous to be all signified in the derivation of the name, and where no one of them is of such preponderant importance as to justify its being singled out to be so indicated, we may avail ourselves of a subsidiary resource. Though we cannot indicate the distinctive properties of the Kind, we may indicate its nearest natural affinities, by incorporating into its name the name of the proximate natural group of which it is one of the species. On this principle is founded the admirable binary nomenclature of botany and zoology. In this nomenclature the name of every species consists of the name of the genus, or natural group next above it, with a word added to distinguish the particular species. The last portion of the compound name is sometimes taken from some _one_ of the peculiarities in which that species differs from others of the genus; as Clematis _integrifolia_, Potentilla _alba_, Viola _pal.u.s.tris_, Artemisia _vulgaris_; sometimes from a circ.u.mstance of an historical nature, as Narcissus _poeticus_, Potentilla _tormentilla_ (indicating that the plant was formerly known by the latter name), Exac.u.m _Candollii_ (from the fact that De Candolle was its first discoverer); and sometimes the word is purely conventional, as Thlaspi _bursa-pastoris_, Ranunculus _thora_; it is of little consequence which; since the second, or as it is usually called, the specific name, could at most express, independently of convention, no more than a very small portion of the connotation of the term. But by adding to this the name of the superior genus, we may make the best amends we can for the impossibility of so contriving the name as to express all the distinctive characters of the Kind. We make it, at all events, express as many of those characters as are common to the proximate natural group in which the Kind is included.
If even those common characters are so numerous or so little familiar as to require a further extension of the same resource, we might, instead of a binary, adopt a ternary nomenclature, employing not only the name of the genus, but that of the next natural group in order of generality above the genus, commonly called the Family. This was done in the mineralogical nomenclature proposed by Professor Mohs. "The names framed by him were not composed of two, but of three elements, designating respectively the Species, the Genus, and the Order; thus he has such species as _Rhombohedral Lime Haloide_, _Octohedral Fluor Haloide_, _Prismatic Hal Baryte_."[22] The binary construction, however, has been found sufficient in botany and zoology, the only sciences in which this general principle has. .h.i.therto been successfully adopted in the construction of a nomenclature.
Besides the advantage which this principle of nomenclature possesses, in giving to the names of species the greatest quant.i.ty of independent significance which the circ.u.mstances of the case admit of, it answers the further end of immensely economizing the use of names, and preventing an otherwise intolerable burden on the memory. When the names of species become extremely numerous, some artifice (as Dr. Whewell[23]
observes) becomes absolutely necessary to make it possible to recollect or apply them. "The known species of plants, for example, were ten thousand in the time of Linnus, and are now probably sixty thousand. It would be useless to endeavour to frame and employ separate names for each of these species. The division of the objects into a subordinated system of cla.s.sification enables us to introduce a Nomenclature which does not require this enormous number of names. Each of the genera has its name, and the species are marked by the addition of some epithet to the name of the genus. In this manner about seventeen hundred generic names, with a moderate number of specific names, were found by Linnus sufficient to designate with precision all the species of vegetables known at his time." And though the number of generic names has since greatly increased, it has not increased in anything like the proportion of the multiplication of known species.
CHAPTER VIII.
OF CLa.s.sIFICATION BY SERIES.
1. Thus far, we have considered the principles of scientific cla.s.sification so far only as relates to the formation of natural groups; and at this point most of those who have attempted a theory of natural arrangement, including, among the rest, Dr. Whewell, have stopped. There remains, however, another, and a not less important portion of the theory, which has not yet, as far as I am aware, been systematically treated of by any writer except M. Comte. This is, the arrangement of the natural groups into a natural series.[24]
The end of Cla.s.sification, as an instrument for the investigation of nature, is (as before stated) to make us think of those objects together, which have the greatest number of important common properties; and which therefore we have oftenest occasion, in the course of our inductions, for taking into joint consideration. Our ideas of objects are thus brought into the order most conducive to the successful prosecution of inductive inquiries generally. But when the purpose is to facilitate some particular inductive inquiry, more is required. To be instrumental to that purpose, the cla.s.sification must bring those objects together, the simultaneous contemplation of which is likely to throw most light upon the particular subject. That subject being the laws of some phenomenon or some set of connected phenomena; the very phenomenon or set of phenomena in question must be chosen as the groundwork of the cla.s.sification.
The requisites of a cla.s.sification intended to facilitate the study of a particular phenomenon, are, first, to bring into one cla.s.s all Kinds of things which exhibit that phenomenon, in whatever variety of forms or degrees; and secondly, to arrange those Kinds in a series according to the degree in which they exhibit it, beginning with those which exhibit most of it, and terminating with those which exhibit least. The princ.i.p.al example, as yet, of such a cla.s.sification, is afforded by comparative anatomy and physiology, from which, therefore, our ill.u.s.trations shall be taken.
2. The object being supposed to be, the investigation of the laws of animal life; the first step, after forming the most distinct conception of the phenomenon itself, possible in the existing state of our knowledge, is to erect into one great cla.s.s (that of animals) all the known Kinds of beings where that phenomenon presents itself; in however various combinations with other properties, and in however different degrees. As some of these Kinds manifest the general phenomenon of animal life in a very high degree, and others in an insignificant degree, barely sufficient for recognition; we must, in the next place, arrange the various Kinds in a series, following one another according to the degrees in which they severally exhibit the phenomenon; beginning therefore with man, and ending with the most imperfect kinds of zoophytes.
This is merely saying that we should put the instances, from which the law is to be inductively collected, into the order which is implied in one of the four Methods of Experimental Inquiry discussed in the preceding Book; the fourth Method, that of Concomitant Variations. As formerly remarked, this is often the only method to which recourse can be had, with a.s.surance of a true conclusion, in cases in which we have but limited means of effecting, by artificial experiments, a separation of circ.u.mstances usually conjoined. The principle of the method is, that facts which increase or diminish together, and disappear together, are either cause and effect, or effects of a common cause. When it has been ascertained that this relation really subsists between the variations, a connexion between the facts themselves may be confidently laid down, either as a law of nature or only as an empirical law, according to circ.u.mstances.
That the application of this Method must be preceded by the formation of such a series as we have described, is too obvious to need being pointed out; and the mere arrangement of a set of objects in a series, according to the degrees in which they exhibit some fact of which we are seeking the law, is too naturally suggested by the necessities of our inductive operations, to require any lengthened ill.u.s.tration here. But there are cases in which the arrangement required for the special purpose, becomes the determining principle of the cla.s.sification of the same objects for general purposes. This will naturally and properly happen, when those laws of the objects which are sought in the special inquiry enact so princ.i.p.al a part in the general character and history of those objects--exercise so much influence in determining all the phenomena of which they are either the agents or the theatre--that all other differences existing among the objects are fittingly regarded as mere modifications of the one phenomenon sought; effects determined by the co-operation of some incidental circ.u.mstance with the laws of that phenomenon. Thus in the case of animated beings, the differences between one cla.s.s of animals and another may reasonably be considered as mere modifications of the general phenomenon, animal life; modifications arising either from the different degrees in which that phenomenon is manifested in different animals, or from the intermixture of the effects of incidental causes peculiar to the nature of each, with the effects produced by the general laws of life; those laws still exercising a predominant influence over the result. Such being the case, no other inductive inquiry respecting animals can be successfully carried on, except in subordination to the great inquiry into the universal laws of animal life. And the cla.s.sification of animals best suited to that one purpose, is the most suitable to all the other purposes of zoological science.
3. To establish a cla.s.sification of this sort, or even to apprehend it when established, requires the power of recognising the essential similarity of a phenomenon, in its minuter degrees and obscurer forms, with what is called the _same_ phenomenon in the greatest perfection of its development; that is, of identifying with each other all phenomena which differ only in degree, and in properties which we suppose to be caused by difference of degree. In order to recognise this ident.i.ty, or in other words, this exact similarity of quality, the a.s.sumption of a type-species is indispensable. We must consider as the type of the cla.s.s, that among the Kinds included in it, which exhibits the properties const.i.tutive of the cla.s.s, in the highest degree; conceiving the other varieties as instances of degeneracy, as it were, from that type; deviations from it by inferior intensity of the characteristic property or properties. For every phenomenon is best studied (_cteris paribus_) where it exists in the greatest intensity. It is there that the effects which either depend on it, or depend on the same causes with it, will also exist in the greatest degree. It is there, consequently, and only there, that those effects of it, or joint effects with it, can become fully known to us, so that we may learn to recognise their smaller degrees, or even their mere rudiments, in cases in which the direct study would have been difficult or even impossible. Not to mention that the phenomenon in its higher degrees may be attended by effects or collateral circ.u.mstances which in its smaller degrees do not occur at all, requiring for their production in any sensible amount a greater degree of intensity of the cause than is there met with. In man, for example, (the species in which both the phenomenon of animal and that of organic life exist in the highest degree,) many subordinate phenomena develop themselves in the course of his animated existence, which the inferior varieties of animals do not show. The knowledge of these properties may nevertheless be of great avail towards the discovery of the conditions and laws of the general phenomenon of life, which is common to man with those inferior animals. And they are, even, rightly considered as properties of animated nature itself; because they may evidently be affiliated to the general laws of animated nature; because we may fairly presume that some rudiments or feeble degrees of those properties would be recognised in all animals by more perfect organs, or even by more perfect instruments, than ours; and because those may be correctly termed properties of a cla.s.s, which a thing exhibits exactly in proportion as it belongs to the cla.s.s, that is, in proportion as it possesses the main attributes const.i.tutive of the cla.s.s.
4. It remains to consider how the internal distribution of the series may most properly take place: in what manner it should be divided into Orders, Families, and Genera.
The main principle of division must of course be natural affinity; the cla.s.ses formed must be natural groups: and the formation of these has already been sufficiently treated of. But the principles of natural grouping must be applied in subordination to the principle of a natural series. The groups must not be so const.i.tuted as to place in the same group things which ought to occupy different points of the general scale. The precaution necessary to be observed for this purpose is, that the _primary_ divisions must be grounded not on all distinctions indiscriminately, but on those which correspond to variations in the degree of the main phenomenon. The series of Animated Nature should be broken into parts at the points where the variation in the degree of intensity of the main phenomenon (as marked by its princ.i.p.al characters, Sensation, Thought, Voluntary Motion, &c.) begins to be attended by conspicuous changes in the miscellaneous properties of the animal. Such well-marked changes take place, for example, where the cla.s.s Mammalia ends; at the points where Fishes are separated from Insects, Insects from Mollusca, &c. When so formed, the primary natural groups will compose the series by mere juxtaposition, without redistribution; each of them corresponding to a definite portion of the scale. In like manner each family should, if possible, be so subdivided, that one portion of it shall stand higher and the other lower, though of course contiguous, in the general scale; and only when this is impossible is it allowable to ground the remaining subdivisions on characters having no determinable connexion with the main phenomenon.
Where the princ.i.p.al phenomenon so far transcends in importance all other properties on which a cla.s.sification could be grounded, as it does in the case of animated existence, any considerable deviation from the rule last laid down is in general sufficiently guarded against by the first principle of a natural arrangement, that of forming the groups according to the most important characters. All attempts at a scientific cla.s.sification of animals, since first their anatomy and physiology were successfully studied, have been framed with a certain degree of instinctive reference to a natural series, and have accorded in many more points than they have differed, with the cla.s.sification which would most naturally have been grounded on such a series. But the accordance has not always been complete; and it still is often a matter of discussion, which of several cla.s.sifications best accords with the true scale of intensity of the main phenomenon. Cuvier, for example, has been justly criticized for having formed his natural groups with an undue degree of reference to the mode of alimentation, a circ.u.mstance directly connected only with organic life, and not lending to the arrangement most appropriate for the purposes of an investigation of the laws of animal life, since both carnivorous and herbivorous or frugivorous animals are found at almost every degree in the scale of animal perfection. Blainville"s cla.s.sification has been considered by high authorities to be free from this defect; as representing correctly, by the mere order of the princ.i.p.al groups, the successive degeneracy of animal nature from its highest to its most imperfect exemplification.
5. A cla.s.sification of any large portion of the field of nature in conformity to the foregoing principles, has. .h.i.therto been found practicable only in one great instance, that of animals. In the case even of vegetables, the natural arrangement has not been carried beyond the formation of natural groups. Naturalists have found, and probably will continue to find it impossible to form those groups into any series, the terms of which correspond to real gradations in the phenomenon of vegetative or organic life. Such a difference of degree may be traced between the cla.s.s of Vascular Plants and that of Cellular, which includes lichens, alg, and other substances whose organization is simpler and more rudimentary than that of the higher order of vegetables, and which therefore approach nearer to mere inorganic nature. But when we rise much above this point, we do not find any sufficient difference in the degree in which different plants possess the properties of organization and life. The dicotyledons are of more complex structure, and somewhat more perfect organization, than the monocotyledons: and some dicotyledonous families, such as the Composit, are rather more complex in their organization than the rest. But the differences are not of a marked character, and do not promise to throw any particular light upon the conditions and laws of vegetable life and development. If they did, the cla.s.sification of vegetables would have to be made, like that of animals, with reference to the scale or series indicated.
Although the scientific arrangements of organic nature afford as yet the only complete example of the true principles of rational cla.s.sification, whether as to the formation of groups or of series, those principles are applicable to all cases in which mankind are called upon to bring the various parts of any extensive subject into mental co-ordination. They are as much to the point when objects are to be cla.s.sed for purposes of art or business, as for those of science. The proper arrangement, for example, of a code of laws, depends on the same scientific conditions as the cla.s.sifications in natural history; nor could there be a better preparatory discipline for that important function, than the study of the principles of a natural arrangement, not only in the abstract, but in their actual application to the cla.s.s of phenomena for which they were first elaborated, and which are still the best school for learning their use. Of this the great authority on codification, Bentham, was perfectly aware: and his early _Fragment on Government_, the admirable introduction to a series of writings unequalled in their department, contains clear and just views (as far as they go) on the meaning of a natural arrangement, such as could scarcely have occurred to any one who lived anterior to the age of Linnus and Bernard de Jussieu.
FOOTNOTES:
[1] Supra, book iii. ch. ii. 3, 4, 5.
[2] Mr. Bailey has given by far the best statement of this theory. "The general name," he says, "raises up the image sometimes of one individual of the cla.s.s formerly seen, sometimes of another, not unfrequently of many individuals in succession; and it sometimes suggests an image made up of elements from several different objects, by a latent process of which I am not conscious." (Letters on the Philosophy of the Human Mind, 1st series, letter 22.) But Mr. Bailey must allow that we carry on inductions and ratiocinations respecting the cla.s.s, by means of this idea or conception of some one individual in it. This is all I require.
The name of a cla.s.s calls up some idea, through which we can, to all intents and purposes, think of the cla.s.s as such, and not solely of an individual member of it.
[3] I have entered rather fully into this question in chap. xvii. of _An Examination of Sir William Hamilton"s Philosophy_, headed "The Doctrine of Concepts or General Notions," which contains my last views on the subject.
[4] Other examples of inappropriate conceptions are given by Dr. Whewell (_Phil. Ind. Sc._ ii. 185) as follows:--"Aristotle and his followers endeavoured in vain to account for the mechanical relation of forces in the lever, by applying the _inappropriate_ geometrical conceptions of the properties of the circle: they failed in explaining the _form_ of the luminous spot made by the sun shining through a hole, because they applied the _inappropriate_ conception of a circular _quality_ in the sun"s light: they speculated to no purpose about the elementary composition of bodies, because they a.s.sumed the _inappropriate_ conception of _likeness_ between the elements and the compound, instead of the genuine notion of elements merely _determining_ the qualities of the compound." But in these cases there is more than an inappropriate conception; there is a false conception; one which has no prototype in nature, nothing corresponding to it in facts. This is evident in the last two examples, and is equally true in the first; the "properties of the circle" which were referred to, being purely fantastical. There is, therefore, an error beyond the wrong choice of a principle of generalization; there is a false a.s.sumption of matters of fact. The attempt is made to resolve certain laws of nature into a more general law, that law not being one which, though real, is inappropriate, but one wholly imaginary.
[5] Professor Bain.
[6] This sentence having been erroneously understood as if I had meant to a.s.sert that belief is nothing but an irresistible a.s.sociation, I think it necessary to observe that I express no theory respecting the ultimate a.n.a.lysis either of reasoning or of belief, two of the most obscure points in a.n.a.lytical psychology. I am speaking not of the powers themselves, but of the previous conditions necessary to enable those powers to exert themselves: of which conditions I am contending that language is not one, senses and a.s.sociation being sufficient without it.
[7] Mr. Bailey agrees with me in thinking that whenever "from something actually present to my senses conjoined with past experience, I feel satisfied that something has happened, or will happen, or is happening, beyond the sphere of my personal observation," I may with strict propriety be said to reason: and of course to reason inductively, for demonstrative reasoning is excluded by the circ.u.mstances of the case.
(_The Theory of Reasoning_, 2nd ed. p. 27.)
[8] _Novum Organum Renovatum_, pp. 35-37.
[9] _Nov. Org. Renov._, pp. 39, 40.
[10] P. 217, 4to edition.
[11] "E, ex, extra, extraneus, tranger, stranger."
Another etymological example sometimes cited is the derivation of the English _uncle_ from the Latin _avus_. It is scarcely possible for two words to bear fewer outward marks of relationship, yet there is but one step between them; _avus_, _avunculus_, _uncle_.
So _pilgrim_, from _ager_: _per agrum_, _peragrinus_, _peregrinus_, _pellegrino_, _pilgrim_.
[12] P. 226-7.
[13] _Essays_, p. 214.
[14] Ibid. 215.