an organism, acts, by its polar forces, on all the other units, and is re-acted on by them. When, too, we learn that glass has its molecular constitution changed by light, and that substances so rigid and stable as metals, have their atoms re- arranged by forces radiated in the dark from adjacent objects; we are obliged to conclude that the ex- cessively-unstable units of which organisms are built, must be sensitive in a transcendant degree, to all the forces pervading the organisms composed of them — must be tending ever to re-adjust, not only their relative positions, but their molecular structures, into equilibrium with these forces. Hence, if ag- gregates of the same species are differently conditioned, and re-act differently on their component units, their component units will be rendered somewhat different; and they will become the more different the more widely the re- actions of the aggregates upon them differ, and the greater the num - ber of generations through which these different re-actions of the aggregates upon them are continued.
If, then, unlikenesses of function among individuals of the same species, produce unlikenesses between the physiological units of one individual and those of another; it becomes com- prehensible that when groups of units derived from two indi- viduals are united, the group formed will be more unstable than either of the groups was before their union: the mixed units will be less able to resist those re- distributing forces which cause evolution; and may so have restored to them, the capacity for development which they had lost.
This view harmonizes with the conclusion which we saw reason to draw, that fertilization does not depend on any intrinsic peculiarities of sperm-cells and germ-cells; but depends on their derivation from different individuals. It explains the fact that nearly-related individuals are less likely to have offspring than others; and that their offspring, when they have them, are frequently feeble. And it gives us a key to the converse fact, that the crossing of varieties results in unusual fertility and vigour.
19 290 THE INDUCTIONS OF BIOLOGY.
Bearing- in mind that the slightly- different orders of phy- siological units which an organism inherits from its parents, are subject to the same set of forces; and that when the organism is fully developed, this set of forces, becoming con- stant, tends slowly to re-mould the two orders of units into the same form; we see how it happens that self-fertilization becomes impossible in the higher organisms, while it remains possible in the lower organisms. In long-lived creatures that have tolerably-definite limits of growth, this assimilation of the somewhat- unlike physiological units, is liable to go on to an appreciable extent; whereas in organisms which do not continuously subject their component units to constant forces, there will be much less of this assimilation. And where the assimilation is not considerable, the segregation of mixed units, may cause the sperm-cells and germ-cells developed in the same individual, to be sufficiently different to produce, by their union, fertile germs; and several generations of self- fertilizing descendants may succeed one another, before the two orders of units have had their unlikenesses so far diminish- ed, that they will no longer do this. The same principles explain for us the variable results of union between nearlyrelated organisms. According to the contrasts among the physiological units they inherit from parents and ancestors; according to the unlike proportions of the contrasted units which they severally inherit; and according to the degrees of segregation of such units in different sperm-cells and germ-cells; it may happen that two kindred individuals will produce the ordinary number of offspring, or will produce none; or will at one time be fertile and at another not; or will at one time have offspring of tolerable strength, and at another time feeble offspring.
To the like causes are also ascribable the phenomena of Variation. These are unobtrusive while the tolerably-uni- form conditions of a species maintain tolerable uniformity among the physiological units of its members; but they become obtrusive when differences of conditions, entailing GENESIS, HEREDITY, AND VARIATION. 291 considerable functional differences, have entailed decided dif- ferences among the physiological units; and when the differ- ent physiological units, differently mingled in every individual, come to bo variously segregated and variously combined.
Did space permit, it might be shown that this hypothesis is a key to many further facts — to the fact that mixed races are comparatively plastic under new conditions; to the fact that pure races show predominant influences when crossed with mixed races; to the fact that while mixed breeds are often of larger growth, pure breeds are the more hardy — have functions less-easily thrown out of balance. But with- out further argument, it will, I think, be admitted, that the power of this hypothesis to explain so many phenomena, and to bring under a common bond phenomena that seem so little allied, is strong evidence of its truth. And such evidence gains greatly in strength on observing that this hypothesis brings the facts of Genesis, Heredity, and Variation into har- mony wath first principles. When we see that these plastic physiological units, which we find ourselves obliged to assume, are just such more integrated, more heterogeneous, more un- stable, and more multiform atoms, as would result from con- tinuance of the steps through v/hich organic matter is reached — when we see that the differentiations of them assumed to oc- cur in differently- conditioned aggregates, and the equilibra- tions of them assumed to occur in aggregates which maintain constant conditions, are but corollaries from those universal principles implied by the persistence of force — when we see that the maintenance of life in the successive generations of a species, becomes a consequence of the continual incidence of new forces on the species, to replace the forces that are ever being rhythmically equilibrated in the propagation of the species — and when we thus see that these apparently-excep- tional phenomena displayed in the multiplication of organic beings, fall into their places as results of the general laws of Evolution; we have weighty reasons for entertaining the hypothesis w^hich affords us this interpretation.
CHAPTEE XI.
CLASSIFICATION.
§ 98. That orderly arrangement of objects called Classi- fication, has two purposes; which, though not absolutely dis- tinct, 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 alpha- betical 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 accord- ing to their subjects, he neglects various superficial similari- ties and distinctions, and groups them according to certain primary and secondary and tertiary attributes, which sever- ally 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, Ge- CLASSIFICATION. 293 ology, 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 essen- tial distinctions. Arrangement according to any single con- spicuous 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 ar- rangement according to combinations of attributes, which, though fundamental, are not conspicuous, requires analysis; and does not suggest itself till analysis has made some pro- gress. 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 Scien- tific Works before Historical Works; nor of the sub- divi- sions 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 un- like, 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 294 THE INDUCTIONS OF BIOLOGY.
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 com- plex 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 cha- racters— 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 pro- babilities 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 be- comes 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 corre- spondence between our conceptions and the realities.
CLASSIFICATION. 295 § 99. Biological classifications illustrate well these phases, througli wliicli classifications in general necessarily pass. In early attempts to arrange organic beings' in some sys- tematic 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 abandon- ment of a linear arrangement for an arrangement in di- vergent 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 Na- tural History became a science, had divisions similarly super- ficial and simple. Beasts, Birds, Fishes, and Creeping-things, are names of groups marked off from one another by con- spicuous differences of appearance and^ modes of life — crea- tures 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 depend- ing 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 296 THE INDUCTIONS OF BIOLOGY.
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, La- biates, &c., under other names, but with limits not very dif- ferent 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 respect- ing 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: CLASSIFICATION.
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 circum- ference, always concentric; coty- ledons 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 mani- fested. On studying the definitions of these primary, se- condary, 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 Lin- nseus. 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, Zoo- pliyta.
This arrangement of classes, is obviously based on ap- parent gradations of rank; and the placing of the orders similarly betrays an endeavour to make successions, begin- ning 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 Lin- naeus, 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 con- ception 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 CLASSIFICATION. 200 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.
THE INDUCTIONS OF BIOLOGY.
But though Cuvier emancipated himself from the concep- tion of a serial progression throughout the Animal- King- dom; sundry of his contemporaries and successors remained fettered by the old error. Less regardful of the differently- co-ordinated sets of attributes displayed by the different sub- kingdoms; and swayed by the belief in a progressive develop- ment, 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 articu- lations.
Feel, but obtain from their sensa- tions 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 sym- metrical, the parts being in pairs.
YEETEBEATA.
Feel; acquire preservable ideas; perform with them operations by which they obtain others; are intel- ligent in different degrees. A ver- ► tebral column; a brain and a spinal marrow; distinct senses; the mus- cles attached to the internal skelej ton; form symmetrical, the parts being in pairs.
CLASSIFICATION. 301 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 embryo- logical 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.