. ' • • I f ~ n’ c ‘ . *' v. , • f • : ' : ■ . . • ■ . . ■ • >X'V 8.V*TA7 S\ ecial causes of these phenomena have not yet been determined. Considering that the electric contrasts are most marked where active secretions are going on — considering, too, that while they do not exist between external parts which are similarly related to the vascular currents, they do exist between external parts which are dissimilarly related to the vascular currents — and considering also that they are extremely difficult to detect where there are no appre¬ ciable movements of fluids ; it may be that they are due simply to the friction of heterogeneous substances, which is universally a cause of electric disturbance. But whatever be the interpretation, the fact remains the same, that there is throughout the living organism, an unceasing production of differences between the electric states of different parts ; and consequently an unceasing restoration of electric equilibrium by the establishment of currents among these parts. Besides these general, and not conspicuous, electrical phe¬ nomena which appear to be common to all organisms, vegetal as well as animal, there are certain special and strongly marked ones. I refer, of course, to those which have made the Torpedo and the Gymnotas objects of so much interest. In these creatures we have a genesis of electricity that is not incidental on the performance of their different functions by the different organs ; but one which is itself a function, having an organ appropriate to it. The character of this organ in both these fishes, and its largely- developed con¬ nexions with the nervous centres, have raised the suspicion, which various experiments have thus far justified, that in it there takes place a transformation of what we call nerve- force into the force known as electricity : this conclusion being more especially supported by the fact, that substances, such as morphia and strychnia, which are known to be powerful THE EE- ACTIONS OF ORGANIC MATTER ON FORCES. 49 nervous stimulants, greatly increase the violence and rapidity of the electric discharges. But whether general or special, and in whatever manner produced, these evolutions of electricity are among the re-actions of organic matter, called forth by the actions to which it is subject. Though these re-actions are not direct, but seem rather to be remote consequences of those changes wrought by external agencies on the organism, they are yet incidents in that general re-distribution of motion, which these external agencies initiate ; and as such must here be noticed. § 21. To these known modes of motion, has next to bo added an unknown one. Ileat, Light, and Electricity aro emitted by inorganic matter when undergoing changes, as well as by organic matter. But there is a kind of force mani¬ fested in some classes of living bodies, wdiich we cannot identify with any of the forces manifested by bodies that are not alive, — a force which is thus unknown, in the sense that it cannot be assimilated with an}r otherwise-recognized class. I allude to what is called nerve-force. This is habitually generated in all animals, save the lowest, by incident forces of every kind. The gentle and violent mechanical contacts, which in ourselves produce sensations of touch and pressure — the additions and abstractions of mole¬ cular vibration, which in ourselves produce sensations of heat and cold ; produce in all creatures that have nervous systems, certain nervous disturbances — disturbances which, as in ourselves, are either communicated to the chief nervous centre, and there constitute consciousness, or else result in merely physical processes that are set going elsewhere in the organism. In special parts distinguished as organs of sense, other external actions bring about other nervous re-actions ; that show themselves either as special sensations, or as ex¬ citements which, without the intermediation of consciousness^ 60 THE DATA OF BIOLOGY. beget actions in muscles, or other organs. Besides neural discharges that follow the direct incidence of external forces, there are others ever being caused by the incidence of forces which, though originally external, have become internal by absorption into the organism of the agents exerting them. For thus may be classed those neural discharges that from moment to moment result from modifications of the tissues, wrought by substances carried to them in the blood. That the unceasing change of matter which oxygen and other agents produce throughout the system, is accompanied by a genesis of nerve- force, is shown by various facts ; — by the fact that nerve-force is no longer generated, if oxygen be with¬ held, or the blood prevented from circulating ; by the fact that when the chemical transformation is diminished, as during sleep with its slow respiration and circulation, there is a diminution in the quantity of nerve-force ; in the fact that an excessive expenditure of nerve- force, involves excessive re¬ spiration and circulation, and excessive waste of tissue. To these proofs that nerve-force is evolved in greater or less quan¬ tity, according as the conditions to rapid molecular change throughout the body, are well or ill fulfilled ; may be added proofs that certain special molecular actions, are the causes of these special re-actions. The effects of alcohol, ether, chloroform, and the vege to -alkalies, put beyond doubt the inference, that the overthrow of molecular equilibrium by chemical affinity, when it occurs at certain places in the body, results in the overthrow of equilibrium in the nerves pro¬ ceeding from these places — results, that is, in the propagation through these nerves, of the change called a nervous dis¬ charge. Indeed, looked at from this point of view, the two classes of nervous changes — the one initiated from without and the other from within — are seen to merge into one class. Both of them may be traced to metamorphosis of tissue. There can be little doubt that the sensations of touch and pressure, are consequent on accelerated changes of matter, produced by mechanical disturbance of the mingled THE HE- ACTIONS OF ORGANIC MATTER ON FORCES. 51 fluids and solids composing tlie parts affected. There is abundant evidence that the sensation of taste, is due to the chemical actions set up by particles which find their way through the membrane covering the nerves of taste ; for, as Prof. Graham points out, sapid substances all belong to the class of crystalloids, which are able rapidly to permeate animal tissue, while colloids, which cannot pass through animal tissue, are all insipid. Similarly with the sense of smell. Substances which excite this sense, are necessarily more or less volatile : and their volatility being the result of their molecular mobility, implies that they have in a high degree, the power of getting at the olfactory nerves by pene¬ trating their mucous investment. Again, the facts which photography has familiarized us with, make it clear that those nervous impressions called colours, are primarily due to certain changes wrought by light in the substance of the retina. And though, in the case of hearing, we cannot so clearly trace the connexion of cause and effect ; yet as we see that the auditory apparatus is one fitted to intensify those vibrations constituting sound, and to convey them to a recep¬ tacle containing fluid in which nerves are immersed ; it can scarcely be doubted that the sensation of sound proximately results from atomic re-arrangements caused in these nerves by the vibrations of the fluid : knowing, as we do, that the re-arrangement of atoms is in all cases aided by agita¬ tion. Perhaps, however, the best proof that nerve- force, whether peripheral or central in its origin, results from chemical transformation, lies in the fact that most of the chemical agents which powerfully affect the nervous system, affect it whether applied at the centre or the periphery. Vari¬ ous acids, mineral and vegetal, are tonics — the stronger ones being usually the stronger tonics ; and this which we call their acidity, implies a power in them of acting on the nerves of taste, while the tingling or pain that follows their absorp¬ tion through the skin, implies that the nerves of touch are acted on by them. Similarly with certain vegeto-alkalies 52 THE DATA OF BIOLOGY. which are peculiarly bitter. These by their bitterness, show that they affect the extremities of the nerves ; while by their tonic properties, they show that they affect the nervous centres — the most intensely bitter among them, strychnia, being the most powerful nervous stimulant. However true it may be that this relation is not a regular one, since opium, hashish, and some other drugs, which work marked effects on the brain, are not remarkably sapid — however true it may be that there are relations between particular substances and particular parts of the nervous system ; yet such instances do but qualify, without negativing, the general proposition. The truth of this proposition can scarcely be doubted when, to the evidence above given, is added the fact that various condiments and aromatic] drugs are given as nervous stimu¬ lants ; and the fact that anaesthetics, besides the general effects they produce when inhaled or swallowed, produce local effects of like kind when absorbed through the skin ; and the fact that ammonia, which in consequence of its extreme molecular mobility, so quickly and so violently excites the nerves be¬ neath the skin, as well as those of the tongue and the nose, is a rapidly-acting stimulant when taken internally. Whether vre shall ever know anything more of this nerve- force, than that it is some species of molecular disturbance that is propagated from end to end of a nerve, it is impossi¬ ble to say. Whether a nerve is merely a conductor, which delivers at one of its extremities an impulse received at the other ; or whether, as some now think, it is itself a generator of force wdiich is initiated at one extremitv and accumulates in its course to the other extremity ; are also questions which cannot yet be answered. All we know is, that forces capable of working molecular changes in nerves, are capable of calling forth from them manifestations of activity — dis¬ charges of some force, which, though probably allied to elec¬ tricity, is not identical with it. And our evidence that nerve- force is thus originated, consists not only of such facts as the above, but also of more conclusive facts established by direct TIIE RE-ACTIONS OF ORGANIC MATTER ON FORCES. 53 experiments on nerves — experiments which show that nerve- force is generated when the cut end of a nerve is either me¬ chanically irritated, or acted on by some chemical agent, or subject to the galvanic current — experiments which thus prove that nerve-force is liberated by whatever disturbs the molecular equilibrium of nerve-substance. And this is all which it is necessary for us here to understand. § 22. The most important of these re-actions called forth from organisms by surrounding actions, remains to be noticed. To the above various forms of insensible motion thus caused, we have to add sensible motion. On the production of this mode of force, more especially depends the possibility of all vital phenomena. It is, indeed, usual to regard the power of generating sensible motion, as confined to one out of the two organic sub-kingdoms ; or, at any rate, as possessed by but few members of the other. On looking closer into the matter, however, we see that plant-life as well as animal-life, is uni¬ versally accompanied by certain manifestations of this power ; and that plant-life could not otherwise continue. Through the humblest, as well as through the highest, ve¬ getal organisms, there are ever going on certain re-distribu¬ tions of matter. In protopliytcs the microscope shows us an internal transposition of parts, which when not active enough to be immediately visible, is proved to exist by the changes of arrangement that become manifest in the course of hours and days. In the individual cells of many higher plants, an active movement among the contained granules may be wit¬ nessed. And well-developed cryptogams in common with all phanerogams, exhibit this genesis of mechanical motion still more conspicuously in the circulation of sap. It might, in¬ deed, be concluded d priori, that through plants displaying much differentiation of parts, an internal movement must be going on ; since, without it, the mutual dependence of organs having unlike functions would seem impossible. Be¬ sides these motions of fluids kept up internally, plants, espe- 64 THE DATA OF BIOLOGY. cially of tlie lower orders, are able to move their external parts in relation to each other, and also to move about from place to place. Illustrations in abundance will occur to all students of recent Natural History — such illustrations as the active locomotion of the zoospores of many Algcc, the rhyth¬ mical bendings of the Oscillatorice, the rambling progression of the Diatomacece. In fact many of these smallest vegetals, and many of the larger ones in their early stages, display a mechanical activity not distinguishable from that of the simplest animals. Among well- organized plants, which are never locomotive in their adult states, we still not unfre- quently meet with relative motions of parts. To such fami¬ liar cases as those of the Sensitive plant and the Yenus, fly-trap, many others may be added. Yvrhen its base is irritated, the stamen of the Berberry flower leans over and touches the pistil. If the stamens of the common wild Cistus be gently brushed with the finger, they spread themselves— bending away from the seed-vessel. And some of the orchid- flowers, as Mr Darwin has recently shown, shoot out masses of pollen on to the entering bee, wdien its trunk is thrust down in search of honey. Though the power of moving is not, as wre see, a character¬ istic of animals alone, yet in them, considered as a class, it is manifested to an extent so marked, as practically to become one of their distinctive characters — indeed, we may say, their most distinctive character. For it is by their immensely greater ability to generate mechanical motion, that animals are enabled to perform those actions which constitute their visible lives ; and it is by their immensely greater ability^ to generate mechanical motion, that the higher orders of animals are most obviously distinguished from the lower orders. Though, on remembering the seemingly active movements of infusoria, some will perhaps question this last-named con¬ trast ; yet, on comparing the quantities of matter propelled through given spaces in given times, they will see that the momentum evolved is far less in the protozoa than in the THE RE-ACTIONS OF ORGANIC MATTER ON FORCES. 55 leleozoa. These sensible motions of animals are effected by various organs under various stimuli. In the humblest forms, and even in some of the more developed ones which inhabit the water, locomotion results from the vibrations of cilia : the contractility resides in these waving hairs that grow from the surface. Some of the Acalephce> and their allies the Polypes, move when mechanically irritated : the long pendant tentacle of a Physalia is suddenly drawn up if touched ; and, as well as its tentacles, the whole body of a Ilyclra collapses if roughly handled, or jarred by some shock in its neighbourhood. In all the higher animals how¬ ever, and to a smaller degree in many of the lower, sensible motion is generated by a special tissue, under the special ex¬ citement of a neural discharge. Though it is not strictly true that such animals show no sensible motions otherwise caused ; since all of them have certain ciliated membranes, and since the circulation of fluid in them is partially due to osmotic and capillary actions ; yet, generally speaking, we may say that their movements are effected only by muscles that contract only through the agency of nerves. What special transformations of force generate these various mechanical changes, we do not, in most cases, know. Those re-distributions of fluid, with the alterations of form sometimes caused by them, that result from osmose, are not, indeed, quite incomprehensible. Certain motions of plants which, like those of the “ animated oat,” follow contact with water, are easily interpreted ; as are also such other vegetal motions as those of the Touch-me-not, the Squirting Cucumber, and the Caipobolus. But we have as yet no clue to the mode in which molecular movement is transformed into the movement of masses, in animals. We cannot refer to known causes the rhythmical action of a Medusa’s disc, or that slow decrease of bulk that spreads throughout the mass of an Alcyonium , when one of its component individuals has been irritated. ISTor are we any better able to say how the insensible motion transmitted through a nerve, elves rise to sensible motion in 56 TIIE DATA OF BIOLOGY. a muscle. It is true that Science has given to Art, several methods of changing insensible into sensible motion. By ap¬ plying heat to water we vaporize it ; and the movement of its expanding vapour, we transfer to solid matter ; but it is clear that the genesis of muscular movement is in no way analogous to this. The force evolved during chemical transformations in a galvanic battery, wrn communicate to a soft iron magnet through a wire coiled round it ; and it wrould be quite possi¬ ble, by placing near to each other several magnets thus excited, to obtain, through the attraction of each for its neighbours, an accumulated movement made up of their separate movements, and thus to mechanically imitate a mus¬ cular contraction ; but from what Tve know of organic mat¬ ter, and the structure of muscle, there is no reason to suppose that anything analogous to this takes place in it. We can, however, through one kind of molecular change, produce sensible changes of aggregation such as possibty might, when occurring in organic substance, cause sensible motion in it : I refer to allotropic change. Sulphur, for example, as¬ sumes different crystalline and non-crystalline forms at dif¬ ferent temperatures ; and may be made to pass backwards and forwards from one form to another, by slight variations of temperature : undergoing each time an alteration of bulk. We know that this allotropism, or rather its analogue iso¬ merism, prevails among colloids — inorganic and organic. We also know that some of these metamorphoses among col¬ loids, are accompanied by visible re-arrangements : instance hydrated silicic acid, which, after passing from its soluble state to the state of an insoluble jelly, begins, in a few days, to contract, and to give out part of its contained water. Now, considering that such isomeric changes of organic as w~ell as inorganic colloids, are often very rapidly produced by very slight causes, it seems not impossible that some of the colloids constituting muscle, may be thus changed by a nervous dis¬ charge — resuming their previous condition when the dis¬ charge ceases. And it is conceivable that by structural THE RE-ACTIONS OF ORGANIC MATTER ON FORCES. 57 arrangements, minute sensible motions so caused, may be ac¬ cumulated into large sensible motions. There is, however, no evidence to support this supposition. § 23. But the truths which it is here our business espe¬ cially to note, are quite independent of hypotheses or inter¬ pretations. It is sufficient for the ends we have in view, to observe that organic matter does exhibit these several conspi¬ cuous re- actions, when acted on by incident forces : it is not requisite that we should know how these re-actions originate. In the last chapter were set forth the several modes in which incident forces cause re-distributions of organic mat¬ ter ; and in this chapter have been set forth the several modes in which is manifested the motion accompanying this re-dis¬ tribution. There w~e contemplated under its several aspects, the general fact, that in consequence of its extreme instability, organic matter undergoes extensive molecular re-arrange¬ ments, on very slight changes of conditions. And here we have contemplated under its several aspects, the correlative general fact, that during these extensive molecular re-arrange¬ ments, there are necessarily evolved large amounts of force. In the one case the atoms of which organic matter consists, are regarded as changing from positions of unstable equili¬ brium to positions of stable equilibrium ; and in the other case they are regarded as giving out in their falls from unstable to stable equilibrium, certain momenta — momenta that may be manifested as heat, light, electricity, nerve- force or mechanical motion, according as the conditions determine. I will add only that these evolutions of force are rigor¬ ously dependent on these changes of matter. It is a corol¬ lary from that primordial truth which, as we have seen, tmderlies all other truths, ( First Principles, §§ 76, 141,) that whatever amount of power an organism expends in any shape, is the correlate and equivalent of a power that was taken into it from without. On the one hand, it 63 TIIE DATA OF BIOLOGY. follows from the persistence of force, that each portion of mechanical or other energy which an organism exerts, im¬ plies the transformation of as much organic matter as con¬ tained this energy in a latent state. And on the other hand, it follows from the persistence of force that no such trans¬ formation of organic matter containing this latent energy can take place, without the energy being in one shape or other manifested. CHAPTER IV * PROXIMATE DEFINITION OF LIFE. § 24. To those who accept the general doctrine of Evolu¬ tion, it needs scarcely he pointed out that classifications are subjective conceptions, which have no absolute demarcations in Nature corresponding to them. They are appliances by which we limit and arrange the matters under investigation ; and so facilitate our thinking. Consequently, when we at¬ tempt to define anything complex, or make a generalization of facts other than the most simple, we can scarcely ever avoid including more than we intended, or leaving out some¬ thing that should be taken in. Thus it happens that on seeking a definition of Life, we have great difficulty in find¬ ing one that is neither more nor less than sufficient. Let ns look at a few of the most tenable definitions that have been given. While recognizing the respects in which they are defective, we shall see what requirements a more com¬ plete one must fulfil. * This chapter and the following two chapters originally appeared in Part III. of the Principles of Psychology : forming a preliminary which, though indis¬ pensable to the argument there developed, was somewhat parenthetical. Having now to deal with the general science of Biology before the more special one of Psychology, it becomes possible to transfer these chapters to their proper place. They have been carefully revised. 60 THE DATA OF BIOLOGY. Selielling said that Life is the tendency to individuation. This formula, until studied, conveys little meaning. But it needs only to consider it as illustrated by the facts of develop¬ ment, or by the contrasts between lower and higher forms of life, to recognize its value ; especially in respect of compre¬ hensiveness. As before shown, however, ( First Principles , § 56), it is objectionable, partly on the ground that it refers, not so much to the functional changes constituting Life, as to the structural changes of those aggregations of matter which manifest Life ; and partly on the ground that it includes under the idea Life, much that we usually exclude from it : for instance — crystallization. The definition of Bicherand, — “ Life is a collection of phenomena which succeed each other during a limited time in an organized body,” — is liable to the fatal criticism, that it equally applies to the decay which goes on after death. For this, too, is “ a collection of phenomena which succeed each other during a limited time in an organized body.” “ Life,” according to De Blainville, “ is the two-fold internal movement of composition and decomposition, at once general and continuous.” This conception is in some re¬ spects too narrow, and in other respects too wide. On the one hand, while it expresses what physiologists distinguish as vegetative life, it excludes those nervous and muscular functions which form the most conspicuous and distinctive classes of vital phenomena. On the other hand, it describes not only the integrating and disintegrating processes going on in a living body, but it equally w^ell describes those going on in a galvanic battery ; which also exhibits a “ two-fold in¬ ternal movement of composition and decomposition, at once general and continuous.” Elsewhere, I have myself proposed to define Life as “ the co-ordination of actions ; ”* and I still incline towards this de¬ finition as one answering to the facts with tolerable precision. * See Westminster Review for April, 1S52. — Art. IV, “A Theory of Popu¬ lation.” PROXIMATE DEFINITION OF LIFE. 61 It includes all organic changes, alike of the viscera, the limbs, and the brain. It excludes the great mass of inor¬ ganic changes ; which display little or no co-ordination. By making co-ordination the specific characteristic of vitality, it involves the truths, that an arrest of co-ordination is death, and that imperfect co-ordination is disease. More¬ over, it harmonizes with our ordinary ideas of life in its dif¬ ferent gradations : seeing that the organisms which we rank as low in their degree of life, are those which display but little co-ordination of actions ; and seeing that from these up to man, the recognized increase in degree of life corresponds with an increase in the extent and complexity of co-ordina¬ tion. But, like the others, this definition includes too much ; for it may be said of the Solar System, with its regularly- recurring movements and its self-balancing perturbations, that it, also, exhibits co-ordination of actions. And how¬ ever plausibly it may be argued that, in the abstract, the motions of the planets and satellites are as properly compre¬ hended in the idea of life, as the changes going on in a motionless, unsensitive seed ; yet, it must be admitted that they are foreign to that idea as commonly received, and as here to be formulated. It remains to add the definition since suggested by Mr Gf. II. Lewes — “ Life is a series of definite and successive changes, both of structure and composition, which take place within an individual without destroying its identity.” The last fact which this statement has the merit of bringing into view — the persistence of a living organism as a whole, in spite of the continuous removal and replacement of its parts — is important. But otherwise it may be argued, that since changes of structure and composition, though probably the causes of muscular and nervous actions, are not the muscular and nervous actions themselves, the definition excludes the more visible movements with which our idea of life is most associated ; and further, that in describing vital changes as a series , it scarcely includes the fact that manv of them, as THE DATA OF BIOLOGY. 62 Nutrition, Circulation, Respiration, and Secretion, in tlieir many subdivisions, go on simultaneously. Thus, however well each of these definitions expresses the phenomena of life under some of its aspects, no one of them is more than approximately true. It may turn out, that to find a formula which will bear every test is impossible. Meanwhile, it is possible to frame a more adequate formula than any of the foregoing. As we shall presently find, these all omit an essential peculiarity of vital changes in general — a peculiarity which, perhaps more than any other, distinguishes them from non-vital changes. Before specify¬ ing this peculiarity, however, it will be well to trace our way, step by step, to as complete an idea of Life as may be reached from our present stand-point : by doing which, we shall both see the necessity for each limitation as it is made, and ulti¬ mately be led to feel the need for a further limitation And here, as the best mode of determining vrhat are those general characteristics which distinguish vitality from non¬ vitality, -we shall do well to compare the two most unlike kinds of vitality, and see in what they agree. Manifestly, that which is essential to Life must be that which is common to Life of all orders. And manifestly, that which is common to all forms of Life, will most readily be seen on contrasting those forms of Life which have the least in common, or are the most unlike.* § 25. Choosing assimilation, then, for our example of bodily life, and reasoning for our example of that life known as intelligence ; it is first to be observed, that they are both processes of change. Without change, food cannot be taken into the blood nor transformed into tissue : without * This paragraph replaces a sentence that, in The Principles of Psychology, referred to a preceding chapter on “Method;” in which the mode of procedure here indicated, was set forth as a mode to be systematically pursued in the choice of hypotheses. Should opportunity ever permit, this chapter on Method will be embodied, along with other matter on the same topic, in a General Introduction to First Principles. mOXOxATE DEFINITION OF LIFE. 63 change, there can be no getting from premisses to conclusion. And it is this conspicuous manifestation of change, which forms the substratum of our idea of Life in general. Doubt¬ less we see innumerable changes to which no notion of vital¬ ity attaches : inorganic bodies are ever undergoing changes of temperature, changes of colour, changes of aggregation. But it will be admitted that the great majority of the phe¬ nomena displayed by inorganic bodies, are statical and not dynamical ; that the modifications of inorganic bodies are mostly slow and unobtrusive ; that on the one hand, when we see sudden movements in inorganic bodies, we are apt to assume living agency, and on the other hand, when we see no movements in organic bodies, we are apt to assume death. From all which considerations it is manifest, that be the requisite qualifications what they may, a definition of Life must be a definition of some kind of change or changes. On further comparing assimilation and reasoning, with a view of seeing in what respect the change displayed in both differs from non- vital change, we find that it differs in being not simple change, but change made up of successive changes. The transformation of food into tissue, involves mastication, deglutition, chymification, chylification, absorption, and those various actions gone through after the lacteal ducts have poured their contents into the blood. Carrying on an argu¬ ment necessitates a long chain of states of consciousness ; each implying a change of the preceding state. Inorganic changes, however, do not in any considerable degree exhibit this peculiarity. It is true that from meteorologic causes, inanimate objects are daily, sometimes hourly, undergoing modifications of temperature, of bulk, of hygrometric and electric condition. Not only, however, do these modifications lack that conspicuousness and that rapidity of succession which vital ones possess, but vital ones form an additional series. Living as well as not-living bodies are affected by atmospheric influences ; and beyond the changes which these produce, living bodies exhibit other changes, more nu- (54 THE DATA OF BIOLOGY. merous and more marked. So that though organic change O O o is not rigorously distinguislied from inorganic change by presenting successive phases — though some inanimate objects, as watches, display phases of change both quick and nu¬ merous — though all objects are ever undergoing change of some kind, visible or invisible — though there is scarcely any object which does not, in the lapse of time, undergo a con¬ siderable amount of change that is fairly divisible into phases; yet, ATital change so greatly exceeds other change in its dis¬ play of varying phases, that we may consider this as prac¬ tically one of its characteristics. Life, then, as thus roughly differentiated, may be regarded as change presenting succes¬ sive phases ; or otherwise, as a series of changes. And it should be observed, as a fact in harmony with' this concep¬ tion, that the higher the life the more conspicuous the varia¬ tions. On comparing inferior with superior organisms, these last will be seen to display more rapid changes, or a more lengthened series of them, or both. Contemplating afresh our two typical phenomena, we may see that vital change is further distinguished from non- vital change, by being made up of many simultaneous changes. Assimilation is not simply a series of actions, but includes many actions going on together. During mastication the stomach is busy with the food already swallowed ; on which it is both pouring out solvent fluids and expending muscular efforts. While the stomach is still active, the intestines are performing their secretive, contractile, and absorbent func¬ tions ; and at the same time that one meal is being digested, the nutriment obtained from a previous meal is undergoing that transformation into tissue which constitutes the final act of assimilation. So also is it, in a certain sense, with mental changes. Though the states of consciousness which make up an argument occur in series, yet, as each of these states is complex — implies the simultaneous excitement of those many faculties by which the perception of any object or relation lias been effected; it is obvious that each such change in 65 PROXIMATE DEFINITION OF LIFE. consciousness implies many component changes. In this respect too, however, it must be admitted that the distinction between animate and inanimate is not precise. No mass of dead matter can have its temperature altered, without at the same time undergoing an alteration in bulk, and sometimes also in hygrometric state. An inorganic body cannot be oxidized, without being at the same time changed in weight, colour, atomic arrangement, temperature, and electric condition. And in some vast and mobile aggre- gates like the sea, the simultaneous as well as the successive changes displayed, outnumber those going on in an animal. Nevertheless, speaking generally, a living thing is distin¬ guished from a dead thing, by the multiplicity of the changes at any moment taking place in it. Add to which, that by this peculiarity, as by the previous one, not only is the vital more or less clearly marked off from the non-vital ; but creatures possessing high vitality are marked off from those possessing low vitality. It needs but to contrast the many organs co-operating in a mammal, with the few in a polype, to see that the actions which are progressing together in the body of the first, as much exceed in number the actions pro¬ gressing together in the body of the last, as these do those in a stone. As at present analyzed, then, Life consists of simultaneous and successive changes. Continuing the comparison, we next find that vital changes, both visceral and cerebral, differ from other changes in their heterogeneity. Neither the simultaneous acts nor the serial acts, which together constitute the process of digestion, are at all alike. The states of consciousness comprised in any ratiocination are not repetitions of each other, either in com¬ position or in modes of dependence. Inorganic processes, on the other hand, even when like organic ones in the number of the simultaneous and successive changes they involve, are unlike them in the homogeneity of these changes. In the case of the sea, just referred to, it is observable that count¬ less as are the actions at any moment going on, they are 4 66 THE DATA OF BIOLOGY. mostly mechanical actions that are to a great degree similar; and in this respect widely differ from the actions at any mo¬ ment taking place ia an organism : which not only belong to the several classes, mechanical, chemical, thermal, electric, but present under each of these classes, innumerable unlike actions. Even where life is nearly simulated, as by the working of a steam-engine, we may see that considerable as is the number of simultaneous changes, and rapid as are the successive ones, the regularity with which they soon recur in the same order and degree, renders them unlike those varied changes exhi¬ bited by a living creature. Still, it will be found that this peculiarity, like the foregoing ones, does not divide the two classes of changes with precision ; inasmuch as there are inanimate things which exhibit considerable heterogeneity of change : for instance, a cloud. The variations of state which this undergoes, both simultaneous and successive, are many and quick ; and they differ widely from each other both in quality and quantity. At the same instant there may occur in a cloud, change of position, change of form, change of size, change of density, change of colour, change of tem¬ perature, change of electric state ; and these several kinds of change are continuously displayed in different degrees and combinations. Yet notwithstanding this, when we consider that very few inorganic objects manifest heterogeneity of change in a marked manner, while all organic objects mani¬ fest it ; and further, that in ascending from low to high forms of life, we meet with an increasing variety in the kinds and amounts of changes displayed ; we see that there is here a further leading distinction between organic and inorganic actions. According to this modified conception, then, Life is made up of heterogeneous changes both simultaneous and successive. If now we look for some point of agreement between the assimilative and logical processes, by which they are distin¬ guished from those inorganic processes that are most like them in the heterogeneity of the simultaneous and successive PROXIMATE DEFINITION OF LIFE. C7 changes they comprise, we discover that they are distinguish¬ ed by the combination subsisting among their constituent changes. The acts that make up digestion are mutually de¬ pendent. Those composing a train of reasoning are in close connection. And generally, it is to be remarked of vital changes, that each is made possible by all, and all are affected by each. Respiration, circulation, absorption, secretion, in their many sub-divisions, are bound up together. Muscular contraction involves chemical change, change of temperature, and change in the excretions. Active thought influences the operations of the stomach, of the heart, of the kidneys. But we miss this union among inorganic processes. Life-like as may seem the action of a volcano in respect of the heterogeneity of its many simultaneous and successive changes, it is not life¬ like in respect of their combination. Though the chemical, mechanical, thermal, and electric phenomena exhibited, have some inter-dependence ; yet the emission of stones, mud, lava, flame, ashes, smoke, steam, usually takes place irregularly in quantity, order, intervals, and mode of conjunction. Even here, however, it cannot be said that inanimate things pre sent no parallels to animate ones. A glacier may be instanced as showing nearly as much combination in its changes as a plant of the lowest organization. It is ever growing and ever decaying ; and the rates of its composition and decom¬ position preserve a tolerably constant ratio. It moves ; and its motion is in immediate dependence on its thawing. L emits a torrent of water, which, in common with its motion, undergoes annual variations, as plants do. Luring part of the year the surface melts and freezes alternately ; and on these changes are dependent the variations in movement, and in efflux of water. Thus we have growth, decay, changes of temperature, changes of consistence, changes of velocity changes of excretion, all going on in connexion ; and it may be as truly said of a glacier as of an animal, that by cease¬ less integration and disintegration it gradually undergoes an entire change of substance without losing its individuality. 68 THE DATA OF BIOLOGY. This exceptional instance, however, will scarcely be held to obscure that broad distinction from inorganic processes, which organic processes derive from the combination among their constituent changes. And the reality of this distinction becomes yet more manifest when we find that, in common with previous ones, it not only marks off the living from the not-living, but also things which live little from things which live much. For while the changes going on in a plant or a zoophyte are so imperfectly combined that they can continue after it has been divided into two or more pieces, the com¬ bination among the changes going on in a mammal is so close that no part cut off from the rest can live, and any con¬ siderable disturbance of one function causes a cessation of the others. Life, therefore, as we now regard it, is a com¬ bination of heterogeneous changes, both simultaneous and successive. Once more looking for a characteristic common to these two kinds of vital action, we perceive that the combinations of heterogeneous changes which constitute them, differ from the few combinations which they otherwise resemble, in re¬ spect of definiteness. The associated changes going on in a glacier, admit of indefinite variation. Under a conceivable alteration of climate, its thawing and its progression may be stopped for myriads of years, without disabling it from again displaying these phenomena under appropriate conditions. By a geological convulsion, its motion may be arrested with¬ out an arrest of its thawing ; or by an increase in the in¬ clination of the surface it slides over, its motion may be accelerated without accelerating its rate of dissolution. Other things remaining the same, a more rapid deposit ot enow may cause an indefinite increase of bulk ; or, conversely the accretion may entirely cease, and yet all the other actions continue until the mass disappears. Here, then, the combina¬ tion has none of that definiteness which, in a plant, marks the mutual dependence of assimilation, respiration, and cir¬ culation. ; much less has it that definiteness seen in the PROXIMATE DEFINITION OF LIFE. 09 mutual dependence of tlie chief animal functions : no one of which can be varied without varying the rest: no one of which can go on unless the rest go on. It is this definiteness of combination which distinguishes the changes occurring in a living body from those occurring in a dead one. Decom¬ position exhibits both simultaneous and successive changes, which are to some extent heterogeneous, and in a sense com¬ bined ; but they are not combined in a definite manner. They vary according as the surrounding medium is air, water, or earth. They alter in nature with the temperature. If the local conditions are unlike, they progress differently in different parts of the mass, without mutual influence. They may end in producing gases, or adipocire, or the dry substance of which mummies consist. They may occupy a few days, or thousands of years. Thus, neither in their simultaneous nor in their suc¬ cessive changes, do dead bodies display that definiteness of combination which characterizes living ones. It is true that in some inferior creatures the cycle of successive changes admits of a certain indefiniteness — that it may be apparently suspended for a long period by desiccation or freezing ; and may afterwards go on as though there had been no breach in its continuity. But the circumstance that only a low order of life permits the cycle of its changes to be thus modified, serves but to suggest that, like the pre¬ vious characteristics, this characteristic of definiteness in its combined changes, distinguishes high vitality from low vital¬ ity, as it distinguishes low vitality from inorganic processes. Hence, our formula as further amended reads thus : — Life is a definite combination of heterogeneous changes, both simul¬ taneous and successive. Finally, we shall still better express the facts, if, instead ol saying a definite combination of heterogeneous changes, we say the definite combination of heterogeneous changes. As it at present stands, the definition is defective both in allow¬ ing that there may be other definite combinations of hetero¬ geneous changes, and in directing attention to the hetero- 70 THE DATA OF BIOLOGY. geneous changes rather than to the definiteness of their combination. Just as it is not so much its chemical elements which constitute an organism, as it is the arrangement of them into special tissues and organs ; so it is not so much its heterogeneous changes which constitute Life, as it is the de¬ finite combination of them. Observe what it is that ceases when life ceases. In a dead body there are going on hetero¬ geneous changes, both simultaneous and successive. What then has disappeared ? The definite combination has dis¬ appeared. Mark, too, that however heterogeneous the simul¬ taneous and successive changes exhibited by an inorganic object, as a volcano, we much less tend to think of it as living, than we do a watch or a steam-engine, which, though displaying homogeneous changes, displays them definitely combined. So dominant an element is this in our idea of Life, that even when an object is motionless, yet, if its parts be definitely combined, we conclude either that it has had life, or has been made by something having life. Thus then, we conclude that Life is — the definite combination of hetero¬ geneous changes, both simultaneous and successive. § 26. Such is the conception at which we arrive without changing our stand-point. It is, however, an incomplete conception. This ultimate formula (which is to a consider¬ able extent identical with one above given — “ the co-ordina¬ tion of actions ; ” seeing that “ definite combination ” is synonymous with “ co-ordination,” and “ changes both si¬ multaneous and successive ” are comprehended under the term “ actions ; ” but which differs from it in specifying the fact, that the actions or changes are “ heterogeneous ”) — this ultimate formula, I say, is after all but proximately correct. It is true that it does not fail by including the growth of a crystal ; for the successive changes this implies cannot be called heterogeneous. It is true that the action of a galvanic battery is not comprised in it ; since here, too, heterogeneity is not exhibited by the successive changes. It is true that by PROXIMATE DEFINITION OF LIFE. 71 tliis same qualification the motions of the Solar System are excluded ; as are also those of a watch and a steam-engine. It is true, moreover, that while, in virtue of their heteroge¬ neity, the actions going on in a cloud, in a volcano, in a glacier, fulfil the definition ; they fall short of it in lacking definiteness of combination. It is further true that this de¬ finiteness of combination, distinguishes the changes taking place in an organism during life, from those which commence at death. And beyond all this it is true that, as well as serving to mark off, more or less clearly, organic actions from inorganic actions, each member of the definition serves to mark off the actions constituting high vitality from those constituting low vitality; seeing that life is high in propor¬ tion to the number of successive changes occurring between birth and death ; in proportion to the number of simultaneous changes ; in proportion to the heterogeneity of the changes ; in proportion to the combination subsisting among the changes ; and in proportion to the definiteness of their com¬ bination. Nevertheless, answering though it does to so many requirements, this definition is essentially defective. It does not convey a complete idea of the thing contem¬ plated. The definite combination of heterogeneous changes , both simultaneous and successive, is a formula which fails to call up an adequate conception. And it fails from omitting the most distinctive peculiarity — the peculiarity of which wre have the most familiar experience, and with w’hich our notion of Life is, more than with any other, associated. It remains now to supplement the definition by the addition of this peculiarity. CHAPTER Y. TIIE CORRESPONDENCE BETWEEN LIFE AND ITS CIRCUMSTANCES. § 27. We habitually distinguish between a live object and a dead one, by observing whether a change which we make in the surrounding conditions, or one which Nature makes in them, is or is not followed by some perceptible change in the object. Ey discovering that certain things shrink when touched, or fly away when approached, or start when a noise is made, the child first roughly discriminates between the living and the not-living ; and the man when in doubt whether an animal he is looking at is dead or not, stirs it with his stick ; or if it be at a distance, shouts, or throws a stone at it. Vegetal and animal life are alike primarily recognized by this process. The tree that puts out leaves when the spring brings a change of temperature, the flower which opens and closes with the rising and 'setting of the sun, the plant that droops when the soil is dry, and re-erects itself when watered, are considered alive because of these in¬ duced changes ; in common with the zoophyte which contracts on the passing of a cloud over the sun, the worm that comes to the surface when the ground is continuously shaken, and the hedgehog that rolls itself up when attacked. Not only, however, do we habitually look for some response when an external stimulus is applied to a living organism, but we perceive a fitness in the response. Dead as well as living things display changes under certain changes of con- CORRESPONDENCE BETWEEN LIFE AND ITS CIRCUMSTANCES. To dilion : instance, a lump of carbonate of soda that effervesces when dropped into sulphuric acid ; a cord that contracts when wetted ; a piece of bread that turns brown when held near the fire. But in these cases, we do not see a connexion between the changes undergone, and the preservation of the things that undergo them ; or, to avoid any teleological im¬ plication — the changes have no apparent relations to future external events which are sure or likely to take place. In vital changes, however, such relations are manifest. Light being necessary to vegetal life, we see in the action of a plant which, when much shaded, grows towards the unshaded side, an appropriateness which we should not see did it grow otherwise. Evidently the proceedings of a spider, which rushes out when its web is gently shaken and stays within when the shaking is violent, conduce better to the obtainment of food and the avoidance of danger than were they reversed. The fact that we feel surprise when, as in the case of a bird fas¬ cinated by a snake, the conduct tends towards self-destruction, at once shows how generally we have observed an adaptation of living changes to changes in surrounding circumstances. INfote further the kindred truth, rendered so familiar by infinite repetition that we forget its significance, that there is invariably, and necessarily, a conformity between the vital functions of any organism, and the conditions in which it is placed — between the processes going on inside of it, and the processes going on outside of it. We know that a fish can¬ not live in air, or a man in water. An oak growing in the ocean, and a seaweed on the top of a hill, are incredible combinations of ideas. We find that every animal is limited to a certain range of climate ; every plant to certain zones of latitude and elevation. Of the marine flora and fauna, each species is found exclusively between such and such depths. Some blind creatures flourish only in dark caves ; the limpet only where it is alternately covered and uncovered by the tide ; the red-snow alga rarely elsewhere than in the arctic regions or among alpine peaks. 74 THE DATA OF BIOLOGY. Grouping together the cases first named, in which a parti¬ cular change in the circumstances of an organism is followed by a particular change in it, and the cases last named, in which the constant actions occurring within an organism im¬ ply some constant actions occurring without it ; we see that in both, the changes or processes displayed by a living body are specially related to the changes or processes in its en¬ vironment. And here we have the needful supplement to our conception of Life. Adding this all-important charac¬ teristic, our conception of Life becomes — The definite com¬ bination of heterogeneous changes, both simultaneous and / successive, in correspondence with external co-existences and sequences . That the full significance of this addition may be seen, it will be necessary to glance at the correspondence under some of its leading aspects.* § 28. Neglecting minor requirements, tne actions going * Speaking of “the general idea of life,” M. Comte says : — “ Cette idee sup¬ pose, en effet, non-seulement celle d’un etre organise de maniere a comporter l’ctat vital, mais aussi celle, non moins indispensable, d’un certain ensemble d’influences exterieure propres a son accomplissement. line telle harmonic entre l’etre vivant et le milieu correspondent, caracterise evidemment la condition fon- damentale de la vie.” Commenting on de Blainville’s definition of life, which he adopts, he says: — “ Cette lumineuse definition ne me parait laisser rien d’ impor¬ tant a desirer, si ce n’est une indication plus directe et plus explicite de ces deux conditions fondamentales co-relatives, necessairement inseparables de l’etat vivant, un organisme determine et un milieu convenable.” It is strange that M. Comte should have thus recognized the necessity of a harmony between an organism and its environment, as a condition essential to life, and should not have seen that the continuous maintenance of such inner actions as will counterbalance outer actions, constitutes life. It is the more strange that he should have been so near this truth and yet missed it, since, besides his wide range of thought, M. Comte is often remarkable for his clear intuitions. Lest by saying this, I should deepen a misconception into which some have fallen, let me take the opportunity of stating, that though I believe some of M. Comte’s minor generalizations to he true, and though I recognize the profundity of many incidental observations he makes, I by no means accept his system. Those general doctrines in which I agree with him, are those which he holds in common with sundry other thinkers. Willi all those general doctrines which are distinctive of his philosophy, I disagree — with all those at least that I have definite knowledge of ; for beyond the first half of his “ Course of Positive Philosophy,” I know his opinions only by hearsay. CORRESPONDENCE BETWEEN LIFE AND ITS CIRCUMSTANCES. 75 on in a plant pre-suppose a surrounding medium containing at least carbonic acid and water, together with a due supply of light and a certain temperature. 'Within the leaves carbon is being assimilated and oxygen given off ; without them, is the gas from which the carbon is abstracted, and the imponderable agents that aid the abstraction. 13 e the nature of the process what it may, it is clear that there are external elements prone to undergo special re-arrangements under special conditions. It is clear that the plant in sunshine presents these conditions and so effects these re-arrange- ments. And thus it is clear that the changes which consti¬ tute the plant’s life, are in correspondence with co-existences in its environment. If, again, we ask respecting the lowest protozoon, how it lives ; the answer is, that while on the one hand its sub¬ stance is ever undergoing oxidation, it is on the other hand ever absorbing nutriment ; and that it may continue to exist, the assimilation must keep pace with, or exceed, the oxidation. If further we ask under what circumstances these combined changes are possible ; there is the obvious reply, that the medium in which the protozoon is placed, must contain oxy¬ gen and food — ox}Tgen in such quantity as to produce some disintegration ; food in such quantity as to permit that dis¬ integration to be made good. In other words — the two antagonistic processes taking place internally, imply the pre¬ sence externally of materials having affinities that can give rise to these processes. Leaving those lowest animal forms revealed by the mi¬ croscope, which simply take in through their surfaces the nutriment and ox}7genated fluids coming in contact with them, we pass to those somewhat higher forms which have their tissues partially specialized into assimilative and re¬ spiratory. In these we see a correspondence between certain actions in the digestive sac, and the properties of certain sur¬ rounding bodies. That a creature of this order may continuo to live, it is necessary not only that there be masses of sub- 76 THE DATA OF BIOLOGY. stance in the environment capable of transformation into its own tissue ; but that the introduction of these masses into its stomach, shall be followed by the secretion of a solvent fluid that will reduce them to a fit state for absorption. Special outer properties must be met by special inner properties. When, from the process by which food is digested, we turn to the processes by which it is seized, we perceive the same general truth. The stinging and contractile power of a polype’s tentacle, correspond to the sensitiveness and strength of the creatures serving it for prey. Unless that external change which brings one of these creatures in con¬ tact with the tentacle, were quickly followed by those inter¬ nal changes which result in the coiling and drawing up of the tentacle, the polype would die of inanition. The funda¬ mental processes of integration and disintegration within it, ■faould get out of correspondence with the agencies and pro¬ cesses without it ; and the life would cease. Similarly, it may be shown that when the creature be¬ comes so large that its tissue cannot be efficiently supplied * with nutriment by mere absorption through its limiting membranes, or duly oxygenated by contact with the fluid that bathes its surface, there arises a necessity for a circu¬ latory system by which nutriment and oxygen may be dis¬ tributed throughout the mass ; and the functions of this sys¬ tem, being subsidiary to the two primary functions, form links in the correspondence between internal and external ac¬ tions. The like is obviously true of all those subordinate functions, secretory and excretory, that facilitate oxidation and assimilation — functions in which we may trace, both co- temporaneous changes answering to co-existences in the en¬ vironment, and successive changes answering to those changes of composition, of temperature, of light, of moisture, of pres¬ sure, which the environment undergoes. Ascending from the visceral actions to the muscular and nervous actions, we find the correspondence displayed in a manner still more obvious. Every act of locomotion implies CORRESPONDENCE BETWEEN LIFE AND ITS CIRCUMSTANCES. 77 the expenditure of certain internal mechanical forces, adapted in amounts and directions to balance or out-balance certain external ones. The recognition of an object is impossible without a harmony between the changes constituting per¬ ception, and particular properties co-existing in the environ¬ ment. Escape from enemies supposes motions within the organism, related in kind and rapidity to motions without it. Destruction of prey requires a particular combination of sub¬ jective actions, fitted in degree and succession to overcome a group of objective ones. And so with those countless au¬ tomatic processes exemplified in works on animal instinct. In the highest order of vital changes, the same fact is equally manifest. The empirical generalization that guides the farmer in his rotation of crops, serves to bring his actions into concord with certain of the actions going on in plants and soil. The rational deductions of the educated navigator who calculates his position at sea, constitute a series of mental acts by which his proceedings are conformed to surrounding circumstances. Alike in the simplest inferences of the child, and the most complex ones of the man of science, we find a correspondence between simultaneous and successive changes <- in the organism, and co-existences and sequences in its envi¬ ronment. § 29. This general formula, which thus includes the lowest vegetal processes as well as the highest manifestations of hu- ' man intelligence, will perhaps call forth some criticisms which it is desirable here to meet. It may be thought that there are still a few inorganic ac¬ tions included in the definition ; as for example that displayed by the mis-named storm-glass. The feathery crystallization which, on a certain change of temperature, takes place in the solution contained by this instrument, and which afterwards dissolves to reappear in new forms under new conditions, may be held to present simultaneous and successive changes that are to some extent heterogeneous, that occur with some de» TIIE DATA OF BIOLOGY. rs finiteness of combination, and, above all, occur in correspond* ence with external changes. In this case vegetal life is sim¬ ulated to a considerable extent ; but it is merely simulated. The relation between the phenomena occurring in the storm- glass and in the atmosphere respectively, is really not a cor¬ respondence at all, in the proper sense of the word. Outside there is a certain change ; inside there is a change of atomic arrangement. Outside there is another certain change ; in¬ side there is another change of atomic arrangement. But subtle as is the dependence of each, internal upon each ex¬ ternal change, the connexion between them does not, in the abstract, differ from the connexion between the motion of a straw and the motion of the wind that disturbs it. In either case a change produces a change, and there it ends. The alteration wrought by some environing agency on an inani¬ mate object, does not tend to induce in it a secondary altera¬ tion, that anticipates some secondary alteration in the en¬ vironment. But in every living body there is a tendency towards secondary alterations of this nature ; and it is in their production that the correspondence consists. The dif¬ ference may be best expressed by symbols. Let A be a change in the environment ; and B some resulting change in an inorganic mass. Then A having produced B, the ac¬ tion ceases. Though the change A in the environment, is followed by some consequent change a in it ; no parallel se¬ quence in the inorganic mass simultaneously generates in it some change b that has reference to the change a. But if we take a living body of the requisite organization, and let the change A impress on it some change C ; then, while in the environment A is occasioning a, in the living body C will be occasioning c : of which a and c will show a certain con¬ cord in time, place, or intensity. And while it is in the con¬ tinuous production of such concords or correspondences that Life consists, it is by the continuous production of them that Life is maintained. The further criticism that maybe expected, concerns cer- CORRESPONDENCE BETWEEN LIFE AND ITS CIRCUMSTANCES. 79 tain verbal imperfections in tlie definition, which it seem3 impossible to avoid. It may bo fairly urged that the word correspondence will not include, without straining, the various relations to be expressed by it. It may be asked : — IIow can the continuous processes of assimilation and respiration, cor¬ respond with the co-cxistcnce of food and oxygen in the en¬ vironment P or again : — IIow can the act of secreting some defensive fluid, correspond with some external danger which may never occur ? or again : — IIow can the dynamical phe¬ nomena constituting perception, correspond with the statical phenomena of the solid body perceived ? The only reply to these questions, is, that we have no word sufficiently general to comprehend all forms of this relation between the organ¬ ism and its medium, and yet sufficiently specific to convey an adequate idea of the relation ; and that the word correspond¬ ence seems the least objectionable. The fact to be expressed in all cases, is, that certain changes, continuous or discon¬ tinuous, in the organism, are connected after such a manner that, in their amounts, or variations, or periods of occurrence, or modes of succession, they have a reference to external ac¬ tions, constant or serial, actual or potential — a reference such that a definite relation among any members of the one group, implies a definite relation among certain members of the other group ; and the word correspondence appears the best fitted to express this fact. § 30. The presentation of the phenomena under this ge¬ neral form, suggests how our definition of Life mav be reduced to its most abstract shape ; and perhaps its best shape. By regarding the respective elements of the definition as relations, we avoid both the circumlocution and the verbal inaccuracy ; and that we may so regard them with propriety is obvious. If a creature’s rate of assimilation is increased in consequence of a decrease of temperature in the environment ; it is that the relation between the food consumed and heat produced, is so re-adjusted by multiplying both its members, that the 80 TIIE DATA. OF BIOLOGY. altered relation in the surrounding medium between the quantity of heat absorbed from, and radiated to, bodies of a given temperature, is counterbalanced. If a sound or a scent wafted to it on the breeze, prompts the stag to dart away from the deer-stalker ; it is that there exists in its neighbour¬ hood a relation between a certain sensible property and cer¬ tain actions dangerous to the stag, while in its organism there exists an adapted relation between the impression this sensible property produces, and the actions by which danger is escaped. If inquiry has led the chemist to a law, enabling him to tell how much of any one element will combine with so much of another ; it is that there has been established in him specific mental relations, which accord with specific chemical relations in the things around. Seeing, then, that in all cases we may consider the external phenomena as simply in relation, and the internal phenomena also as simply in re¬ lation ; the broadest and most complete definition of Life will be — The continuous adjustment of internal relations to external relations * * While it is simpler, this modified formula has the further advantage of being somewhat more comprehensive. To say that it includes not only those definite combinations of simul¬ taneous and successive changes in an organism, which cor¬ respond to co-existences and sequences in the environment, but also those structural arrangements which enable the or¬ ganism to adapt its actions to actions in the environment, may perhaps be going too far ; for though these structural arrangements present internal relations adjusted to external relations, yet the continuous adjustment of relations can scarcely be held to include a fixed adjustment already mado. Clearly, Life, which is made up of dynamical phenomena, cannot be defined in terms that shall at the same time define the apparatus manifesting it, which presents only statical phenomena. Lut while this antithesis serves to remind us that the fundamental distinction between the organism and *' In further elucidation of this general doctrine, see First Principles , § 25. CORRESPONDENCE BETWEEN LIFE AND ITS CIRCUMSTANCES. 81 its actions, is as wide as that between Matter and Motion, it at the same time draws attention to the fact, that if the structural arrangements of the adult are not properly in¬ cluded in the definition, yet the developmental processes by which those arrangements were established, are included. For that process of evolution during which the organs of the embryo are fitted to their prospective functions, is from, be¬ ginning to end the gradual or continuous adjustment of in¬ ternal relations to external relations. Moreover, those struc¬ tural modifications of the adult organism, which, under change of climate, change of occupation, change of food, slowly bring about some re-arrangement in the organic balance, must simi¬ larly be regarded as continuous adjustments of internal re¬ lations to external relations. So that not only does the de¬ finition, as thus expressed, comprehend all those activities, bodily and mental, which constitute our ordinary idea of Life; but it also comprehends, both those processes of development by which the organism is brought into general fitness for these activities, and those after-j)rocesses of adaptation by which it is specially fitted to its special activities. Nevertheless, superior as it is in simplicity and comprehen¬ siveness, so abstract a formula as this is scarcely fitted for our present purpose. Reserving its terms for such use as oc¬ casion may dictate, it will be best commonly to employ its more concrete equivalent — to consider the internal relations as “ definite combinations of simultaneous and successive changes ; ” the external relations as “ co-existences and se¬ quences ; ” and the connexion between, them as a “ corre¬ spondence.’ * CHAPTER VI. TIIE DEGREE OF LIFE VARIES AS THE DEGREE OF CORRESPONDENCE. § 31. Already it lias been shown respecting eacli other qualification included in the foregoing definition, that the life is high in proportion as that* qualification is well fulfilled ; and it is now to be remarked, that the same thing is especially true respecting this last qualification — the correspondence be¬ tween internal and external relations. It is manifest a priori , that since changes in the physical state of the environment, as also those mechanical actions and those variations of available food which occur in it, are liable to stop the processes going on in the organism ; and since the adaptive changes in the organism have the effects of directly or indirectly counter¬ balancing these changes in the environment ; it follows that the life of the organism will be short or long, low or high, according to the extent to which changes in the environment are met by corresponding changes in the organism. Allow¬ ing a margin for perturbations, the life will continue only while the correspondence continues ; the completeness of the life will be proportionate to the completeness of the corre¬ spondence ; and the life will be perfect only when the corre¬ spondence is perfect. Hot to dwell in general statements, however, let us contemplate this truth under its concrete aspects. § 32. In life of the lowest order, we find that only the 83 TIIE LIFE VARIES AS TIIE CORRESPONDENCE. most prevalent coexistences and sequences in tlie environ¬ ment, liave any simultaneous and successive changes answer¬ ing to them in the organism. A plant’s vital processes display adjustment solely to the continuous coexistence of certain elements and forces surrounding its roots and leaves ; and vary only with the variations produced in these ele¬ ments and forces by the sun — are unaffected by the countless mechanical and other changes occurring around ; save when accidentally arrested by these. The life of a worm is made up of actions referring almost exclusively to the tangible pro¬ perties of adjacent things. All those visible and audible changes which happen near it, and are connected with other changes that may presently destroy it, pass unrecognized — produce in it no adapted changes : its only adjustment of in¬ ternal relations to external relations of this order, is seen when it escapes to the surface on feeling the vibrations pro¬ duced by an approaching mole. Adjusted as are the pro¬ ceedings of a bird, to a far greater number of coexistences and sequences in the environment, cognizable by sight, hearing, scent, and their combinations ; and numerous as are the dangers it shuns and the needs it fulfils, in virtue of this ex¬ tensive correspondence ; it exhibits no such actions as those by which a human being counterbalances variations in tem¬ perature and supply of food, consequent on the seasons. And when we see the plant eaten, the worm trodden on, the bird dead from starvation ; we see alike that the death is an arrest of such correspondence as existed ; that it occurred when there was some change in the environment to which the or¬ ganism made no answering change ; and that thus, both in shortness and simplicity, the life was incomplete in propor¬ tion as the correspondence was incomplete. Progress towards more prolonged and higher life, evidently implies an ability to respond to less general coexistences and sequences. Each step upwards must consist in adding to the previously- adjusted relations which the organism exhibits, some further relation parallel to a further relation in the environment. And the 84 THE DATA OF BIOLOGY. greater correspondence thus established, must, other things equal, show itself both in greater complexity of life, and greater length of life — a truth which will be duly realized on remembering that enormous mortality which prevails among lowly-organized creatures, and that gradual increase of longevity and diminution of fertility which we meet with on ascending to creatures of higher and higher development. It must, however, be remarked, that while length and com¬ plexity of life are, to a great extent, associated — while a more extended correspondence in the successive changes, commonly implies increased correspondence in the simul¬ taneous changes ; yet it is not uniformly so. Between the two great divisions of life — animal and vegetal — this contrast by no means holds. A tree may live a thousand years, though the simultaneous changes going on in it answer only to the few chemical affinities in the air and the earth, and though its serial changes answer only to those of day and night, of the weather and the seasons. A tortoise, which exhibits in a given time nothing like the number of internal actions adjusted to external ones, that are exhibited by a do°-, yet lives far longer. The tree by its massive trunk, and the tortoise by its hard carapace, are saved the necessity of re¬ sponding to those many surrounding mechanical actions which organisms not thus protected must respond to or die ; or rather — the tree and the tortoise display in their structures, certain simple statical relations adapted to meet countless dynamical relations external to them. But notwithstanding the qualifications suggested by such cases, it needs but to compare a microscopic fungus with an oak, an animalcule with a shark, a mouse with a man, to recognize the fact that this increasing correspondence of its changes with those of the environment, which characterizes progressing life, ha¬ bitually shows itself at the same time in continuity and in complication. Even were not the connexion between length of life and complexity of life thus conspicuous, it would still be true THE LIFE VARIES AS TIIE CORRESPONDENCE. 85 that the degree of life varies with the degree of correspond¬ ence. For if the lengthened existence of a tree be looked upon as tantamount to a considerable degree of life ; then it must be admitted that its lengthened display of correspond¬ ences is tantamount to a considerable degree of correspond¬ ence. If otherwise it be held, that notwithstanding its much shorter existence, a dog must rank above a tortoise in degree of life, because of its superior activity ; then it is implied that its life is higher, because its simultaneous and successive changes are more complex and more rapid — because the correspondence is greater. And since we regard as the high¬ est life, that which, like our own, shows great complexity in the correspondences, great rapidity in the succession of them, and great length in the series of them ; the equivalence between decree of life and degree of corresDondence, is un- questionable. § 33. In further elucidation of this general truth, and especially in explanation of the irregularities just referred to, it requires to be observed, that as the life becomes higher the environment itself becomes more complex. Though, literally, the environment means all surrounding space with the coexistences and sequences contained in it ; yet, practi¬ cally, it often means but a small part of this. The environ¬ ment of an entozoon, can scarcely be said to extend beyond the body of the animal in which the entozoon lives. That of a fresh- water alga is, virtually, limited to the ditch inhab¬ ited by the alga. And understanding the term in this re¬ stricted sense, we shall see that the superior organisms inhabit the more complicated environments. Thus, contrasted with that found on land, the lower life is that found in the sea ; and it has the simpler environment. Marine creatures are affected by a smaller number of co¬ existences and sequences than terrestrial ones. Being very nearly of the same specific gravity as the surrounding medium, they have to contend with less various mechanical 86 THE DATA OF BIOLOGY. actions. The zoophyte rooted to a stone, and the acalephe passively borne along in the current, need to undergo no internal changes such as those by which the caterpillar meets the varying effects of gravitation, while creeping over and under the leaves. Again, the sea is liable to none of those extreme and rapid alterations of temperature which the air suffers. Night and day produce no appreciable modifications in it ; and it is comparatively little affected by the seasons. Thus its contained fauna show no marked cor¬ respondences similar to those by which air-breathing crea¬ tures counterbalance thermal changes. Further, in respect to the supply of nutriment the conditions are more simple. The lower tribes of animals inhabiting the water, like the plants inhabiting the air, have their food brought to them. The same current which brings oxygen to the oyster, also brings it the microscopic organisms on which it lives : the disintegrating matter and the matter to be inte¬ grated, coexist under the simplest relation. It is otherwise with land animals. The oxygen is everywhere ; but that which is needed to neutralize its action is not everywhere : it has to be sought ; and the conditions under which it is to be obtained are more or less complex. So too with that liquid by the agency of which the vital processes are carried on. To marine creatures, water is ever present, and by the lowest is passively absorbed ; but to most creatures living on the earth and in the air, it is made available only through those nervous changes constituting perception, and those muscular ones by which drinking is effected. Simi¬ larly, the contrast might be continued with respect to the electric and hygrometric variations ; and the greater multi¬ plicity of optical and acoustic phenomena with which ter¬ restrial life is surrounded. And tracing upwards from the amphibia th(3 widening extent and complexity which the environment, as practically considered, assumes— observing further how increasing heterogeneity in the flora and fauna of the globe, itself progressively complicates the environment 87 THE LIFE VA111ES AS THE CORRESPONDENCE. of eacli species of organism — it might finally be shown that the same general truth is displayed in the history of mankind : whose advance in civilization has been simultaneous with their advance from the less varied requirements of the torrid zone to the more varied requirements of the temperate zone ; whose chief steps have been made in regions presenting a complicated physical geography ; and who, in the course of their progress, have been adding to their physical environ¬ ment a social environment that has been growing even more involved. Thus, speaking generally, it is clear that those re¬ lations in the environment to which relations in the organism must correspond, themselves increase in number and intricacy as the life assumes a higher form. § 34. To make yet more manifest the fact, that the degree of life varies as the degree of correspondence, I may here point out, that those other distinctions successively noted when contrasting vital changes with non-vital changes, are all implied in this last distinction — their correspondence with external coexistences and sequences. And to this may be added the supplementary fact, that the increasing fulfil¬ ment of those other distinctions which we found to accompany increasing life, is involved in the increasing fulfilment of this last distinction. To descend to particulars: — We saw that living organisms are characterized by successive changes ; and that as the life becomes higher, the successive changes become more numerous. Well, the environment is full of successive changes, both positive and relative ; and the greater the correspondence, the greater the number of suc¬ cessive changes an organism must display. We saw that life presents simultaneous changes ; and that the more elevated it is, the more marked the multiplicity of them. Well, besides countless phenomena of coexistence in the environ¬ ment, there are often many changes occurring in it at the same moment ; and hence increased correspondence with it, supposes an increased display of simultaneous changes in the S3 THE DATA OF BIOLOGY. organism. Similarly with the heterogeneity of the changes. In the environment the relations are very varied in their kinds ; and hence, as the organic actions come more and more into correspondence with them, they also must become very varied in their kinds. So again is it, even with definiteness of combination. For though the inorganic bodies of which the environment mainly consists, do not present definitely- combined changes, yet they present definitely-combined properties ; and though the minor meteorologic variations of the environment, do not show much definiteness of combination, yet those resulting from day and night and the seasons do. Add to which, that as the environment of each organism comprehends all those other organisms existing within its sphere of life — as the most important and most numerous surrounding changes with which each animal has to deal, are the definitely- combined changes exhibited by other animals, whether prey or enemies ; it results that definiteness of combination is a general characteristic of the external changes with which internal ones have to correspond. Hence, increase of correspondence involves increased definiteness of combination. So that throughout, the correspondence of the internal relations wuth the external ones, is the essential thing ; and all the special characteristics of the internal relations, are but the collateral results of this correspondence. § 35. As affording the simplest and most conclusive proof that the degree of life varies as the degree of correspondence, it remains to point out that perfect correspondence would be perfect life. Were there no changes in the environment but such as the organism had adapted changes to meet ; and were it never to fail in the efficiency with which it met them ; there would be eternal existence and universal knowledge. Death by natural decay, occurs because in old age the relations be¬ tween assimilation, oxidation, and genesis of force goino- on in the organism, gradually fall out of correspondence with the relations between oxygen and food and absorption of heat by THE LIFE VARIES AS THE CORRESPONDENCE. 89 the environment. Death from disease, arises either when the organism is congenitally defective in its power to balance the ordinary external actions by the ordinary internal actions, or when there has taken place some unusual external action to which there was no • answering internal action. Death by accident, implies some neighbouring mechanical changes of which the causes are either unobserved from inattention, or are so intricate that their results cannot be foreseen ; and consequently certain relations in the organism are not adjusted to the relations in the environment. Manifestly, if, to every outer coexistence and sequence by which it was ever in any degree affected, the organism presented an answering process or act ; the simultaneous changes would be indefinitely nu¬ merous and complex, and the successive ones endless — the correspondence would be the greatest conceivable, and the life the highest conceivable, both in degree and in length. § 36. Before closing the chapter, it will be useful to compare the definition of Life here set forth, with the defini¬ tion of Evolution set forth in First Principles. Living bodies being bodies which display in the highest degree the structural changes constituting Evolution ; and Life being made up of the functional changes that accompany these structural changes ; we ought to find a certain harmony between the definitions of Evolution and of Life. Such a harmony is not wanting. The first distinction we noted between the kind of change shown in Life, and other kinds of change, was its serial character : we saw that vital change is substantially unlike non-vital change, in being made up of successive changes. Now since organic bodies display in so much higher a de¬ gree than inorganic bodies, those continuous differentiations and integrations which constitute Evolution ; and since the re-distributions of matter thus carried so far in a compara¬ tively short period, imply concomitant re-distributions of mo¬ tion ; it is clear that in a given time, organic bodies must. 5 90 TIIE DATA OF BIOLOGY. ■undergo changes so comparatively numerous as to render the successiveness of tlieir changes a marked characteristic. And it will follow a priori, as we found it to do a posteriori, that the organisms exhibiting Evolution in the highest degree, exhibit the longest or the most rapid successions of changes, or both. Again, it was shown that vital change is distinguished from non- vital change by being made up of many simultaneous changes ; and also that creatures possess¬ ing high vitality are marked off from those possessing low vitality, by the far greater number of their simultaneous changes. Here too there is entire congruity. In First Principles , § 116, we reached the conclusion, that a force falling on any aggregate is divided into several forces ; that when the aggregate consists of parts that are unlike, each part becomes a centre of unlike differentiations of the inci¬ dent force ; and that thus the multiplicity of such differen¬ tiations must increase with the multiplicity of the unlike parts. It follows necessarily, therefore, that organic aggre¬ gates, which as a class are distinguished from inorganic aggregates by the greater number of their unlike parts, must be also distinguished from them by the greater number of simultaneous changes they display ; and further that the higher organic aggregates, having more numerous unlike parts than the lower, must undergo more numerous simul¬ taneous changes. We next found that the changes occurring in living bodies, are contrasted with those occurring in other bodies, as being much more heterogeneous ; and that the changes occurring in the superior living bodies, are similarly contrasted with those occurring in inferior ones. Well, heterogeneity of function is the correlate of hetero¬ geneity of structure ; and heterogeneity of structure is the leading distinction between organic and inorganic aggre¬ gates, as well as between the more highly organized and the more lowly organized. By reaction, an incident force must be rendered multiform in proportion to the multiformity of the aggregate on which it falls ; and hence those most mul- TIIE LIFE VAUIES AS THE CORRESPONDENCE. 91 liform aggregates which display in the highest degree the phenomena of Evolution structurally considered, must at the same time be aggregates which display in the highest de¬ gree the multiform actions which constitute Evolution functionally considered. These heterogeneous changes, exhibited simultaneously and in succession by a living or¬ ganism, prove, on further inquiry, to be distinguished by their combination from certain non-vital changes which simulate them. Here, too, the parallelism is maintained. It was shown in § 56 of First Principles, that an essential characteristic of Evolution is the integration of parts, which accompanies their differentiation — an integration that is shown both in the consolidation of each part, and in the consolidation of all the parts into a whole. How, manifestly, combination among the changes going on in different com¬ bined parts, must be proportionate to the degree of com¬ bination among these parts : the more mutually-dependent the parts, the more mutually-dependent must be their actions.. Hence, animate bodies having greater co-ordin¬ ation of parts than inanimate ones, must exhibit greater co-ordination of changes. And this greater co-ordination of their changes must not only distinguish organic from inor¬ ganic aggregates ; but must, for the same reason, distinguish higher organisms from lower ones, as we found that it did. Yet once more, it was pointed out that the changes constituting Life, differ from other changes in the definiteness of their combination ; and that a distinction like in kind, though less in degree, holds between the vital changes of superior creatures and those of inferior creatures. These, also, are contrasts in harmony with the contrasts disclosed by the analysis of Evolution. We saw {First Principles, §§ 54, 55) that during Evolution, there is an increase of definiteness as well as an increase of heterogeneity. We saw that the integration accompanying differentiation, has necessarily the effect of increasing the distinctness with which the parts are marked off from each other ; and that so, out of the inco- 92 TIIE DATA OF BIOLOGY. herent and indefinite, there arises the coherent and definite. But a coherent whole made up of definite parts definitely combined, must exhibit more definitely combined changes than a whole made up of parts that are neither definite in themselves nor in their combination. Hence, if living bodies display more than other bodies this structural definiteness, then, definiteness of combination must be a characteristic of the changes constituting Life ; and must also distinguish the vital changes of higher organisms from those of lower organ¬ isms. Finally, however, we discovered that all these peculiarities are subordinate to the one fundamental pecu¬ liarity, that vital changes take place in correspondence with external co-existences and sequences ; and that the highest possible Life is reached, when there is some inner relation of actions fitted to meet every outer relation of actions by which the organism can be affected. But this conception of the highest possible Life, is in perfect harmony with the con¬ ception, before arrived at, of the ultimate limit of Evolution. When treating of equilibration as exhibited in organic phenomena ( First Principles , §§ 133, 134), it was pointed out, that the continual tendency is towards the establishment of a balance between inner and outer changes. It was shown that “ the final structural arrangements must be such as will meet all the forces acting on the aggregate, by equivalent antagonistic forces/5 and that “ the maintenance of such a moving equilibrium ” as an organism displays, i: requires the habitual genesis of internal forces correspond¬ ing in number, directions, and amounts, to the external incident forces — as many inner functions, single or com¬ bined, as there are single or combined outer actions to be met.’5 It was shown, too, that the relations among concep¬ tions and ideas, are ever in progress towards a better balance between mental actions and those actions in the environment to which conduct must be adjusted. So that that main¬ tenance of a correspondence between inner and outer rela¬ tions, which we have here found to constitute Life, and the 93 THE LIFE VARIES AS THE CORRESPONDENCE. perfection of which is the perfection of Life, answers com¬ pletely to that state of organic moving equilibrium which we saw arises in the course of Evolution, and tends ever to become more complete. There is much significance in this complete parallelism. That two inquiries starting from different points and carried on in different ways, should lead to conclusions so entirely harmonizing with each other, cannot fail further to confirm these conclusions; if further confirmation of them be needed, CHAPTER Y1L THE SCOPE OF BIOLOGY, § 37. We are now in a position to map-out the boundaries and divisions of our subject. Grouping together the general results arrived at in the first three chapters, and joining with them the results which the last three chapters have brought us to, we shall be prepared to comprehend the science of Biology as a whole ; and to see how its truths may best be classified. In the chapters treating of Organic Matter, the Actions of Forces on it, and its Reactions on Forces, the generalizations reached were these : — that organic matter is specially sensi¬ tive to surrounding agencies ; that in consequence of the extreme instability of the compounds it contains, minute dis¬ turbances can cause in it large amounts of re-distribution ; and that during the fall of its unstably-arranged atoms into stable arrangements, there are given out proportionately large amounts of motion. We saw that organic matter is so constituted, that small incident actions are capable of initiat¬ ing great reactions — setting up extensive structural modifica¬ tions, and liberating large quantities of power. In the chapters just concluded, the changes of which Life is made up, were shown to be so adjusted as to balance outer changes. And the general process of the adjustment wo found resolves itself into this ; that if in the environment there are any related actions, A and B, by which the or- THE SCOPE OF BIOLOGY. 95 ganism is affected, then if A produces in the organism some change a, there follows in the organism some change b, fitted in time, direction, and amount to meet the action B — a change which is often required to be much larger than it3 antecedent. Mark, noAV, the relation between these two final results. On the one hand, for the maintenance of that correspondence between inner and outer actions which constitutes Life, an organism must be susceptible to small changes from small external forces (as in sensation), and must be able to initiate large changes in opposition to large external forces (as in muscular action). On the other hand, organic matter is at once extremely sensitive to disturbing agencies of all kinds, and is capable of suddenly evolving motion in great amounts. That is to say, the constitution of organic matter specially adapts it to receive and produce the internal changes required to balance external changes. This being the general character of the vital Functions, and of the Matter in which they are performed, the science of Biology becomes an account of all the phenomena attend¬ ant on the performance of such Functions by such Matter — an account of all the conditions, concomitants, and conse¬ quences, under the various circumstances fallen into by living bodies. If all the functional phenomena which living bodies present, are, as we have concluded, incidents in the main¬ tenance of a correspondence between inner and outer ac¬ tions ; and if all the structural phenomena which living bodies present, are direct or indirect concomitants of func¬ tional phenomena ; then the entire Science of Life, must con¬ sist in a detailed interpretation of all these functional and structural phenomena in their relations to the phenomena of the environment. Immediately or mediately, proximately or remotely, every trait exhibited by organic bodies, as distinguished from inorganic bodies, must be referable to this continuous adjustment between their actions and the actions going on around them. Such being the extent and nature of our subject-matter, it may be thus divided. 96 THE DATA OF BIOLOGY. 1. An account of the structural phenomena presented h j organisms. And this subdivides into : — * a . The structural phenomena presented by individual organisms. b. The structural phenomena presented by successions of organisms. 2. An account of the functional phenomena which or¬ ganisms present. And this, too, admits of sub-division into : — a . The functional phenomena of individual organisms. b. The functional phenomena of successions of organisms. 8. An account of the actions of Structure on Function, and the re-actions of Function on Structure. And like the others, this is divisible into : — a. The actions and re-actions as exhibited in individual organisms. b. The actions and re-actions as exhibited in successions of organisms. 4. An account of the phenomena attending the production of successions of organisms : in other words — the phenomena of Genesis. There is, indeed, another mode of grouping the facts of Biology, with which all are familiar. According as they are facts of animal or vegetal life, they may be classed under the heads of Zoology and Botany. But this di¬ vision, though convenient and indeed necessary for practi¬ cal purposes, is one that does not here concern us. Dealing with organic structures and functions in connexion, with their causes, conditions, concomitants, and consequences, Biology cannot divide itself into Animal-Biology and Yege- tal-Biology ; since the same fundamental classes of phe¬ nomena are common to both. Becogniziug this familiar distinction only as much as convenience obliges us to do, let us now pass on to consider, more in detail, the classification of biologic phenomena, above set down in its leading outlines. § 88. The facts of structure which an individual or- THE SCOPE OF BIOLOGY. 97 ganism exhibits, are of two chief kinds. In order of con- ejhcuousness, though not in order of time, there come first those ultimate arrangements of parts which characterize the organism in its mature state — an account of which, commonly called Anatomy, is more properly called Morphology. And second, there come those successive modifications through which the organism passes in its development from the germ to the adult form — an account of which is called Embryology. The facts of structure which any succession of individual organisms exhibits, admit of similar classification. On the one hand, we have those inner and outer differences of shape, that are liable to arise between the adult members of suc¬ cessive generations descended from a common stock — differ¬ ences which, though usually not marked between adjacent generations, may in course of many generations become great. And on the other hand, we have those developmental modi¬ fications through which such modifications of the descended forms are reached. The interpretation of structure, as exhibited in individual organisms and successions of organisms, is aided by two sub¬ sidiary divisions of biologic inquiry, named Comparative Anatomy (properly Comparative Morphology) and Compara¬ tive Embryology. These cannot properly be regarded as in themselves parts of Biology • since the facts embraced under them are not substantive phenomena, but are simply inci¬ dental to substantive phenomena. All the facts of structural Biology are comprehended under the two foregoing sub¬ divisions ; and the comparison of these facts as presented in different classes of organisms, is simply a method of inter¬ preting the real relations and dependencies of the facts compared. Nevertheless, though Comparative Morphology and Com¬ parative Embryology do not disclose additional series of con¬ crete or special facts, they lead to the establishment of certain abstract or general facts. By them it is made manifest that underneath the superficial differences of groups and classes 98 TIIE DATA OF BIOLOGY. and types of organisms, there are hidden fundamental simi¬ larities ; and that the courses of development in such groups and classes and types, though in many respects’ divergent, are in some essential respects, coincident. The wide truths thus disclosed, come under the heads of General Morphology and General Embryology. By contrasting the structures of organisms, there is also achieved that grouping of the like and separation of the unlike, called Classification. First by observation of ex¬ ternal characters ; second by observation of internal charac¬ ters ; and third by observation of the phases of development ; it is ascertained what organisms are most similar in all particulars ; what organisms are like each other in every important attribute ; what organisms have common primor¬ dial characters. Whence there finally results such an ar¬ rangement of organisms, that if certain structural attributes of any one be given, its other structural attributes may be empirically predicted ; and which prepares the way for that interpretation of their relations and genesis, which forms an important part of rational Biology. § 39. The second main division of Biolog3r, above de¬ scribed as embracing the functional phenomena of organisms, is that which is in part signified by Physiology : the remain¬ der being what we distinguish as Psychology. Both of these fall into subdivisions that may best be treated separ¬ ately. That part of Physiology which is concerned with the molecular changes going on in organisms, is known as Organic Chemistry. An account of the modes in which the force generated in organisms by chemical change, is trans¬ formed into other forces, and made to work the various or¬ gans that carry on the functions of Life, comes under the head of Organic Physics. Psychology, wdiich is mainly concerned with the adjustment of vital actions to actions in the environment (in contrast with Physiology, which is mainly concerned with vital actions apart from THE SCOPE OF BIOLOGY. m actions in the environment) consists of two quite distinct por¬ tions. Objective Psycholog}^ deals with those functions of the nervo-muscular apparatus by which such organisms as possess it, are enabled to adjust inner to outer relations ; and includes also, the study of the same functions as externally manifested in conduct. Subjective Psychology deals with the sensations, perceptions, ideas, emotions, and volitions that are the direct or indirect concomitants of this visible adjustment of inner to outer relations — considers these several hinds of conscious¬ ness in their genesis, and their connexions of co-existence and succession. Consciousness under its different modes and forms, being a subject-matter radically distinct in nature from the subject-matter of Biology in general ; and the method of self-analysis, by which alone the laws of dependence among changes of consciousness can be found, being a method un¬ paralleled by anything in the rest of Biology ; we are obliged to regard Subjective Psychology as a separate study —not absolutely, of course, but relatively to the mind of each student. And since it would be very inconvenient to dis¬ sociate Objective Psychology from Subjective Psychology, we are practically compelled to deal with the two as forming an independent sub-science, to be treated apart from the lower divisions of Biology. Obviously, the functional phenomena presented in succes¬ sions of organisms, similarly divide into physiological and psychological. Under the physiological, come the modifications of bodily actions that arise in the course of generations, as concomitants of structural modifications ; and these may be modifications, qualitative or quantitative, in the molecular changes classed as chemical, or in the organic actions classed as physical, or in both. Under the psychological, come the qualitative and quantitative modifica¬ tions of instincts, feelings, conceptions, and mental changes in general, that occur in creatures having more or less intelligence, when certain of their conditions are changed. This, like the preceding department of Psychology, has in 100 THE DATA OF BIOLOGY. tlie abstract two different aspects — the objective and the sub¬ jective. Practically, however, the objective, which deala with these mental modifications as exhibited in the changing habits and abilities of successive generations of creatures, is the only one that admits of scientific investigation ; since the corresponding alterations in consciousness, cannot be im¬ mediately known to any but the subjects of them. Evidently, convenience requires us to class this part of Psychology along with the other parts, in a distinct sub- science. Light is thrown on functions, as well as on structures, by comparing organisms of different kinds. Comparative Physiology and Comparative Psychology, are the names given to those collections of facts respecting the homologies and analogies, bodily and mental, that are brought to light by this kind of inquiry. These classified observations concern¬ ing likenesses and differences of functions, are helpers to interpret functions in their essential natures and relations. Hence Comparative Physiology and Comparative Ps}Tchology are names of methods, rather than names of true subdivisions of Piology. Here, however, as before, the comparison of special truths, besides facilitating their interpretation, brings to light certain general truths. Contrasting bodily and mental functions as exhibited in various orders of organisms, shows that there exists, more or less extensively, a community of processes and methods. Hence result two groups of abstract proposi¬ tions, constituting General Physiology and General Psy¬ chology. § 40. In these various divisions and sub-divisions of the first two great departments of Piology, the phenomena of Structure are considered separately from the phenomena of Function, so far as separate treatment of them is possible. The third great department of Biology deals with them in their necessary connexions. It comprehends the determin- THE SCOPE OF BIOLOGY. 101 ution of functions by structures, and tlie determination of structures by functions. As displayed in individual organisms, the action of struc¬ tures on functions is to be studied, not only in the broad and familiar fact that the general kind of life an organism leads is necessitated by the main characters of its organization, but in the more special and less conspicuous fact, that between members of the same species, minor differences of structure lead to minor differences of power to perform certain kinds of action, and of tendency to perform such kinds of action. Con¬ versely, under the re-actions of function on structure as displayed in individual organisms, come the facts showing that functions, when fulfilled to their normal extents, main¬ tain integrity of structure in their respective organs ; and that within certain limits, the increase of functions is followed by such structural changes in their respective organs, as enables the organs to discharge better their extra functions. Inquiry into the action of structure on function as dis¬ played in successions of organisms, introduces us to such phenomena as l\Ir Darwin’s “Origin of Species” deals with. In this category come all proofs of the general truth, that when an individual is enabled by a certain structural pecu¬ liarity, to perform better than others of its species some advantageous action ; and when it bequeaths more or less of its structural peculiarity to descendants, among whom those which have it most markedly, are best able to thrive and propagate ; there arises through this continuous action of structure on function, a visibly modified type of structure, ha vine: a more or less distinct function. In the cor- relative class of facts, which come under the category of re¬ actions of function on structure as exhibited in successions of organisms, are to be placed all those modifications of struc¬ ture which arise in races, when changes of conditions entail changes in the balance of their functions. Here is to be studied the way in which altered function externally necessi- 102 THE DATA OF BIOLOGY. tatcd, works, by re-act ion, altered structure ; and howinsucceed- ing generations, this altered structure may be made continu¬ ally more marked by tliis altered function. Though logically distinct, these two sub-divisions of biologic inquiry cannot in practice be carried on apart. A speciality of struc¬ ture which leads to an excess of function in any direction, is, by the perpetual re-action of function, rendered ever more decided. A speciality of function, by calling forth a corre¬ sponding speciality of structure, produces an increasingly efficient discharge of such function. Whichever of the two initiates the change, there goes on between them an unceas¬ ing action and re-action, producing in them co-ordinate modifications. § 41. The fourth great division of Biology, comprehend¬ ing the phenomena of Genesis, may be conveniently separated into three sub-divisions. Tinder the first, comes a description of all the special modes whereby the multiplication of organisms is carried on: which modes range themselves under the two chief heads oi sexual and asexual. An account of Sexual Multiplication in¬ cludes the various methods by which germs and ova are fertilized, and by which, after fertilization, they are furnished with the materials, and maintained in the conditions, needful for their development. An account of Asexual Multiplica¬ tion includes the various methods by which, from the same fertilized germ or ovum, there are produced many organisms that are partially or totally independent of each other. The second of these sub-divisions deals with the phenomena of Genesis in the abstract. It takes for its subject-matter, such general questions as — What is the end subserved by the union of sperm-cell and germ -cell ? Why cannot all multi¬ plication be carried on after the asexual method? What are the laws of hereditary transmission ? What are tho causes of variation ? The third sub-division is devoted to still more abstract THE SCOPE OF BIOLOGY. ioa aspects of the phenomena. Recognizing the general facts of multiplication, without reference to their modes or immediate causes, it concerns itself simply with the different rates of multiplication in different kinds of organisms, and different individuals of the same kind. Generalizing the numerous contrasts and variations of fertility, it seeks a rationale of them in their relations to other organic phenomena. § 42. Such appears to he the natural arrangement of divisions and sub-divisions which Eiology presents, when re¬ garded from the highest point of view, as the Science of Life — the science which has for its subject-matter, the cor¬ respondence of organic relations, with the relations amid which organisms exist. This, however, is a classification of the parts of Biology when fully developed ; rather than a classification of the parts of Biology as it now stands. Several of the sub-divisions above named have no recognized existence ; and sundry of the others are in quite rudimentary states. It is therefore impossible now to fill in, even in the roughest way, more than a part of the outlines hero sketched. Our course of inquiry being thus in great measure de¬ termined by the present state of knowledge, we are com¬ pelled to follow an order widely different from this ideal one. It will be necessary first to give an account of those empiri¬ cal generalizations which naturalists and physiologists have established : arranging them rather with a view to facility of comprehension than to logical sequence ; and append¬ ing to those wdiich admit of it, such deductive interpreta¬ tions as First Principles furnish us with. Having done this, We shall be the better prepared for dealing with the lead¬ ing truths of Biology, in connexion with the doctrine cf Evolution. PART II. THE INDUCTIONS OF BIOLOGY. CHAP TEE T, GROWTH. $ 43. Peru Ars the widest and most familiar induction of Biology, is that organisms grow. "While, however, this is a characteristic so habitually and markedly displayed by plants and animals, as to be carelessly thought peculiar to them, it is really not so. Under appropriate conditions, increase of size takes place in inorganic aggregates, as well as in organic aggregates. Crystals grow ; and often far more rapidly than living bodies. Where the requisite materials are supplied in the requisite forms, growth may be witnessed in non-crystal¬ line masses : instance the fungus-like accumulation of carbon that takes place on the wick of an unsnuffed candle. On an immensely larger scale, we have growth in geologic formations : the slow accumulation of deposited sediment into a stratum, is not distinguishable from growth in its widest acceptation. And if we go back to the genesis of celestial bodies, assuming them to have arisen by Evolution, these, too, must have gradually passed into their concrete shapes through processes of growth. Growth is indeed a concomi¬ tant of Evolution ; and if Evolution of one kind or other is universal, growth is universal — universal, that is, in the sense that all aggregates display it in some way at somo period. The essential community of nature between organic growth and inorganic growth, is, however, most clearly seen 108 THE INDUCTIONS OF BIOLOGY. on observing that they both result in the same way. The segregation of different kinds of detritus from each other, a3 well as from the water carrying them, and their aggregation into distinct strata, is but an instance of a universal tend- enc}^ towards the union of like units and the parting of un¬ like units ( First Principles, § 123). The deposit of a crystal from a solution, is a differentiation of the previously mixed atoms ; and an integration of one class of atoms into a solid body, and the other class into a liquid solvent. Is not the growth of an organism a substantially similar process ? Around a plant there exist certain elements that are like the elements which form its substance ; and its increase of size is effected by continually integrating these surrounding like elements with itself. Nor does the animal fundament¬ ally differ in this respect from the plant or the crystal. Its food is a portion of the environing matter, that contains some compound atoms like some of the compound atoms constitut¬ ing its tissues; and either through simple imbibition or through digestion, the animal eventually integrates with it¬ self, units like those of which it is built up, and leaves behind the unlike units. To prevent misconception, it may be well to point out that growth, as here defined, must be distinguished from certain apparent and real augmentations of bulk which simulate it. Thus, the long, white potato- shoots thrown out in the dark, are produced at the expense of the substances which the tuber contains : they illustrate not the accumulation of organic matter, but simply its re¬ arrangement. Certain animal-embryos, again, during their early stages, increase considerably in size without assimil¬ ating any solids from the environment ; and they do this by absorbing the surrounding water. Even in the highest organisms, as in children, there appears sometimes to occur a rapid gain in dimensions, that does not truly measure the added quantity of organic matter ; but is in part 'due to changes analogous to those just named. Alterations of this GROWTH. 101) kind must not be confounded with that growth, properly so called, of which we have here to treat. The next general fact to be noted respecting organic growth, is, that it has limits. Here there appears to be a distinction between organic and inorganic growth ; but this distinction is by no means definite. Though that aggrega¬ tion of inanimate matter which simple attraction produces, may go on without end ; yet there appears to be an end to that more definite kind of aggregation wdiich results from polar attraction. Different elements and compounds, habitu¬ ally form crystals more or less unlike in their sizes ; and each seems to have a size that is not usually exceeded without a tendency arising to form new crystals rather than to increase the old. On looking at the organic kingdom as a whole, we see that the limits between which growth ranges, are very wide apart. At the one extreme, we have monads so minute as to be rendered but imperfectly visible by micro¬ scopes of the highest power ; and at the other extreme, we have trees of 300 feet high, and animals of 100 feet long. It is true that though in one sense this contrast may be legitimately drawn, yet in another sense it may not ; sinco these largest organisms are made by the combination of units that are individually like the smallest. A single plant of the genus Protococcus, is of the same structure as one of the many cells united together to form the thallus of some higher Alga, or the leaf of a phsenogam. Each separate shoot of a pliaenogam is usually the bearer of many leaves. And a tree is an assemblage of numerous united shoots. One of these great teleophytes is thus an ag¬ gregate of aggregates of aggregates of units, which sever¬ ally resemble protophytes in their sizes and structures ; and a like building up is traceable throughout a consider¬ able part of the animal kingdom. Even, however, when we bear in mind this qualification, and make our com¬ parisons between organisms of the same degree of compo- 110 THE INDUCTIONS OF BIOLOGY. sition, we still find the limit of growth to have a great range. The smallest branched flowering plant is extremely insignificant by the side of a forest tree ; and there is an enormous difference in bulk between the least and the great¬ est mammal. But on comparing members of the same species, we discover the limit of growth to be much less vari¬ able. Among the Protozoa and Protophyta , each kind has a tolerably constant adult size; and among the most complex organisms, the differences between those of the same kind that have reached maturity, are usually not very great. The compound plants do, indeed, sometimes present marked contrasts between stunted and well-grown individuals ; but the higher animals diverge but inconsiderably from the average standards of their species. On surveying the facts with a view of empirically general¬ izing the causes of these differences, we are soon made aware that by variously combining and conflicting with each other, these causes produce great irregularities of result. It be¬ comes manifest that no one of them can be traced to its consequences, unqualified by the rest. Hence the several statements contained in the following paragraphs, must be taken as subject to mutual modification. Let us consider first, the connexion between degree of growth and complexity of structure. This connexion being involved with many others, becomes apparent only on so averaging the comparisons, as to eliminate differences among the rest. Nor does it hold at all where the conditions are radically dissimilar ; as between plants and animals. But bearing in mind these qualifications, we shall see that organization has a determining influence on increase of mass. Of plants the lowest, classed as Thallogens, usually attain no considerable size. Lichens, Algoe, and Fun¬ gi, count among their numbers but few bulky species : the largest, such as certain Algm found in antartic seas, not serving greatly to raise the average. Though among Acrogens there are some, as the Tree-ferns, which attain a GROWTH. ill considerable height, the majority are but of humble growth. The Endogens, including at one extreme small grasses and at the other tall palms, show us an average and a maximum greater than that reached by the Acrogens. And the En- dogens are exceeded by the Exogens ; among which are found the monarchs of the vegetal kingdom. Pass¬ ing to animals, we meet the fact that the size attained by Vertebrata is usually much greater than the size attained by Invertebrata. Of invertebrate animals the smallest, classed as Protozoa , are also the simplest ; and the largest, be¬ longing* to the Annulosa and Mollmca , are among the most complex of their respective types. Of vertebrate animals we see that the greatest are Mammals ; and that though, in past epochs, there were reptiles of vast bulk, their bulk did not equal that of the whale. Between rep tiles and birds, and between land -vertebrates and aquatic vertebrates, the relation does not hold : the conditions of existence be¬ ing in these cases widely different. But among fishes as a class, and among reptiles as a class, it is observable that, speaking generally, the larger species are framed oil the higher types. The critical reader, who has men¬ tally checked these statements in passing them, has doubtless already seen that this relation is not a dependence of or¬ ganization on growth, but a dependence of growth on or¬ ganization. The majority of Exogens are smaller than some Endogens ; many Endogens are exceeded in size by certain Acrogens ; and even among Thallogens, the least developed of plants, there are kinds of a size which many plants of the highest order do not reach. Similarly among animals : there are plenty of Crustaceans less than Actiniae ; numerous reptiles are smaller than some fish ; the majority of mam¬ mals are inferior in bulk to the largest reptiles ; and in the contrast between a mouse and a well-grown Medusa , we see a creature that is elevated in the scale of organization, ex¬ ceeded in mass by one that is extremely degraded. Clearly then, it cannot be held that high organization is habitually 112 THE INDUCTIONS OF BIOLOGY. accompanied by great size. The proposition here illustrated is the converse one, that great size is habitually accompanied by high organization. The conspicuous fact that the largest species of both animals and vegetals belong to the highest classes ; and that throughout their various sub-classes the higher usually contain the more bulky forms ; shows this connexion as clearly as we can expect it to be shown, amid so many modifying causes and conditions. The relation ^between growth and supply of available nutriment, is too familiar a relation to need proving. There are, however, some aspects of it that must be contemplated be¬ fore its implications can be fully appreciated. Among plants, which are all constantly in contact with the gaseous, liquid, and solid matters to be incorporated with their tissues ; and which, in the same locality, receive not very unlike amounts of light and heat ; differences in the supplies of available nutriment, have but a subordinate connexion with differences of growth. Though in a cluster of herbs spring¬ ing up from the seeds let fall by a parent, the greater size of some than of others is doubtless due to better nutrition, consequent on accidental advantages ; jTet no such inter¬ pretation can be given of the contrast in size between these herbs and an adjacent tree. Other conditions here come into play : one of the most important probably being, an absence in the one case, and presence in the other, of an ability to se¬ crete such a quantity of ligneous fibre as will produce a stem capable of supporting a large growth. Among animals, however, which (excepting some Entozoa) differ from plants in this, that instead of bathing their surfaces, the matters they subsist on are dispersed, and have to be obtained ; the relation between available food and growth^ is shown with more regularity. The Protozoa , living on microscopic fragments of organic matter contained in the surrounding water, are unable, during their brief lives, to accumulate any considerable quantity of nutriment. Polypes and Molluscoida , having for food these scarcely visible mem- GROWTH. 113 bers of the animal kingdom, are, though large compared with their prey, small as measured by other standards : even when aggregated into groups of many individuals, which severally catch food for the common weal, they are often so inconspicuous as readily to be passed over by the unobservant. And if from this point upwards we survey the successive grades of animals, it becomes manifest that, in proportion as the size is great, the masses of nutriment are either large, or, what is practically the same thing, are so abundant and so grouped as that large quantities may be readily taken in. Though, for example, the greatest of mammals, the arctic whale, feeds on such comparatively small creatures as the acalephes and molluscs floating in the seas it inhabits, its method of gulping in whole shoals of them and filtering away the accompanying water, enables it to secure great quantities of food. We may then with safety say, that, other things equal, the growth of an animal depends on the abundance and sizes of the masses of nutriment which its powers enable it to appropriate. Perhaps it may be needful to add that, in interpreting this statement, the number of competitors must be taken into account. Clearly, not the absolute, but the relative, abundance of fit food is the point ; and this relative abundance very much depends on how many individuals are competing for the food. Thus all who have had experience of fishing in Highland lochs, know that where the trout are numerous they are small, and that where they are comparatively large they are compara¬ tively few. What is the relation between growth and expenditure of force ? is a question which next presents itself. Though there is reason to believe such a relation exists, it is not very readily traced : involved as it is with so many other rela¬ tions. Some contrasts, however, may be pointed out, that appear to give evidence of it. Passing over the vegetal kingdom, throughout which the expenditure of force is too small to allow of such a relation being visible ; let us seek in 6 114 THE INDUCTIONS OF BIOLOGY. the animal kingdom, some case where classes otherwise allied, are contrasted in their locomotive activities. Let ns compare birds on the one hand, with reptiles and mammals on the other. It is an accepted doctrine that birds are organized on a type closely allied to the reptilian type, but superior to it ; and though in many respects the organization of birds is inferior to that of mammals, yet in other respects, as in the greater heterogeneity and integration of the skeleton, the more complex development of the respiratory system, and the higher temperature of the blood, it may be held that birds stand above mammals. Hence were growth de¬ pendent only on organization, we might infer that the limit of growth among birds should not be much short of that among mammals ; and that the bird- type should admit of a larger growth than the reptile-type. Again, we see no mani¬ fest disadvantages under which birds labour in obtaining food, but from which reptiles and mammals are free. On the contrary, birds are able to get at food that is fixed beyond the reach of reptiles and mammals ; and can catch food that is too swift of movement to be ordinarily caught by reptiles and mammals. Nevertheless, the limit of growth in birds, falls far below that reached by reptiles and mammals. With what other contrast between these classes, is this contrast connected ? May we not suspect that it is connected with the contrast between their amounts of locomotive exertion ? Whereas mammals (excepting bats, which are small), are during all their movements supported by solid surfaces or dense liquids ; and whereas reptiles (excepting the ancient pterodactyles, which were not very large), are similarly re¬ stricted in their spheres of movement ; the majority of birds move more or less habitually through a rare medium, in which they cannot support themselves without relatively great efforts. The conclusion that there exists this inverse ratio between growth and expenditure of force, is enforced by the significant fact, that those members of the class Aves, as the Dinornis and Epiornis , which approached in size to GROWTH. 115 the larger Mammalia and Heptilia , were creatures incapable of flight — creatures which did not expend this excess of force in locomotion. Further evidence that there is an antagonism between the increase of bulk and the quantity of motion evolved by an organism, is supplied by the ge¬ neral experience, that human beings and domestic animals, when overworked wdiile growing, are prevented from attain¬ ing the ordinary dimensions. One other general truth concerning degrees of growth, must be set down. It is a rule, having exceptions of no great importance, that large organisms commence their separate existences as masses of organic matter more or less considerable in size, and commonly with organizations more or less advanced; and that throughout each organic sub-kingdom, there is a certain general, though irregular, relation between the initial and the final bulks. Yegetals exhibit this relation much less clearly and constantly than animals. Yet though, among the plants that begin life as minute spores, there are some which , under their special conditions, grow to considerable sizes, the immense majority of them remain small. While, conversely, the great Endogens and Exogens, when thrown off from their parents, have already the formed organs of young plants, to which are attached large stores of highly nutritive matter. That is to sajq where the young jflant consists merely of a centre of development, the ultimate growth is commonty insignificant ; but where the growth is to become great, there exists to start with, a well-developed embryo and a stock of assimilable matter. Through¬ out the animal kingdom, this relation is tolerably regular. Save among classes that escape the ordinary requirements of animal life, small germs or eggs do not give rise to bulky creatures. Where great bulk is to be reached, the young proceeds from an egg of considerable bulk, or is born of con¬ siderable bulk ready-organized and partially active. In the class fishes, for instance, a certain average proportion obtains between the sizes of the ova and the sizes of the adult indi- 116 THE INDUCTIONS OF BIOLOGY. dividuals ; and among the highest fishes, as sharks, the eggs are comparatively few and comparatively large. Rep¬ tiles have eggs that are smaller in number, and relatively greater in mass, than those of fishes ; and throughout this class, too, there is a general ratio between the bulk of the egg and the bulk of the adult creature. As a group, birds show us a further limitation in the number of their eggs, and a further increase in their relative sizes ; and from the minute eggs of the humming-bird up to the immense ones of the Epiornis , holding several quarts, we see that, speaking ge¬ nerally, the greater the eggs, the greater the birds. Finally, among mammals (omitting the marsupials) the young are born, not only of comparatively large sizes, but with ad¬ vanced organizations ; and throughout this sub-division of the vertebrata, as throughout .the others, there is a mani¬ fest connexion between the sizes at birth and the sizes at maturity. As having a kindred meaning, there must finally be noted the fact, that the young of these highest animals, besides starting in life with bodies of considerable sizes, almost fully organized, are, during sub¬ sequent periods of greater or less length, supplied with nutri¬ ment — in birds by feeding, and in mammals by suckling and afterwards by feeding. That is to say, beyond the mass and organization directly bequeathed, a bird or mammal obtains a further large mass at but little cost to itself. Were an exhaustive treatment of the topic intended, it would be needful to give a paragraph to each of the many incidental circumstances by which growth may be aided or restricted. Such facts as that an entozoon is limited by the size of the creature, or even the organ, in which it thrives ; that an epizoon, though getting abundant nutriment with¬ out appreciable exertion, is restricted to that small bulk at which it escapes ready detection by the animal it infests ; that sometimes, as in the weazel, smallness is a condition to successful pursuit of the animals preyed upon ; and that at other times, the advantage of resembling certain other crea- GROWTH. 117 tures, and so deceiving enemies or prey, becomes an indirect cause of restricted size. But the present purpose is simply to set down those most general relations between growth and other organic phenomena, which induction leads us to. Having done this, let us go on to inquire whether these general relations can be deductively established. § 44. That there must exist a certain dependence of growth on organization, may be shown d priori. When wo consider the phenomena of Life, either by themselves or in their relations to surrounding phenomena, we see that, other things equal, the larger the aggregate the greater is the needful complexity of structure. In plants, even of the highest type, there is a com¬ paratively small mutual dependence of parts : a gathered flower-bud will unfold and flourish for days, if its stem be immersed in water ; and a shoot cut off from its parent- tree and stuck in the ground, will grow. The respective parts having vital activities that are not widely unlike, it is pos¬ sible for great bulk to be reached without that structural complexity required for combining the actions of parts. Even here, however, we see that for the attainment of great bulk, there requires such a degree of organization as shall co-ordinate the functions of roots and branches — we see that such a size as is reached by trees, is not possible without an efficient vascular system enabling the remote organs to utilize each other’s products. And we see that such a co-existence of large growth with low organization, as occurs in some of the marine Algce, occurs where the conditions of existence do not necessitate any considerable mutual dependence of parts — where the near approach of the plant to its medium in specific gravity, precludes the need of a well -developed stem, and where all the materials of growth being derived from the water by each portion of the thallus, there requires no apparatus for transferring materials from part to part. Among animals which, with but few 118 THE INDUCTIONS OF BIOLOGY. exceptions, are, by tbe conditions of tbeir existence, required to take in nutriment through one specialized part of the body, it is clear that there must be a means whereby other parts of the body, to be supported by this nutriment, must have it conveyed to them. It is clear that for an equally efficient maintenance of their nutrition, the parts of a large mass must have a more elaborate propelling and conducting apparatus ; and that in proportion as these parts undergo greater waste, a yet higher development of the vascular system is necessitated. Similarly with the pre-requisites to those mechanical motions which animals are required to perform. The parts of a mass cannot be made to move, and have their movements so co-ordinated as to produce locomo¬ tive and other actions, without certain structural arrange¬ ments ; and, other things equal, a given amount of such activity requires more involved structural arrangements in a large mass than in a small one. There must at least be a co-ordinating apparatus presenting greater contrasts in its central and peripheral parts. The qualified dependence of growth on organization, is equally implied when we study it in connexion with that adjustment of inner to outer relations which constitutes Life. In plants this is not conspicuous, because the adjustment of inner to outer relations is but small. Still, it is visible in the fact that the condition on which only a plant can grow to a great size, is, that it shall, by the development of a massive trunk, present inner relations of forces fitted to counter¬ balance those outer relations of forces, which tend continually and occasionally to overthrow it ; and this formation of a core of regularly- arranged woody fibres, is an advance in organization. Throughout the animal kingdom, this connexion of phenomena is manifest. To obtain materials for growth ; to avoid injuries, which interfere with growth ; and to escape those enemies which bring growth to a sudden end ; implies in the organism, the means of fitting its movements to meet numerous external co-existences and sequences— GROWTH. 119 implies such, various structural arrangements as shall make possible these variously-adapted actions. It cannot he questioned that, everything else remaining constant, a more complex animal, capable of adjusting its conduct to a greater number of surrounding contingences, will be the better able to secure food and evade damage, and so to increase bulk. And evidently, without any qualification, we may say that a large animal, living under such complex conditions of exist¬ ence as everywhere obtain, is not possible without compara¬ tively high organization. While, then, this relation is traversed and obscured by sundry other relations, it cannot but exist. Deductively we see that it must be modified, as inductively we saw that it is modified, by the circumstances amid wdiich each kind of or¬ ganism is placed ; but that it is always a factor in determin¬ ing the result. § 45. That growth is, ccetcris paribus , dependent on the sup¬ ply of assimilable matter, is a proposition so continually illus¬ trated by special experience, as well as so obvious from general experience, that it would scarcely need stating, were it not re¬ quisite to notice the qualifications with which it must be taken. The materials which each organism requires for building itself up, are not of one kind, but of several kinds. As a vehicle for transferring matter through their structures, all organisms require water as well as solid constituents ; and how¬ ever abundant the solid constituents, there can be no growth in the absence of wrater. Among the solids supplied, there must be a proportion ranging within certain limits. A plant round which carbonic acid, water, and ammonia exist in the right quantities, may yet be arrested in its growth by a deficiency of silica. The total absence of lime from its food, may stop the formation of a mammal’s skeleton : thus dwarfing, if not eventually destroying, the mammal ; and this, no matter what quantities of other needful colloids and crystalloids are furnished. 1?Q THE INDUCTIONS OF BIOLOGY. Again, the truth that, other things equal, growth varies according to the supply of nutriment, has to be qualified by the condition, that the supply shall not exceed the ability to appropriate it. In the vegetal kingdom, the assimilating surface being external, and admitting of rapid expansion by the formation of new roots, shoots, and leaves, the effect of this limitation is not conspicuous : by artificially supplying plants with those materials which they have usually the most difficulty in obtaining, we can greatly facilitate their growth ; and so can produce striking differences of size in the same species. Even here, however, the effect is confined within the limits of the ability to appropriate ; since in the absence of that solar light and heat, by the help of which the chief appropriation is carried on, the additional materials of growth are useless. In the animal kingdom this restriction is rigorous. The absorbent surface being, in the great majority of cases, internal ; having a comparatively small area, which cannot be greatly enlarged without re¬ construction of the whole body ; and being in connexion with a vascular system, which must also be re- constructed before any considerable increase of nutriment can be made available ; it is clear that beyond a certain point, very soon reached, increase of nutriment will not cause increase of growth. On the contrary, if the quantity of nutriment taken in, is greatly beyond the absorbent power, the excess, becoming an obstacle to the regular working of the organism, may retard growth rather than advance it. While then it is certain, d priori , that there cannot be growth in the absence of such substances as those of which an organism consists ; and while it is equally certain that the amount of growth must primarily be governed by the supply of these substances ; it is not less certain that extra supply will not produce extra growth, beyond a point very soon reached. Deduction shows to be necessary, as induction makes familiar, the truths that, the value of food for purposes of growth depends not on the quantity of the various 121 GROWTH. organizable materials it contains, but on the quantity of the material most needed ; that given a right proportion of materials, the pre-existing structure of the organism limits their availability ; and that the higher the structure, the sooner is this limit reached. § 46. But why should the growth of every organism be finally arrested ? Though the rate of increase may, in each case, be necessarily restricted within a narrow range of varia¬ tion — though the increment that is possible in a given time, cannot exceed a certain amount ; yet why should the incre¬ ments decrease, and finally become insensible ? Why should not all organisms, when supplied with sufficient materials, continue to grow as long as they live P To find an answer to this question, we must first revert to the nature and functions of organic matter. In the first three chapters of Part I., it was shown that plants and animals mainly consist of substances in states of unstable equilibrium— substances which have been raised to* this unstable equilibrium by the expenditure of the forces we know as solar radiations, and which give out these forces in other forms, on falling into states of stable equilibrium. Leaving out the water, which serves as a vehicle for these materials and a medium for their changes ; and excluding those mineral matters that play either passive or subsidiary parts ; organisms are built up of compounds which are stores of force. Those complex colloids and crystalloids which, as united together, form organized bodies, are the same colloids and crystalloids which give out, on their decomposition, the forces expended by organized bodies. Thus these nitrogeneous and carbonaceous substances, being at once the materials for organic growth and the sources of organic force ; it results that as much of them as is used up for the genesis of force, is taken away from the means of growth ; and as much as is economized by diminishing the genesis of force, is available for growth. Given that limited quantity 122 THE INDUCTIONS OF BIOLOGY. of nutritive matter which, the pre-existing structure of an organism enables it to absorb ; and it is a necessary corollary from the persistence of force, that the matter accumulated as growth, cannot exceed that surplus which remains unde¬ composed, after the production of the required amounts of sensible and insensible motion. This, which would be rigorously true under all conditions, if exactly the same substances were used in exactly the same proportions, for the production of force and for the formation of tissue, requires, however, to be taken with the qualification, that some of the force- evolving substances are not constituents of tissue ; and that thus, there may be a genesis of force which is not at the expense of potential growth. But since organisms (or at least animal organisms, with wdiich we are here chiefly concerned,) have a certain power of selective absorption, which, partially in an individual and more completely in a race, adapts the proportions of the substances absorbed to the needs of the system ; then if a certain habitual expenditure of force, leads to a certain habitual absorption of force- evolviug matters that are not available for growth ; and if, were there less need for such matters, the ability to absorb matters available for growth would be increased to an equi¬ valent extent ; it follows that the antagonism described, does, in the long run, hold even without this qualification. Hence, growth is substantially equivalent to the absorbed nutriment, minus the nutriment used up in action. This, however, is no answer to the question — why has individual growth a limit ? The antagonism described, does not manifestly account for the fact, that in every domestic animal the increments of growth bear continually decreasing ratios to the mass, and finally come to an end. Nevertheless, it is demonstrable that the excess of absorbed over expended nutriment, must, other things equal, become less as the size of the animal becomes greater. In similarly-shaped bodies, the masses vary as the cubes of the dimensions ; whereas the strengths vary as the squares of the dimensions. See here GROWTH. 123 the solution of the problem. Supposing a creature which a year ago was one foot high, has now become two feet high, while it is unchanged in proportions and structure ; what are the necessary concomitant changes that have taken place in it ? It is eight times as heavy ; that is to say, it has to re¬ sist eight times the strain which gravitation puts on its structure ; and in producing, as well as in arresting, every one of its movements, it has to overcome eight times the inertia. Meanwhile, the muscles and bones have sever¬ ally increased their contractile and resisting powers in pro¬ portion to the areas of their transverse sections ; and hence are severally but four times as strong as they were. Thus, W'hile the creature has doubled in height, and while its ability to overcome forces has quadrupled, the forces it has to overcome have grown eight times as great. Hence, to raise its body through a given space, its muscles have to be contracted with twice the intensity, at a double cost of matter expended. This necessity will be seen still more clearly if we leave out the motor apparatus, and consider only the forces required and the means of supplying them. For since, in similar bodies, the areas vary as the squares of the dimensions, and the masses vary as the cubes ; it follows that the absorbing sur¬ face has become four times as great, while the weight to be moved by the matter absorbed has become eight times as great. If then, a year ago, the absorbing surface could take up twice as much nutriment as was needed for expenditure, thus leaving one-half for growth, it is now able only just to meet expenditure, and can provide nothing for growth. How¬ ever great the excess of assimilation over waste, may be dur¬ ing the early life of an active organism, we see that because a series of numbers increasing as the cubes, overtakes a series increasing as the squares, even though starting from a much smaller number, there must be reached, if the organism lives long enough, a point at which the surplus assimilation is brought down to nothing — a point at which expenditure ba¬ lances nutrition — a state of moving equilibrium. This, 124 THE INDUCTIONS OF BIOLOGY. however, though the chief, is not the sole, varying relation be¬ tween degrees of growth and amounts of expended force. There are two more ; one of which conspires with the last, while the other conflicts with it. Consider in the first place, the cost at which nutriment is distributed through the body, and effete matters removed from it. Each increment of growth being added at the periphery of the organism, the force ex¬ pended in the transfer of matter must increase in a rapid progression — a progression more rapid than that of the mass. But as the dynamic expense of distribution is small compared with the dynamic value of the materials distributed, this item in the calculation is unimportant. Now consider, in the second place, the changing proportion between production and loss of heat. In similar organisms, the quantities of heat generated by similar actions going on throughout their sub¬ stance, must increase as the masses, or as the cubes of the dimensions. Meanwhile, the surfaces from which loss of heat by radiation takes place, increase only as the squares of the dimensions. Though the loss of heat does not therefore in¬ crease only as the squares of the dimensions, it certainly in¬ creases at a smaller rate than the cubes. And to the extent that augmentation of mass results in a greater retention of heat, it effects an economization of force. This advantage is not, however, so important as at first appears. Organic heat is a concomitant of organic action, and is so abundantly pro¬ duced during action, that the loss of it is then of no conse¬ quence : indeed the loss is often not rapid enough to keep the supply from rising to an inconvenient excess. It is only in respect of that maintenance of heat which is needful during quiescence, that large organisms have an advantage over small ones in this relatively diminished loss. Thus these two subsidiary relations between degrees of growth and amounts of expended force, being in antagonism with each other, we may conclude that their differential result does not greatly modify the result of the chief relation previously set forth. Any one who proceeds to test this deduction, will find some GROWTH. 125 seeming incongruities between it and certain facts inductively established. Lest these should mislead him, it will be well to explain them. Throughout the vegetal kingdom, he may remark that there is no limit of growth except what death entails. Passing over a large proportion of plants which never exceed a comparatively small size, because they wholly or partially die down at the end of the year ; and pointing to trees that annually send forth new shoots, even when their trunks are hollowed out by decay ; he may ask — IIow does growth happen here to be unlimited ? The answer is, that plants are only accumulators ; they are in no apprecia¬ ble degree expenders. As they do not undergo a waste which increases as the cubes of the dimensions, while assimilation increases as their squares ; there is no reason why their growth should be arrested by the equilibration of assimilation and waste. Again, should he look among animals for an exact correspondence between the decreasing increments of growth as ascertained by observation and as determined by de¬ duction, he will not find it. And there are sufficient reasons why the correspondence cannot be more than approximate. Besides the fact above noted, that there are other varying relations which complicate the chief one, he must bear in mind that the bodies compared are not truly similar : the proportions of trunk to limbs and trunk to head, vary con¬ siderably. The comparison is still more seriously vitiated by the inconstant ratio between the constituents of which the bodyfis composed. In the flesh of adult mammalia, water forms from 68 to 71 per cent., organic substance from 24 to 28 per cent., and inorganic substance from 3 to 5 per cent.; whereas in the foetal state, the water amounts to 87 per cent., and the solid organic constituents to only 11 per cent. Clearly this change from a state in which the force- evolving matter forms one tenth of the whole, to a state in which it forms two and a half tenths, must greatly interfere with the parallelism between the actual and the theoretical progression. Yet another difficulty may come under his notice. The crocodile 126 THE INDUCTIONS OF BIOLOGY. is said to grow as long as it lives ; and there appears reason to think that some predaceous fishes, such as the pike, do the same. That these animals of comparatively high organ¬ ization, have no definite limits of growth, is, however, an ex¬ ceptional fact due to the exceptional non-fulfilment of those conditions which entail limitation. What kind of life does a crocodile lead? It is a cold-blooded, or almost cold¬ blooded, creature ; that is, it expends very little for the main¬ tenance of heat. It is habitually inert: not chasing prey, but lying in wait for it ; and undergoes considerable exertion only during its occasional brief contests with prey. Such other exertion as is, at intervals, needful for moving from place to place, is rendered small by the small difference between the animal’s specific gravity and that of water. Thus the crocodile expends in muscular action, an amount of force that is insignificant compared with the force commonly expended by land- animals. Hence its habitual assimilation is diminished much less than usual by habitual waste ; and beginning with an excessive disproportion between the two, it is quite possible for the one never quite to lose its advance over the other while life continues. On looking closer into such cases as this and that of the pike, which is similarly cold-blooded, similarly lies in wait, and is similarly able to obtain larger and larger kinds of prey as it increases in size ; we discover a further reason for this absence of a definite limit. The mechanical causes necessitating a limit, are here only partially in action. For a creature living in a medium of nearly the same density as its body, has not constantly to overcome that gravitative force which is the chief resistance to be met by terrestrial animals : it has not to expend for this purpose, a muscular power that is large at the outset, and increases as the cubes of its dimensions. The only force in¬ creasing as the cubes of its dimensions, which it has thus to overcome, is the inertia of its parts. The exceptional con¬ tinuance of growth observed in creatures so circumstanced, is therefore perfectly explicable. GROWTH. 127 § 47. Obviously this antagonism between accumulation and expenditure, must be a leading cause of the contrasts in size between allied organisms that are in many respects similarly conditioned. The life followed by each kind of animal, is one involving a certain average amount of exertion for the obtainment of a given amount of nutriment — an exertion, part of which goes to the gathering or catching of food, part to the tearing and mastication of it, and part to the after¬ processes requisite for separating the nutritive atoms — an exertion which therefore varies according as the food is abund¬ ant or scarce, fixed or moving, according as it is mechani¬ cally easy or difficult to deal with when secured, and accord¬ ing as it is, or is not, readily soluble. Hence, while among animals of the same species having the same mode of life, there will be a tolerably constant ratio between accumulation and expenditure, and therefore a tolerably constant limit of growth ; there is ever}^ reason to expect that different species, following different modes of life, will have unlike ratios be¬ tween accumulation and expenditure, and therefore unlike limits of growth. Though the facts as inductively established, show a general harmony with this deduction, we cannot usually trace this harmony in any specific way ; since the conflicting and con¬ spiring causes which affect growth are so numerous. The only contrast which seems fairly to the point, is the before- named one between the vertebrates which fly, and the most nearly-allied vertebrates which do not fly : the differences in degrees of organization and relations to food, being not such as seriously to affect the comparison. If it be admitted that birds habitually expend more force than mammals and rep¬ tiles, then it will follow a priori , that, other things being tolerably equal, they should have a lower limit of growth than mammals and reptiles ; and this we know to be the fact d posteriori. § 48. One of the chief causes, if not the chief cause, of 128 THE INDUCTIONS OF BIOLOGY. tlie differences between the sizes of organisms, lias yet to be considered. We are introduced to it by pushing the above inquiry a little further. Small animals have been shown to possess an advantage over large ones, in the greater ratio which, other things equal, assimilation bears to expenditure; and we have seen that hence, small animals in becoming largo ones, gradually lose that surplus of assimilative power which they had, and eventually cannot assimilate more than is required to balance waste. But how come these animals while young and small, to have surplus assimilative powers ? Have all animals equal surplus of assimilative powers ? And if not, how far do differences between the surpluses de¬ termine differences between the limits of growth ? W e shall find in the answers to these questions, the interpretation of many marked contrasts in growth that are not due to any of the causes above assigned. For example, an ox immensely exceeds a sheep in mass. Yet the two live from generation to generation in the same fields, eat the same grass and tur¬ nips, obtain these aliments with the same small expenditure of force, and differ scarcely at all in their degrees of organiz¬ ation. Whence arises, then, their striking unlikeness of bulk P We noted when studying the phenomena of growth in¬ ductively, that organisms of the larger and higher types, com¬ mence their separate existences, as masses of organic matter having tolerable magnitudes. Speaking generally, we saw that throughout each organic sub-kingdom, the acquire¬ ment of great bulk occurs only where the incipient bulk and organization are considerable ; and that they are the more considerable in proportion to the complexity of the life which the organism is to lead. The deductive interpretation of this induction may best be commenced by an analogy. A street orange-vendor makes but a trifling profit on each transaction ; and unless more than ordinarily fortunate, he is unable to realize during the day a larger amount than will meet his wants : leav¬ ing him to start on the morrow in the same condition as GROWTH. 129 before. The trade of the huxter in ounces of tea and half- pounds of sugar, is one similarly entailing much labour for small returns. Beginning with a capital of a few pounds, it is impossible for him to have a shop large enough, or goods sufficiently abundant and various, to permit an extensive business : he must be content with the half-pence and pence which he makes by little sales to poor people ; and if, avoid¬ ing bad debts, he is able by strict economy to accumulate anything, it can be but a trifle. A large retail trader is obliged to lay out much money in fitting up an adequate establishment ; he must invest a still greater sum in stock ; and he must have a further floating capital to meet the charges that fall due before his returns come in. Setting out, however, with, means enough for these purposes, he is able to make numerous and comparatively large sales ; and so to get greater and more numerous increments of profit. Similarly, to get returns in thousands, merchants and manu¬ facturers must make their investments in tens of thousands. In brief, the rate at which a man’s wealth accumulates, is measured by the surplus of income over expenditure ; and this, save in exceptionably favourable cases, is determined by the capital with which he begins business. How ap¬ plying the analogy, we may trace in the transactions of an organism, the same three ultimate elements. There is the expenditure required for the obtainment and digestion of food ; there is the gross return in the shape of nutriment as¬ similated, or fit for assimilation ; and there is the difference between this gross return of nutriment and the nutriment that was used up in the labour of securing it — a difference which may be a profit or a loss. Clearly, however, a surplus implies that the force expended is less than the force latent in the assimilated food. Clearly, too, the increment of growth is limited to the amount of this surplus of income over expenditure ; so that large growth implies both that the excess of nutrition over waste shall be relatively considerable, and that the waste and nutrition shall be on extensive scales. 130 THE INDUCTIONS OF BIOLOGY. And clearly, tlie ability of an organism to expend largely and assimilate largely, so as to make a large surplus, presupposes a large physiological capital, in the shape of organic matter more or less complete in its structural arrangements. Throughout the vegetal kingdom, the illustrations of this truth are not conspicuous and regular : the obvious reason being, that since plants are accumulators and in so small a degree expenders, the premises of the above argument are but very partially fulfilled. The food of plants (excepting Fungi and certain parasites) being in a great measure the same for all, and bathing all so that it can be absorbed with¬ out effort, their vital processes result almost entirely in profit. Once fairly rooted in a fit place, a plant may thus from the outset add its entire returns to capital ; and may soon be able to carry on its processes on a large scale, though it does not at first do so. When, however, plants are expenders, namely, during their germination and first stages of growth, their degrees of growth are determined by their amounts of vital capital. It is because the young tree commences life with a ready-formed embryo and store of food sufficient to last for some time, that it is enabled to strike root and lift its head above the surrounding herbage. Throughout the animal kingdom, however, the necessity of this relation is everywhere obvious. The small carnivore preying on small herbivores, can increase in size only by small increments : its organization unfitting it to digest larger creatures, even if it can kill them, it cannot profit by amounts of nutriment ex¬ ceeding a narrow limit ; and its possible increments of growth being small to set out with, and rapidly decreasing, must come to an end before any considerable size is attained. Manifestly the young lion, born of tolerable bulk, suckled un¬ til much bigger, and fed until half- grown, is enabled by the power and organization which he thus gets gratis, to catch and kill animals of size enough to give him the large supply of nutriment needed to meet his large expenditure, and yet leave a large surplus for growth. Thus then is explained GROWTH. 131 the above-named contrast between the ox and the sheep. A calf and a lamb commence their physiological transactions on widely different scales ; their first increments of growth are similarly contrasted in their amounts ; and the two diminish¬ ing series of such increments, end at similarly-contrasted limits. § 49. Such are the several conditions by which the phe¬ nomena of growth are governed. Conspiring and conflicting in endless different ways and degrees, they in every case qualify more or less differently each other’s effects. Hence it happens that we are obliged to state each generalization as true on the average, or to make the proviso — other things equal. Understood, in this qualified form, our conclusions are these. First, that growth being an integration with the organism, of such environing matters as are of like nature with the matters composing the organism, its growth is de¬ pendent on the available supply of such matters : this is alike a truth established by experience, and an inference from the truth given in our forms of thought ( First Principles, § 67). Second, that the available supply of assimilable matter being the same, and other conditions not dissimilar, the degree of growth varies according to the surplus of nutrition over ex¬ penditure — a generalization which is illustrated in some of the broader contrasts between different divisions of organ¬ isms, and is a direct corollary from the persistence of force. Third, that in the same organism, the surplus of nutrition over expenditure is a variable quantity ; and that growth is unlimited or has a definite limit, according as the surplus does or does not progressively decrease. This proj^osition we found on the one hand exemplified by the unceasing growth of organisms that do not expend force ; by the growth, slowly diminishing but never completely ceasing, of organisms that expend comparatively little force ; and by the definitely limited growth of organisms that expend much force ; and 132 THE INDUCTIONS OF BIOLOGY. on the other hand, we found it to follow from a certain rela tive increase of expenditure that necessarily accompanies in crease of bulk, and to be therefore an indirect corollary from the persistence of force. Fourth, that among organisms which are large expenders of force, the size ultimately at¬ tained is, other things equal, determined by the initial size : in proof of which conclusion we have abundant facts, as web. as the d 'priori necessity that the sum-totals of analogous diminishing series, must depend upon the amounts of their initial terms. Fifth, that where the likeness of other cir¬ cumstances permits a comparison, the possible extent of growth depends on the degree of organization : an inference testified to by the larger forms among the various divisions and sub-divisions of organisms ; and inferable d priori from the conditions of existence. CHAPTER 31. DEVELOPMENT.* § 50. Certain general aspects of Development may bo studied apart from any examination of internal structures. These fundamental contrasts between the modes of arrange¬ ment of parts, originating, as they do, the leading external distinctions among the various forms of organization, will be best dealt with at the outset. If all organisms have arisen by Evolution, it is of course not to be expected that such several modes of development can be absolutely demarcated : we may be sure of finding them united by transitional modes. But premising that a classification of modes can but approx¬ imately represent the facts, we shall find our general con¬ ceptions of Development aided by one. Development is primarily central. All organic forms of which the entire history is known, set out with a symmetri¬ cal arrangement of parts round a centre. In organisms of the lowest grade, no other mode of arrangement is ever definitely established ; and in the highest organisms, central development, though subordinate to another mode of de¬ velopment, continues to be habitually shown in the changes of * In ordinary speech, Development is often used as synonymous with Growth. It hence seems needful to say, that Development as here and hereafter used, means increase of structure, and not increase of bulk. It may be added, that the word Evolution, comprehending Growth as well as Development, is to he reserved for occasions when both are implied. 134 THE INDUCTIONS OF BIOLOGY. minute structure. Let us glance at these propositions in the concrete. Leaving out those Rhizopods which are wholly structureless, every plant and animal in its earliest stage, consists of a spherical sac, full of liquid containing organic matter, in which is suspended a nucleated cell, more or less distinct from the rest ; and the first changes that occur in the germ thus constituted, are changes that take place round centres produced by division of the original centre. From this type of structure, the simplest organisms do not depart ; or depart in no definite or conspicuous ways. Among plants, the TJredo and the several tribes of Protococci permanently maintain such a central distribution ; while among animals, it is permanently maintained by crea¬ tures like the Gregarinci , and in a different manner by the Amceba, Actinoplirys , and their allies. In larger organisms, made up chiefly of units that are analogous in structure to these simplest organisms, the formation of units ever continues to take place round points or nuclei ; though the arrangement of these units into groups and wholes may proceed after another method. Central development may be distinguished into unicentral and multicentral; according as the product of the original germ, develops symmetrically round one centre, or develops without subordination to one centre — develops, that is, in subordination to many centres. Unicentral de¬ velopment, as displayed not in the formation of single cells but in the formation of aggregates, is not common. The animal kingdom shows it only in the small group named Thalassicollce : inert, spherical masses of jelly, with scarcely any organization, which are found floating in southern seas. It is feebly represented in the vegetal kingdom by the Vol- vox globator. On the other hand, multicentral devel¬ opment, or development round insubordinate centres, is va¬ riously exemplified in both divisions of the organic world. It is exemplified in two distinct ways, according as the insubor¬ dination among the centres of development is partial or total. DEVELOPMENT. 135 We may most conveniently consider it under tlie heads hence arising. Total insubordination among the centres of development, is shown where the units or cells, as fast as the}r are severally formed, part company and lead independent lives. This, in the vegetal kingdom, habitually occurs among the Proto - phyta; and in the animal kingdom, among the Proto¬ zoa. Partial insubordination is seen in those somewhat advanced organisms, that consist of units which, though they have not separated, have so little mutual depend¬ ence that the aggregate they form is irregular. Among plants, the Thallogens very generally exemplify this mode of development. Lichens, spreading with flat or corrugated edges in this or that direction, as the conditions determine, have no manifest co-ordination of parts. In the Algce, the Nostocs similarly show us an un symmetrical structure. Of Fungi , the sessile and creeping kinds display no further dependence of one part on another, than is implied by their cohesion. And even in such better-organized plants as the Marchantia , the general arrangement shows no reference to a directive centre. Among animals, many of the Sponges may be cited as being thus devoid of that co-ordination implied by symmetry: the Amseba-like units composing them, though they have some subordination to local centres, have no subor¬ dination to a general centre. To distinguish that kind of development in which the whole product of a germ coheres in one mass, from that kind of development in which it does not, Professor Huxley has introduced the words “ con¬ tinuous ” and “ discontinuous and these seem the best fitted for the purpose. Multicentral development, then, is divisible into continuous and discontinuous. F rom central development we pass insensibly to that higher kind of development for which axial seems the most appro¬ priate name. A tendency towards this is vaguely manifested almost everywhere. The great majority even of Protophyta and Protozoa have different longitudinal and transverse di- 136 THE INDUCTIONS OF BIOLOGY. mens ions — have an obscure if not a distinct axial structure. The originally cellular units out of which higher organisms are mainly built up, usually pass into shapes that are subordi¬ nated to lines rather than to points. And in the higher organ¬ isms, considered as wholes, an arrangement of parts in rela¬ tion to an axis is distinct and nearly universal. We see it in the superior orders of Thallogens ; and in all the Acrogens, Endogens, and Exogens. With few exceptions the Ccelente- rata clearly exhibit it ; it is traceable, though less conspicu¬ ously, throughout the Molluscct ; and the Annulosa and Vertebrata uniformly show it with perfect definiteness. This kind of development, like the first kind, is of two orders. The whole germ-product may arrange itself round a single axis, or it may arrange itself round many axes ; the structure may be uniaxial or multiaxial. Each division of the organic kingdom furnishes examples of both these or¬ ders. In such Fungi as exhibit axial development at all, we commonly see development round a single axis. Some of the Algce , as the common tangle, show us this arrange¬ ment. And of the higher plants, many Endogens and small Exogens are uniaxial. Of animals, the advanced are without exception in this category. There is no known ver¬ tebrate in which the whole of the germ-product is not subor¬ dinated to a single axis. In the more fully-organized Annu¬ losa, the like is almost universal ; as it is also in the superior orders of Mollusca. Multiaxial development occurs in most of the plants we are familiar with — every branch of a shrub or tree being an independent axis. But while in the vegetal kingdom, multiaxial development prevails among the highest types; in the animal kingdom, it prevails only among the lowest types. It is extremely general, if not universal, among the Coelenterata ; it is characteristic of the Mollus- coida ; among Molluscs the compound Ascidians exhibit it ; and it is seen, though under another form, in the inferior Annulosa. Development that is axial, like development that is central, DEVELOPMENT. 137 may be either continuous or discontinuous : the parts having different axes may continue united, or they may separate. Instances of each alternative are supplied by both plants and animals. Continuous, multiaxial development, is that which plants usually display ; and need not be illustrated further than by reference to every garden. As cases of it in animals may be named, all the compound Ilydrozoa and Ac- tirnzoa; and such molluscous forms as the Botryllidce. Of multiaxial development that is discontinuous, a familiar instance among plants exists in the common strawberry. This sends out over the neighbouring surface, long slender shoots, bearing at their extremities buds that presently strike roots, and become new individuals ; and these by and by lose their connexions with the original axis. Other plants there are that produce certain specialized buds called bulbils, which separating themselves and falling to the ground, grow into independent plants. Among animals the fresh- water potype very clearly shows this mode of development : the young polypes, budding out from its surface, severally arrange their parts around distinct axes, and eventually detaching themselves, lead separate lives, and produce other polypes after the same fashion. By some of the lower Annulosa , this multiplication of axes from an original axis, is carried on after a different manner : the string of segments spontaneously divides ; and after farther growth, division recurs in one or both of the halves. And in the Aphides , we have a still fur¬ ther modification of this process. Grouping together its several modes as above delineated, we see that Development is 1 r* Unicentral / Central * [ or , r Continuous 1 < or f . < ^ Multicentral < Uniaxial or <■ Discontinuous v Axial < | °F . < f Continuous L Multiaxial * \ or ^ Discontinuous 133 THE INDUCTIONS OF BIOLOGY. Any one adequately acquainted with, the facts, may readily raise objections to this arrangement. He may name forms which do not obviously come under any of these heads. He may point to plants that are for a time multicentral, hut after¬ wards develop axially. And from lower types of animals, he hot choose many in which the continuous and discontinuous modes are both displayed. But, as already hinted, an ar¬ rangement free from such anomalies must he impossible, if the various orders of organization have arisen by Evolution. The one above sketched out, is to be regarded as only a rough grouping of the facts, which helps us to a conception of them in their totality ; and so regarded, it will be of service w7hen vre come to treat of Individuality and Heproduction. § 51. From these most general external aspects of organic development, let us now turn to its internal and more special aspects. When treating of Evolution as a universal process of things, a rude outline of the course of structural changes in organisms was given ( First Principles, §§ 43, 55, 56). Here, however, it will be proper to describe these changes more fully. The bud of any common plant in its earliest stage, consists of a small hemispherical or sub-conical projection. While it increases most rapidly at the apex, this presently deve¬ lops on one side of its base, a smaller projection of like general shape with itself. Here is the rudiment of a leaf ; which pre¬ sently spreads more or less round the base of the central hemisphere or main axis. At the same time that the central hemisphere rises higher, this lateral prominence, also in¬ creasing, gives rise to subordinate prominences or lobes. These are the rudiments of stipules, wrhere the leaves aro stipulated. Meanwhile, towards the other side of the main axis, and somewhat higher up, another lateral prominence arising, marks the origin of a second leaf. By the time that the first leaf has produced another pair of lobes, and the second leaf has produced its primary pair, the central hemi¬ sphere, still increasing at its apex, exhibits the rudiment of a DEVELOPMENT. 139 third leaf. Similarly throughout. While the germ of each succeeding leaf thus arises, the germs of the previous leaves, in the order of their priority, are changing their rude nodu¬ lated shapes into fiattened-out expansions ; which slowly put on those sharp outlines they show when unfolded. Thus from that extremely indefinite figure, a rounded lump, giving- off from time to time lateral lumps, which severally becoming symmetrically lobed, gradually assume specific and involved forms, we pass little by little to that comparatively complex thing — a leaf-bearing shoot. Internally, a bud under¬ goes analogous changes. The layer of substance which forms the surface of the hemisphere, and in which these metamor¬ phoses commence, consists of a transparent, irregularly-aggre¬ gated mass of cells and centres of growth, not formed into a tissue. Especially is this the case at the apex, where the vital activity is the greatest. Here the primitive cellular mass passes without any line of demarcation into the tissues that are developing from it. While, by continued cell-multi¬ plication this layer increases, and doing so most rapidly at the apex thrusts outwards its lateral portions, these begin to exhibit differentiations. “ Gradually,” sa}Ts Schleiden, “ se¬ parate masses of cells, with a distinct and definite outline, appear in this chaos, and they cease to partake of the process of growth going on. At first the epidermis is separated, then the vascular bundles, later the parenchyma.” Similarly with the lateral buds whence leaves arise. In the, at first, un¬ organized mass of cells constituting the rudimentary leaf, there are formed vascular bundles which eventually become the veins of the leaf ; and gradually there appear also, though in ways that have not been specified, the parenchyma and the epithelium. Nor do we fail to find an essentially parallel set of changes, when we trace the histories of the in¬ dividual cells. While the tissues they compose are separ¬ ating, the cells are growing step by step more unlike. Some become flat, some polyhedral, some cylindrical, some prismatic, some spindle-shaped. These develop spiral fibres 140 THE INDUCTIONS OF BIOLOGY. in their interiors ; and those, net-works of fibres. Here a number of cells unite together to form a tube ; and there they become solid by the internal deposition of woody or other matter. Through such changes, too numerous and involved to be here detailed, the originally uniform cells go on diverg¬ ing and re-diverging, until there are produced various forms that seem to have very little in common. The arm of a man makes its first appearance in as simple a way as does the shoot of a plant. According to BischofF, it buds-out from the side of the embryo, as a little tongue-shaped projection, presenting no differences of parts ; and it might serve for the rudiment of some one of the various other organs that also arise as buds. C ontinuing to lengthen, it presently becomes somewhat enlarged at its end ; and is then described as a pedicle bearing a flattened, round- edged lump. This lump is the representative of the future hand ; and the pedicle, of the future arm. By and by, at the edges of this flattened lump, there appear four clefts, dividing from each other the buds of the future fingers ; and the hand as a whole grows a little more distinguishable from the arm. Up to this time, the pedicle has remained one continuous piece ; but it now begins to show a bend at its centre, which indicates the division into arm and forearm. The distinctions thus rudely indicated, gradually increase : the fingers elongate and become jointed ; and the proportions of all the parts, originally very un¬ like those of the complete limb, slowly approximate to them. During its bud-like stage, the rudimentary arm is nothing but a homogeneous mass of simple cells, with¬ out any arrangement. By the diverse changes they gradually undergo, these cells are transformed into bones, muscles, blood-vessels, and nerves. The extreme softness and delicacy of this primary cellular tissue, renders it difficult to trace the initial stages of these differentiations. In consequence of the colour of their contents, the blood-vessels are the first parts to become visible. Afterwards the cartilaginous parts, which are the bases of the future bones, become marked out by the DEVELOPMENT. 141 denser aggregation of their constituent cells, and the produc¬ tion between these of a hyaline substance which unites them into a translucent mass. When first perceptible, the muscles are gelatinous, pale, yellowish, transparent, and indistinguish¬ able from their tendons. The various other tissues of which tie arm consists, beginning with very faintly- marked differ¬ ences, become day by day more definite in their outlines and appearances. In like manner, the units composing these tissues, severally assume increasingly-specific characters. The fibres of muscle, at first made visible in the midst of their gelatinous matrix only by immersion in alcohol, grow more numerous and distinct ; and by and by they begin to exhibit transverse stripes. The bone-cells put on by degrees tneir curious structure of branching canals. And so in their respective ways with the units of skin and the rest. Thus in each of the organic sub-kingdoms, we see this change from an incoherent, indefinite homogeneity, to a coherent, definite heterogeneity, illustrated in a quadruple way. The originally -like units or cells, become unlike in various ways, and in ways more numerous and marked as the development goes on. The several tissues which these several classes of cells form by aggregation, grow little by little distinct from each other ; and little by little put on those structural complexities, that arise from differentiations among their component units. In the shoot, as in the limb, the external form, originally very simple, and having much in common with countless simple forms, organic and in¬ organic, gradually acquires an increasing complexity, and an increasing unlikeness to other forms. And meanwhile, the remaining parts of the organism to which the shoot or limb belongs, having been severally assuming structures divergent from each other and from that of this particular shoot or limb, there has arisen a greater heterogeneity in the organ¬ ism as a whole. § 52. One of the most remarkable inductions of embry- 142 THE INDUCTIONS OF BIOLOGY. ology comes next in order. Yon Baer found that in its earliest stage, every organism has the greatest number of characters in common with all other organisms in their earliest stages ; that at a stage somewdiat later, its structure is like the structures displayed at corresponding phases by a less extensive multitude of organisms ; that at each sub* sequent stage, traits are acquired which successively distin¬ guish the developing embryo from groups of embryos that it previously resembled — thus step by step diminishing the group of embryos which it still resembles ; and that thus the class of similar forms, is finally narrowed to the species of which it is a member. This abstract proposition will per¬ haps not be fully realized by the general reader. It will be best to re-state it in a concrete shape. The germ out of which a human being is evolved, differs in no visible respect from the germ out of which every animal and plant is evolved. The first conspicuous structural change undergone by this human germ, is one characterizing the germs of animals only — differentiates them from the germs of plants. The next distinction established, is a distinction exhibited by all Vertebrata; but never exhibited by Annulosa , Mollusca, or Ccelenterata. Instead of continuing to resemble, as it now does, the rudiments of all fishes, reptiles, birds, and mammals ; this rudiment of a man, assumes a structure that is seen only in the rudiments of mammals. Later, the embryo undergoes changes which exclude it from the group of implacental mammals ; and prove that it belongs to the group of placental mammals. Later still, it grows unlike the embryos of those placental mammals distinguished as ungulate or hoofed ; and continues to resemble only the unguiculate or clawed. By and by, it ceases to be like any foetuses but those of the quacl- rumana ; and eventually the foetuses of only the higher quadrumana are simulated. Lastly, at birth, the infant, belonging to whichever human race it may do, is structurally very much like the infants of all other human races; and only afterwards acquires those various minor peculiarities of DEVELOPMENT. U3 form tliat distinguish the variety of man to which it be¬ longs. The generalization here expressed and illustrated, must not be confounded with an erroneous semblance of it that has obtained considerable currency. An impression has been given by those who have popularized the statements of em¬ bryologists, that during its development, each higher organ¬ ism passes through stages in which it resembles the adult forms of lower organisms — that the embryo of a man is at one time like a fish, and at another time like a reptile. This is not the fact. The fact established is, that up to a certain point, the embryos of a man and a fish continue similar, and that then differences begin to appear and increase — the one embryo approaching more and more towards the form of a fish ; the other diverging from it more and more. And so with the resemblances to the more advanced types. Suppos¬ ing the germs of all kinds of organisms to be simultaneously developing, we may say that all members of the vast mul¬ titude take their first steps in the same direction ; that at the second step one-half of this vast multitude diverges from the other half, and thereafter follows a different course of deve¬ lopment ; that the immense assemblage contained in either of these divisions, very soon again shows a tendency to take two or more routes of development ; that each of the two or more minor assemblages thus resulting, shows for a time but small divergences among its members, but presently again divides into groups which separate ever more widely as they progress ; and so on, until each organism, when nearly com¬ plete, is accompanied in its further modifications only by organisms of the same species ; and last of all, assumes the peculiarities which distinguish it as an individual — diverges to a slight extent to the organisms it is most like. The reader must also be cautioned against accepting this general¬ ization as exact. The likenesses thus successively displayed are not precise but approximate. Only leading characteris¬ tics are the same : not all the details. It is as though in 144 TIIE INDUCTIONS OF BIOLOGY. one of the diverging groups just described, each kind of organism, though haying a general direction of development like that of the others it is for a time travelling with, shows from the first a tendency to leave the general route — a tend¬ ency which presently becomes strongly marked. Making all requisite qualifications, however, these resemblances re¬ main conspicuous ; and the fact that they follow each other in the way described, is a fact of great significance. § 53. This comparison between the course of development in any creature, and the course of development in all other creatures — this arrival at the conclusion that the course of development in each, at first the same as in all others, be¬ comes stage b}r stage differentiated from the courses of all others, brings us within view of an allied conclusion. If wo contemplate the successive stages passed through by any higher organism, and observe the relation between it and its environment at each of these stages ; we shall see that this re¬ lation is modified in. a way analogous to that in which the relation between the organism and its environment is modi¬ fied, as we advance from the lowest to the highest grades. Along with the progressing differentiation of each organism from others, we find a progressing differentiation of it from its environment ; like that progressing differentiation from the en vironment which we meet with in the ascending forms of life. Let us first glance at the way in which the ascending forms of life exhibit this progressing differentiation from the environment. In the first place, it is illustrated in structure. Ad¬ vance from the homogeneous to the heterogeneous, itself in¬ volves an increasing distinction from the inorganic world. In the lowest Protozoa we have a simplicity approaching to that of air, water, or earth ; and the ascent to organisms of greater and greater complexity of structure, is an ascent to organisms that are in that respect more strongly contrasted with the structureless environment. In form, again, DEVELOPMENT. ] 4 5 wo see the same fact. An ordinary characteristic of inor¬ ganic matter is its indefiniteness of form ; and this is also a characteristic of the lower organisms, as compared with the higher. Speaking generally, plants are less definite than animals, both in shape and size — admit of greater modifica¬ tions from variations of position and nutrition. Among ani¬ mals, the simplest Hhizopods are not only structureless but amorphous : the form is never specific, and is constantly changing. Of the organisms resulting from the aggregation of such creatures, we see that while some, as the Foramini- fera , assume a certain definiteness of form, in their shells at least ; others, as the Sponges, are very irregular. The Zoo¬ phytes and the Polyzoa are compound organisms, most of which have a mode of growth not more determinate than that of plants. But among the higher animals, we find not only that the mature shape of each species is very definite, but that the individuals of each species differ very little in size. A parallel increase of contrast is seen in chemi¬ cal composition. With but few exceptions, and those only partial ones, the lowest animal and vegetal forms are inhabit¬ ants of the water ; and water is almost their sole constituent. Desiccated Protophyta and Protozoa shrink into mere dust ; and among the Acalephes, we find but a few grains of solid matter to a pound of water. The higher aquatic plants, in common with the higher aquatic animals, possessing as they do increased tenacity of substance, also contain a greater pro¬ portion of the organic elements ; and so are chemically more unlike their medium. And when we pass to the superior classes of organisms — land-plants and land-animals — we find that, chemically considered, they have little in common either with the earth on which they stand or the air which sur¬ rounds them. In specific gravity too, we may note the like truth. The very simplest forms, in common with the spores and gemmules of higher ones, are as nearly as may be of the same specific gravity as the water in which they Boat ; and though it cannot be said that among aquatic 146 THE INDUCTIONS OF BIOLOGY. creatures, superior specific gravity is a standard of general superiority, }Tet we may fairly say that the superior orders of them, when divested of the appliances by which their specific gravity is regulated, differ more from water in their relative weight than do the lowest. In terrestrial organisms, the contrast becomes extremely marked. Trees and plants, in common with insects, reptiles, mammals, birds, are all of a specific gravity considerably less than that of the earth and immensely greater than that of the air. Yet further, we see the law similarly fulfilled in respect of temperature. Plants generate but extremely small quantities of heat, which are to be detected only by very delicate experiments ; and practically they may be considered as having the same tem¬ perature as their environment. The temperature of aquatic animals is very little above that of the surrounding water : that of the invertebrata being mostly less than a degree above it, and that of fishes not exceeding it by more than two or three degrees ; save in the case of some large red-blooded fishes, as the tunny, which exceed it in temperature by nearly ten degrees. Among insects, the range is from two to ten degrees above that of the air : the excess varying according to their activity. The heat of reptiles is from four to fifteen degrees more than the heat of their medium. While mam¬ mals and birds maintain a heat which continues almost un¬ affected by external variations, and is often greater than that of the air by seventy, eighty, ninety, and even a hundred degrees. Once more, in greater self-mobility a pro¬ gressive differentiation is traceable. The especial character¬ istic by which we distinguish dead matter is its inertness : some form of independent motion is our most general test of life. Passing over the indefinite border-land between the animal and vegetal kingdoms, we may roughly class plants as organisms which, while they exhibit that species of motion implied in growth, are not only devoid of locomotive power, but with some unimportant exceptions are devoid of the power of moving their parts in relation to each other ; and DEVELOPMENT. JA 4/ tlius are less differentiated from tlie inorganic world than animals. Though in those microscopic Protophyta and Pro¬ tozoa inhabiting the water — the spores of algoe, the gemmulcs of sponges, and the infusoria generally — we see locomotion produced by ciliary action ; yet this locomotion, while rapid relatively to the size of the creatures, is absolutely slow. Of the Coelcnterata , a great part are either permanently rooted or habitually stationary ; and so have scarcely any self-mobility but that implied in the relative movements of parts ; while the rest, of which the common jelly-fish will serve as a sam¬ ple, have mostly but little ability to move themselves through the water. Among the nigher aquatic Invertebrata , — cuttle¬ fishes and lobsters, for instance, — there is a very considerable power of locomotion ; and the aquatic Vcrtebrata are, con¬ sidered as a class, much more active in their movements than the other inhabitants of the water. Hut it is only when we come to air-breathing creatures, that we find the vital charac teristic of self- mobility manifested in the highest degree. Flying insects, mammals, birds, travel with a velocity far exceeding that attained by any of the lower classes of ani¬ mals ; and so are more strongly contrasted with their inert environment. . Thus, on contemplating the various grades of organisms in their ascending order, we find them more and more distinguished from their inanimate media, in structure, in form, in chemical composition, in specific gravity, in temperature , in self -mobility . It is true that this general¬ ization does not hold with complete regularity. Organisms which are in some respects the most strongly contrasted with the environing inorganic world, are in other respects less so than inferior organisms. As a class, mammals are higher than birds ; and yet they are of lower temperature, and have smaller powers of locomotion. The stationary oyster is of higher organization than the free-swimming medusa ; and the cold-blooded and less heterogeneous fish, is quicker in its movements than the warm-blooded and more heterogeneous sloth. But the admission that the several aspects under 148 THE INDUCTIONS OF BIOLOGY. wliicli this increasing contrast shows itself, bear variable ratios to each other, does not conflict with the general truth, that as we ascend in the hierarchy of organisms, we meet with not only an increasing differentiation of parts, but also an increasing differentiation from the surrounding medium in sundry other physical attributes. It would seem that this peculiarity has some necessary connexion with superior vital manifestations. One of those lowly gelatinous forms, so transparent and colourless as to be with difficulty dis¬ tinguished from the water it floats in, is not more like its medium in chemical, mechanical, optical, thermal, and other properties, than it is in the passivity with which it sub¬ mits to all the influences and actions brought to bear upon it ; while the mammal does not more widely differ from inanimate things in these properties, than it does in the ac¬ tivity with which it meets surrounding changes by compens¬ ating changes in itself. And between these tvro extremes, we shall observe a constant ratio between these two kinds of contrast. Whence wre may say, that in proportion as an organism is physically like its environment, does it remain a passive partaker of the changes going on in its environment ; while in proportion as it is endowed with powers of counter¬ acting such changes, it exhibits greater unlikeness to its en¬ vironment.* If now7, from this same point of view7, we consider the rela¬ tion borne to its environment by any superior organism in its successive stages, we find an analogous series of con¬ trasts. Of course in respect of degrees of structure, the parallelism is complete. The difference, at first small, be¬ tween the comparatively structureless germ and the com¬ paratively structureless inorganic world, becomes necessarily greater, step by step, as the differentiations of the -germ be¬ come more numerous and definite. IIow of form the like holds, is equally manifest. The sphere, wdiich is * This paragraph originally formed part of a review-article on “ Transcenden¬ tal Physiology,” published in 1857 DEVELOPMENT. j.jQ the point of departure common to all organisms, is the most generalized of figures ; and one that is, under various circum¬ stances, assumed by inorganic matter. "While the incipient organism is spherical, it is not only like many particular in¬ organic masses ; but it is like the rest, in the sense that it has the shape which would result, were all their irregularities averaged. But as it develops, it loses all likeness to inor¬ ganic objects in the environment ; and eventually becomes distinct even from all organic obiects in its environ- ment. In specific gravity , the alteration, though not very marked, is still in the same direction. Development being habitually accompanied by a relative decrease in the quantity of water, and an increase in the quantity of consti¬ tuents that are heavier than water, there results a small au«>- mentation of relative weight. In power of maintain¬ ing a temperature above that of surrounding things, the differentiation from the environment that accompanies deve¬ lopment, is marked. All ova are absolutely dependent for their heat on external sources. Like inorganic bodies, they gain or lose heat according as neighbouring bodies are colder or hotter. The mammalian young is, during its uterine life, dependent on the maternal heat ; and at birth has but a par¬ tial power of making good the loss by radiation. But as it advances in development, it gains an ability to maintain a constant temperature above that of surrounding things : so becoming markedly unlike all surrounding things, save or¬ ganisms of allied nature. Lastly, in self-mobility this increasing contrast is not less decided. Save in a few aber¬ rant tribes, chiefly parasitic, we find the general fact to be, that the locomotive power, totally absent or very small at the outset, increases with the advance towards maturity. The more highly developed the organism becomes, the stronger grows the contrast between its activity and the inertness of the objects amid which it moves. Thus we may say that the development of an individual organism, is at the same time a differentiation of its parts 150 THE INDUCTIONS OF BIOLOGY. from cadi other, and a differentiation of the consolidated whole from the environment ; and that in the last as in the first respect, there is a general analogy between the progres¬ sion of an individual organism, and the progression from the lowest orders of organisms to the highest orders. It may be remarked that some kinship seems to exist between these generalizations and the doctrine of Schelling, that Life is the tendency to individuation. For evidently, in becom¬ ing more distinct from each other, and from their environ¬ ment, organisms acquire more marked individualities. As far as I can gather from outlines of his philosophy, however, it appears that Schelling entertained this conception in a general and transcendental sense, rather than in a special and scientific one. § 54. The deductive interpretations of these general facts of development, in so far as they are at present possible, must be postponed until we arrive at the fourth and fifth divisions of this work ; which will be chiefly occupied with them. There are, however, one or two general aspects of these inductions, which may be here most conveniently dealt with deductively. The general law of development as displayed in organisms, is readily shown to be necessary, if the initial and terminal stages are such as we know them to be. Grant that each organism is at the outset homogeneous, and that when com¬ plete it is relatively heterogeneous ; and of necessity it fol¬ lows that development is a change from the homogeneous to the heterogeneous — a change during which there must be gone through all the infinitesimal gradations of heterogeneity that lie between these extremes. If, again, there is at first indefiniteness, and at last definiteness, the transition cannot but be from the one to the other of these, through all intermedi- ate degrees of definiteness. F urther, if the parts, originally incoherent or uncombined, eventually become relatively co¬ herent or combined ; there must be a continuous increase of • coherence or combination. Hence the general truth that DEVELOPMENT. 151 development is a change from incoherent, indefinite homo¬ geneity, to coherent, definite heterogeneity, becomes a self- evident one, when observation has shown us the state in which organisms begin, and the state in which they end. Just in the same wray that the growth of an entire organ¬ ism, is carried on by abstracting from the environment substances like those composing the organism ; so the pro¬ duction of each organ wfithin the organism, is carried on by abstracting from the substances contained in the organism, those required by this particular organ. Each organ at the expense of the organism as a whole, integrates with itself certain special kinds and proportions of the matters circulat¬ ing around it ; in the same way that the organism as a vdiole, integrates with itself certain special kinds and propor¬ tions of matters at the expense of the environment as a whole. So that the organs are qualitatively differentiated from each other, in a way analogous to that by which the en¬ tire organism is qualitatively differentiated from things around it. Evidently this selective assimilation illustrates the general truth, demonstrable ci priori, that like units tend to segregate. It illustrates, moreover, the further aspect of this general truth, that the pre-existence of a mass of certain units, produces, probably by polar attraction, a tendency for diffused units of the same kind to aggregate with this mass, rather than elsewhere. It has been shown of particular salts, A and B, co-existing in a solution not suf¬ ficiently concentrated to crystallize, that if a crystal of the salt A be put inf o the solution, it will increase by uniting with itself the dissolved atoms of the salt A ; and that similarly, though there otherwise takes place no deposition of the salt B, yet if a crystal of the salt B is placed in the solution, it will exercise a coercive force on the diffused atoms of this salt, and grow at their expense. Ao doubt much organic assimilation occurs in the same way. Particular parts of the organism are com¬ posed of special units, or have the function of secreting special units, which are ever present in them in large quan- 152 TIIE INDUCTIONS OF BIOLOGY. titles. The fluids circulating through the body contain special units of this same order. And these diffused units are continually being deposited along with the groups of like units that already exist. IIow purely physical are the causes of this selective assimilation, is, indeed, conclusively shown by the fact, that abnormal constituents of the blood are segregrated in the same way. Cancer-cells having begun to be deposited at a particular place, continue to be deposited at that place. Tubercular matter, making its appearance at particular points, collects more and more round those points. And similarly in numerous pustular diseases. Where the component units of an organ, or some of them, do not exist as such in the circulating fluids, but are formed out of elements or compounds that exist separately in the circulat¬ ing fluids ; it is clear that the process of differential assimil¬ ation is of a more complex kind. Still, however, it seems not impossible that it is carried on in an analogous way. If there be an aggregate of compound atoms, each of which contains the constituents A, B, C ; and if round this aggre¬ gate the constituents A and B and C are diffused in uncoin- bined states ; it may be suspected that the coercive polar force of these aggregated compound atoms A, B, C, may not only bring into union with themselves adjacent compound atoms A, B, C, but may cause the adjacent constituents A and B and C to unite into such compound atoms, and then aggre¬ gate with the mass. Should this be so, the process of differ¬ ential assimilation, which plaj^s so important a part in organic development, will not be difficult to understand. At present, however, chemical inquiry appears to have furnished no evidence either for or against such an hypothesis. CHAPTER III. FUNCTION. § 55. Does Structure originate Function, or does Func¬ tion originate Structure P is a question about which there ha3 been disagreement. Using the word Function in its widest signification, as the totality of all vital actions, the question amounts to this — does Life produce Organization, or does Organization produce Life P To answer this question is not easy, since we habitually find the two so associated that neither seems possible without the other ; and they appear uniformly to increase and decrease together. If it be said that the arrangement of or¬ ganic substances in particular forms, cannot be the ultimate cause of vital changes, which must depend on the properties of such substances ; it may be replied that, in the absence of structural arrangements, the forces evolved cannot be so directed and combined as to secure that correspondence between inner and outer actions which constitutes Life. Again, to the allegation that the vital activity of every germ whence an organism arises, is obviously antecedent to the development of its structures ; there is the answer that such germ is not absolutely structureless, but consists of a mass of cells, containing a cell that differs from the rest, and initiates the developmental changes. There is, however, one fact implying that Function must be regarded as taking pre¬ cedence of Structure. Of the lowest Iihizopods, which pre- 154: THE INDUCTIONS OF BIOLOGY. sent no distinctions of parts, and nevertheless feed and grow and move about, Prof. Huxley has remarked that they exhibit Life without Organization. The perpetual changes of form which alone distinguish one of these creatures from an inanimate fragment, are no doubt totally irregular and un¬ directed. Still they do, through an average of accidents, subserve the creatures’ nutrition ; and they do imply an ex¬ penditure of force that in some way depends on the consump¬ tion of nutriment. They do, therefore, though in the rudest way, display a vital adjustment of internal to external relations. § 56. Function falls into divisions of several kinds, ac¬ cording to our point of view. Let us take these divisions in the order of their simplicity. Under Function in its widest sense, are included both the statical and the dynamical distributions of force which an organism opposes to the forces brought to bear on it. In a tree, the woody core of trunk and branches, and in an animal, the skeleton, internal or external, may be regarded as pas¬ sively resisting the gravity and momentum which tend habitually or occasionally to derange the requisite relations between the organism and its environment ; and since they resist these forces simply by their cohesion, their functions may be classed as statical. Conversely, the leaves and sap- vessels in a tree, and those organs which in an animal similarly carry on nutrition and circulation, as well as those which generate and direct muscular motion, must be con¬ sidered as dynamical in their actions. From another point of view. Function is divisible 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. In plants we see little beyond the first of these : expenditure being inappreciable, and transfer required only to facilitate FUNCTION. 155 accumulation. In animals, the function of accumulation comprehends those processes by which the materials contain¬ ing latent force are taken in, digested, and separated from other materials ; the function of transfer comprehends those processes by which these materials, and such others as are needful to liberate the forces they contain, are conveyed throughout the organism ; and the function of expenditure comprehends those processes by which the forces are liberated from these materials, and transformed into properly co-ordin¬ ated motions. Each of these three most general divisions, includes several more special divisions. The accu¬ mulation of force may be separated into alimentation and aeration ; of wdiich the first is again separable into the various acts gone through between prehension of food and the transformation of part of it into blood. By the transfer of force is to be understood what we call circulation ; if the meaning of circulation be extended to embrace the duties of both the vascular system and the lymphatics. Under the head of expenditure of force, come nervous actions and mus¬ cular actions : though not absolutely co-extensive with ex¬ penditure, these are almost so. Lastly, there are the subsidiary functions which do not properly fall within any of these general functions, but subserve them by removing the obstacles to their performance : those, namely, of ex¬ cretion and exhalation , whereby waste products are got rid of. Again, disregarding their purposes and considering them analytically, the general physiologist may consider functions in their widest sense as the correlatives of tissues — the actions of epidemic tissue, cartilaginous tissue, clastic tissue, connective tissue, osseous tissue, muscular tissue, nervous tissue, glandular tissue. Once more, physiology in its concrete interpretations, recognizes special functions as the ends of special organs — regards the teeth as having the office of mastication ; the heart as an apparatus to propel blood ; this gland as fitted to produce one requisite 156 TIIE INDUCTIONS OF BIOLOGY. secretion and that to produce another ; each muscle as the agent of a particular motion ; each nerve as the vehicle of a special sensation or a special motor impulse. It is clear that dealing with Biology only in its larger aspects, specialities of function do not concern us ; except in so far as they serve to illustrate, or to qualify, its general¬ ities § 57. The first induction to be here set down, is a familiar and obvious one : the induction, namely, that com¬ plexity of function, is the correlative of complexity of struc¬ ture. The leading aspects of this truth must be briefly noted. Where there are no distinctions of structure, there are no distinctions of function. One of the Ehizopods above instanced as exhibiting life without organization, will serve as an illustration. From the outside of this creature, which has not even a limiting membrane, there are protruded numerous thread-like processes. Originating from any point of the surface, each of these may contract again and disap¬ pear ; or it may touch some fragment of nutriment, which it draws with it, when contracting, into the general mass — thus serving1 as hand and mouth : or it mav come in contact with its fellow-processes at a distance from the body, and become confluent with them ; or it may attach itself to an adjacent fixed object, and help by its contraction to draw the body into a new position. In brief, this structureless speck of animated jelly, is at once all stomach, all skin, all mouth, all limb, and doubtless, too, all lung. In organisms having a fixed distribution of parts, there is a concomitant fixed distribution of actions. Among plants we see that when, instead of a uniform tissue like that of the Algos , everywhere devoted to the same process of assimilation, there arise, as in the Exogens, root and stem and leaves, there arise correspondingly unlike processes. Still more con¬ spicuously among animals, do there result varieties of function when the originally homogeneous mass is replaced by lietero- FUNCTION. 157 geneous organs; since both, singly and by tlicir combinations, do modified parts generate modified changes. Up to the highest organic types, this dependence continues mani¬ fest ; and it may be traced not only under this most general form, but also under the more special form, that in animals haying one set of functions developed to more than usual heterogeneity, there is a correspondingly heterogeneous ap¬ paratus devoted to them. Thus among birds, which have more varied locomotive powers than mammals, the limbs are more widely differentiated ; while mammals, which rise to more numerous and more involved adjustments of inner to outer relations than birds, have more complex nervous systems. § 58. It is a generalization almost equally obvious with the last, that functions, like structures, arise by progressive differentiations. Just as an organ is first an indefinite rudi¬ ment, having nothing but some most general characteristic in common with the form it is ultimately to take ; so a function begins as a kind of action that is like the kind of action it will eventually become, only in a very vague way. And in functional development, as in structural development, the leading trait thus early manifested, is followed success¬ ively by traits of less and less importance This holds equally throughout the ascending grades of organisms, and throughout the stages of each organism. Let us look at cases : confining our attention to animals, in which func¬ tional development is better displayed than in plants The first differentiation established, separates the two fundamentally- opposed functions above named — the accumu¬ lation of force and the expenditure of force. Passing over the, (Protozoa among which, however, such tribes as present fixed distributions of parts show us substantially the same thing), and commencing with the lowest Coelenterata, where definite tissues make their first appearance, we observe that the only marked functional distinction is between the endo- 15* TTltf INDUCTIONS OF BIOT-OOV’. derm, which absorbs nutriment, and tlie ectoderm, which, by its own contractions and those of the tentacles it bears, pro¬ duces motion. That the functions of accumulation and exr penditure are here very incompletely distinguished, may be admitted without affecting the position that this is the first specialization which begins to appear. » These two most general and most radically- opposed functions, become, in the Polyzoa, much more clearly marked-off from each other ; at the same time that each of them becomes partially divided into subordinate functions. The endoderm and ectoderm are no longer merely the inner and outer walls of the same simple sac into which the food is drawn ; but the endoderm forms a true alimentary canal, separated from the ectoderm by a peri- visceral cavity, containing the nutritive matters absorbed from the food. That is to say, the function of accumulating force is exercised by a part distinctly divided from the part mainly occupied in expending force : the space between them, full of absorbed nutriment, effecting in a vague way that transfer of force which, at a higher stage of evolution, becomes a third leading function. Meanwhile, the endoderm no longer discharges the accumulative function in the same way throughout its whole extent ; but its differ¬ ent portions, oesophagus, stomach and intestine, perform different portions of this function. And instead of a con¬ tractility uniformly diffused through the ectoderm, there have arisen in it, some parts which have the office of con¬ tracting (muscles), and some parts which have the office of making them contract (nerves and ganglia). As we pass upwards, the transfer of force, hitherto effected quite incidentally, comes to have a special organ. In the ascidian molluscs, circulation is produced by a muscular tube, open at both ends, which, by a wave of contraction passing along it, sends out at one end the nutrient fluid drawn in at the other ; and which, having thus propelled the fluid for a time in one direction, reverses its movement and propels it in the opposite direction. By such means does this rudimentary FUNCTION. 1 C9 heart generate alternating currents in the crude and dilute nutriment occupying the peri-visceral cavity. IIovv the func¬ tion of transferring force, thus vaguely indicated in these in¬ ferior forms, comes afterwards to be the definitely-separated office of a complicated apparatus made up of many parts, each of which has a particular portion of the general duty, need not be described. It is sufficiently manifest that this o-eneral function becomes more clearly marked-off* from the others, at the same time that it becomes itself oarted into subordinate i unctions In a developing emorvo, tne functions or more strictly the structures whicn are to perform tnem, arise in the same general order. A like primarv distinction very early ap¬ pears between the endoderm and the ectoderm — the part which has the office of accumulating force, and the part out of which grow those organs that are the great expenders of force Between these two there presently becomes visible the rudiment of that vascular system, which has to fulfil the * intermediate duty of transferring force. Of these three general functions, that of accumulating force is carried on from the outset : the endoderm, even while yet incompletely differentiated from the ectoderm, absorbs nutritive matters from the subjacent yelk. The transfer of force is also to some extent effected by the rudimentary vascular system, as soon as its central cavity and attached vessels are sketched out But the expenditure of force (in the higher animals at least) is not appreciably displayed by the ectodermic struc¬ tures that are afterwards to be mainly devoted to it : there * is no sphere for the actions of these parts. Similarly with the chief subdivisions of these fundamental functions. If we look at those discharged by the ectoderm, potentially if not actually, we see that the distinction first established separates the office of transforming other force into mechani ( al motion, from the office of liberating the force to be so transformed — in the midst of the part out of which the mus¬ cular system is to be developed, there is marked-out the 160 THE INDUCTIONS OF BIOLOGY. rudiment of the nervous system. This indication of struc¬ tures which are to share between them the general duty of expending force, is soon followed by changes that foreshadow further specializations of this general duty. In the incipient nervous system, there begins to arise that contrast between the cerebral mass and the spinal cord, which, in the main, answers to the division of nervous actions into directive and executive ; and at the same time, the appearance of vertebral laminae foreshadows the separation of the osseous system, which has to resist the strains of muscular action, from the muscular system, which, in generating motion, en¬ tails these strains. Simultaneously there have been going . on similar actual and potential specializations in the functions of accumulating force and transferring force. And through¬ out all subsequent phases, the method is substantially the same. This progress from general, indefinite, and simple kinds of action, to special, definite, and complex kinds of action, lias been aptly termed by Milne-Edwards, the “ physio¬ logical division of labour.” Perhaps no metaphor can more truly express the nature of this advance from vital activity in its lowest forms to vital activity in its highest forms. And probably the general reader cannot in any other way obtain so clear a conception of functional development in organisms, as he can by tracing out functional development in societies : noting how there first comes a distinction between the governing class and the governed class ; how while in the governing class there slowly grow up such differences of duty as the civil, military, and ecclesiastical, there arise in the governed class, fundamentally industrial differences like those between agriculturists and artizans ; and how there is a continual multiplication of such specialized occupations, and specialized shares of each occupation. § 59. I ully to understand this change from homogeneity to heterogeneity of function, which accompanies the change FUNCTION. 161 from homogeneity to heterogeneity of structure, it is needful to contemplate it under a converse aspect. Standing alone, the above exposition conveys both an inadequate and an erroneous idea. The divisions and subdivisions of function, becoming definite as they become multiplied, do not lead to a more and more complete independence of functions ; as they would do were the process nothing beyond that just de¬ scribed ; but by a simultaneous process they are rendered more mutually dejiendent. While in one respect they are separating from each other, they are in another respect com¬ bining with each other. At the same time that they are being differentiated, they are also being integrated. Some illustrations will make this plain. In animals which display little beyond the primary dif¬ ferentiation of functions, the activity of that part which absorbs nutriment or accumulates force, is not immediately bound up with the activity of that part which, in producing motion, expends force. In the higher animals, however, the performance of the alimentary functions depends on the per¬ formance of various muscular and nervous functions. Masti¬ cation and swallowing are nervo-muscular acts ; the ryth¬ mical contractions of the stomach and the allied vermicular motions of the intestines, result from the stimulation of cer¬ tain muscular coats by the nerve- fibres distributed through them ; the secretion of the several digestive fluids by their respective glands, is due to nervous excitation of them. ; and digestion, besides requiring these special aids, is not properly performed in the absence of a continuous discharge of energy from the great nervous centres. Again, the function of transferring nutriment or latent force, from part to part, though at first not closely connected with the other functions, O v s eventually becomes so. The short contractile tube which propels backwards and forwards the crude dilute blood con¬ tained in the perivisceral cavity of an inferior mollusc, is neither structurally nor functionally much entangled with the creature’s other organs. But on passing upwards through 8 1G2 THE INDUCTIONS OF BIOLOGY tlie higher molluscs, in which this simple tube is replaced by a system of branched tubes, that deliver their contents through their open ends into the tissues at distant parts ; and on coming to those advanced types of animals which have closed arterial and venous systems, ramifying minutely in every corner of every organ ; we find that the vascular apparatus, while it has become structurally interwoven with the whole body, has become unable to fulfil its office without the help of offices that are quite separated from its own. The heart is now a complex pump, worked by powerful muscles that are excited by a local nervous system ; and the general nervous system also, takes a share in regu¬ lating the contractions both of the heart and of all the arteries. On the due discharge of the respiratory function, too, the function of circulation is directly dependent : if the aeration of the blood is impeded, the vascular activity is lowered ; and arrest of the one very soon causes stoppage of the other. Similarly with the duties of the nervo- muscular system. Animals of low organization, in which the differentiation and integration of the vital actions have not been carried far, will move about for a considerable time after being eviscerated, or deprived of those appliances by which force is accumulated and transferred. But animals of high organization are instantly killed by the removal of these appliances, and even by the injury of minor parts of them : a dog’s movements are suddenly brought to an end, by cutting one of the main canals along which the materials that evolve movements are conveyed. Thus while in w ell- developed creatures the distinction of functions is very marked, the combination of functions is very close. From instant to instant, the aeration of blood implies that certain respiratory muscles are being made to contract by certain nerves ; and that the heart is duly propelling the blood to be aerated. From instant to instant digestion pro¬ ceeds only on condition that there is a supply of aerated blood, and a due current of nervous energy through the digestive FUNCTION. 'JG3 organs. That the heart may act, it must from instant to in¬ stant be excited by discharges from certain ganglia ; and the discharges from these ganglia are made possible, only by the conveyance to them, from instant to instant, of the blood which the heart propels. It is not easy to find an adequate expression for this double re-distribution of functions. It is not easy to realize a trans¬ formation through which the functions thus become in one sense separated and in another sense combined, or even in¬ terfused. Here, however, as before, an analogy drawn from social organization helps us. If we observe how the increas¬ ing division of labour in societies, is accompanied by a closer co-operation ; and how the agencies of different social actions, while becoming in one respect more distinct, become in an¬ other respect more minutely ramified through each other ; we shall understand better the increasing physiological co¬ operation that accompanies increasing physiological division of labour. Note, for example, that while local divisions and classes of the community have been grow¬ ing unlike in their several occupations, the carrying on of their several occupations has been growing dependent on the due activity of that vast organization by which sus¬ tenance is collected and diffused. During the early stages of social development, every small group of people, and often every family, obtained separately its own necessaries ; but now, for each necessary, and for each superfluity, there ex¬ ists a combined body of wholesale and retail distributors, which brings its branched channels of supply within reach of all. While each citizen is pursuing a business that does not immediately aim at the satisfaction of his personal wants, his personal wants are satisfied by a general agency that brings from all places commodities for him and his fellow- citizens — an agency which could not cease its special duties for a few days, without bringing to an end his own special duties and those of most others. Consider, again, how each of these differentiated functions is everywhere pervaded by 164 THE INDUCTIONS OF BIOLOGY. certain other differentiated functions. Merchants, manu¬ facturers, wholesale distributors of their several species, to¬ gether with lawyers, bankers, &c., all employ clerks. In clerks we have a specialized class dispersed through various other classes ; and having its function fused with the differ¬ ent functions of these various other classes. Similarly commercial travellers, though having in one sense a separate occupation, have in another sense an occupation forming part of each of the many occupations which it aids. As it is here with the sociological division of labour, so is it with the physiological division of la¬ bour above described. Just as we see in an advanced com¬ munity, that while the magisterial, the clerical, the medical, the legal, the manufacturing, and the commercial activities, have grown distinct, they have yet their agencies mingled together in every locality ; so in a developed organism, we see that while the general functions of circulation, secretion, absorption, excretion, contraction, excitation, &c., have be¬ come differentiated, yet through the ramifications of the sys¬ tems apportioned to them, they are closely combined with each other in every organ. § GO. The physiological division of labour, is usually not carried so far as wholly to destroy the primary physiological community of labour. As in societies the adaptation of special classes to special duties, does not entirely disable these classes from performing each others’ duties on an emergency ; so in organisms, tissues and structures that have become fitted to the particular offices they have ordinarily to discharge, often remain partially able to discharge other offices. It has been pointed out by Dr Carpenter, that “ in cases where the differ¬ ent functions are highly specialized, the general structure retains, more or less, the primitive community of function which originally characterized it.” A few instances will bring home this generalization The roots and leaves of plants are widely diffe.renti- 9 FUNCTION. n * .00 ated in tlieir functions : by tbe roots, water and mineral substances are absorbed ; while the leaves take in, and de¬ compose, carbonic acid. Nevertheless, leaves retain a con¬ siderable power of absorbing water ; and in what are popu¬ larly called “ air-plants,” the absorption of water is wholly carried on by them and by the stems. Conversely, the under¬ ground parts can partially assume the functions of leaves : the exposed tuber of a potato develops chlorophyll on its surface, and in other cases, roots, properly so called, do the like. In trees, the trunks, which have in great measure ceased to produce buds, recommence producing them if the branches are cut off ; and under such circumstances the roots, though not in the habit of developing leaf-bearing organs, send up numerous suckers. Much more various examples of vicarious function may be found among animals. Starting with the extreme case of the common hydra, which can live when the duties of skin and stomach have been interchanged by turning it inside out, we find in all grades, even up to the highest, that absorbent and excret¬ ing organs can partially supply each others’ places. Among well- organized animals, the taking in of nutriment is ef¬ fected exclusively by an internal membrane ; but the external membrane is not wholly without the power to take in nutri¬ ment : when food cannot be swallowed, life may be pro¬ longed by immersing the body in nutritive fluids. The ex¬ cretion of carbonic acid and absorption of oxygen, are mainly performed by the lungs, in creatures which have lungs ; but in such creatures there continues a certain amount of cutane¬ ous respiration, and in soft-skinned batrachians like the frog, this cutaneous respiration is important. Again, when the kidneys are not discharging their" duties, a notable quantity of urea is got rid of by perspiration. Other instances are supplied by the higher functions. In man, the limbs, which among: lower vertebrates are almost wholly organs of locomotion, are specialized into organs of locomo¬ tion and organs of manipulation. Nevertheless, the human 166 TIIE INDUCTIONS OF BIOLOGY. arms and legs do, when needful, fulfil, to some extent, each others’ offices. JNot only in childhood and old age are the arms used for purposes of support, but on occasions of emerg¬ ency, as when mountaineering, they are so used by men in full vigour. And that legs are to a considerable degree capable of performing the duties of arms, is proved by the great amount of manipulatory skill reached by them when the arms are absent. Among the perceptions, too, there are ex¬ amples of partial substitution. The deaf Dr Kitto described himself as having become excessively sensitive to vibrations propagated through the body ; and as so having gained the power of perceiving, through his general sensations, those neighbouring concussions of which the ears ordinarily give notice. Blind people make hearing perform, in part, the office of vision. Instead of identifying the positions and sizes of neighbouring objects by the reflection of light from their surfaces, they do this in a rude way by the reflection of sound from their surfaces. We see, as we might expect to see, that this power of per¬ forming more general functions, is great in proportion as the parts have been but little adapted to their special func¬ tions. In the hydra, where complete transposition of functions is possible, the histological differentiation that has been estab¬ lished, is extremely slight, or even inappreciable. Those parts of plants which show so considerable a power of discharging each others’ offices, are not widely unlike in their minute structures. And the tissues that in animals are to some extent mutually vicarious, are tissues in which the original cellular composition is still conspicuous. But we do not find evidence that the muscular, nervous, or osseous tissues are able in any degree to perform those processes which the less differentiated tissues perform. Nor have we any proof that nerve can partially fulfil the duty of muscle, or muscle that of nerve. We must sav, therefore, that the ability to resume the primordial community of function, FUNCTION. 167 varies inversely as the established specialization of function ; and that it disappears when the specialization of function becomes great* § 61. Something approaching to a priori reasons may be given for the conclusions thus reached d posteriori. They must be accepted for as much as they seem worth. It may be argued that on the hypothesis of Evolution, Life necessarily comes before organization. On this hypo¬ thesis, organic matter in a state of homogeneous aggregation, must precede organic matter in a state of heterogeneous ag¬ gregation. But since the passing from a structureless state to a structured state, is itself a vital process, it follows that vital activitv must have existed while there was vet no 4/ %/ structure : structure could not else arise. That function takes precedence of structure, seems also implied in the definition of Life. If Life consists of inner actions so adjusted as to balance outer actions — if the actions are the substance of Life, while the adjustment of them constitutes its form ; then, may we not say that the actions to be formed must come before that which forms them — that the continu¬ ous change which is the basis of function, must come before the structure which brings function into shape ? Or again, since throughout all phases of Life up to the highest, every advance is the effecting of some better adjustment of inner to outer actions; and since the accompanying new com¬ plexity of structure is simply a means of making possible this better adjustment ; it follows that function is from beginning to end the determining cause of structure. Eot only is this manifestly true where the modification of struc¬ ture arises bv reaction from modification of function ; but it is also true where a modification of structure otherwise pro¬ duced, apparently initiates a modification of function. Eor it is only when such so-called spontaneous modification of structure subserves some advantageous action, that it is per- 108 THE INDUCTIONS OF BIOLOGY. manently establish ed : if it is a structural modification that happens to facilitate the vital activities, “ natural selection ” retains and increases it ; but if not, it disappears. The connexion which we noted between heterogeneity of structure ‘and heterogeneity of function — a connexion made so familiar by experience as to appear scarcely worth specifying — is clearly a necessary one. It follows from the general truth that in proportion to the heterogeneity of any aggregate, is the heterogeneity it will produce in any inci¬ dent force ( First Principles , § 116). The force continually liberated in the organism by decomposition, is here the inci¬ dent force ; the functions are the variously modified forms produced in its divisions by the organs they pass through ; and the more multiform the organs the more multiform must be the differentiations of the force passing through, them. It follows obviously from this, that if structure progresses from the homogeneous, indefinite, and incoherent, to the heterogeneous, definite, and coherent, so too must function. If the number of different parts in an aggregate must deter¬ mine the number of differentiations produced in the forces passing through it — if the distinctness of these parts from each other, must involve distinctness in their reactions, and there¬ fore distinctness between the divisions of the differentiated force ; there cannot but be a complete parallelism between the development of structure and the development of func¬ tion. If structure advances from the simple and general to the complex and special, function must do the same. CHAPTER IV. WASTE AND REPAIR. § 62. Throughout tlie vegetal kingdom, the processes of Waste and Repair are comparatively insignificant in their amounts. Though plants, and especially certain parts of them, do, in the absence of light or under particular con¬ ditions, give out carbonic acid ; yet this carbonic acid, assuming it to indicate consumption of tissue, indicates but a small consumption. Of course if there is little waste, there can be but little repair — that is, little of the interstitial repair which restores the integrity of parts worn by functional acti¬ vity. Nor, indeed, is there displayed by plants in any con¬ siderable degree, if at all, that other species of repair which consists in the restoration of lost or injured organs. Torn leaves and the shoots that are shortened by the pruner, do not reproduce their missing parts ; and though when the branch of a tree is cut off close to the trunk, the place is in the course of years covered over, it is not by any reparative action in the wounded surface, but by the lateral growth of the adjacent bark. Hence, without saying that Waste and Repair do not go on at all in plants, we may fitly pass them over as of no importance. There are but slight indications of waste in those lower orders of animals which, by their comparative inactivity, show themselves least removed from vegetal life. Actinioe kept in an aquarium, do not appreciably dimink h in bulk 170 THE INDUCTIONS OF BIOLOGY. from prolonged abstinence. Even fish, though much more active than most other aquatic creatures, appear to undergo but little loss of substance when kept unfed during con¬ siderable periods. Reptiles, too, maintaining no great temperature, and passing their lives mostly in a state of torpor, suffer but little diminution of mass by waste. When, however, we turn to those higher orders of animals which are active and hot-blooded, we see that waste is rapid : producing when unchecked, a notable decrease in bulk and weight, ending very shortly in death. Besides finding that waste is inconsiderable in creatures that pro¬ duce but little insensible and sensible motion, and that it becomes conspicuous in creatures that produce much insen¬ sible and sensible motion ; we find that in the same crea¬ tures there is most waste when most motion is generated. This is clearly proved by liybernating animals. “ Ya- lentin found that the waking marmot excreted in the average 75 times more carbonic acid, and inhaled 41 times more oxygen than the same animal in the most complete state of hybernation. The stages between waking and most pro¬ found hybernation yielded intermediate figures. A waking hedgehog yielded about 20’5 times more carbonic acid, and consumed 18*4 times more oxygen than one in the state of hy¬ bernation. ” If we take these quantities of absorbed oxygen and excreted carbonic acid, as indicating something like the relative amounts of consumed organic substance, we see that there is a striking contrast between the waste ac¬ companying the ordinary state of activity, and the waste accompanying complete quiescence and reduced temperature. This difference is still more definitely shown by the fact, that tho mean daily loss from starvation in rabbits and guinea-pigs, bears to that from hybernation, the proportion of 18' 3 : 1. Among men and domestic animals, the relation between degree of waste and amount of expended force, though one respecting which there is little doubt, is less distinctly demonstrable ; since waste is not allowed to go on WASTE AND REPAIR. 1T1 uninterfered with. We have however in the lingering lives of invalids who are able to take scarcely any nutriment, but are kept warm and still, an illustration of the extent to which waste diminishes as the expenditure of force declines. .i. Besides the connexion between the waste of the organism as a whole, and the production of sensible and insensible motion by the organism as a whole ; there is a traceable connexion between the waste of special parts and the activi¬ ties of such special parts. Experiments have shown that “ the starving pigeon daily consumes in the average 40 times more muscular substance than the marmot in the state of torpor, and only 11 times more fat, 33 times more of the tissue of the alimentary canal, 18*3 times more liver, 15 times more lung, 5 times more skin.” That is to say, in the hybernating animal the parts least consumed are the almost totally quiescent motor-organs, and the part most consumed is the hydro-carbonaceous deposit serving as a store of force ; whereas in the pigeon, similarly unsupplied with food but awake and active, the greatest loss takes place in the motor-organs. The relation between special activity and special waste, is illustrated too in the daily experiences of all : not indeed in the measurable decrease of the active parts in bulk or weight, for this we have no means of ascertaining ; but in the diminished ability of such parts to perform their functions. That legs exerted for many hours in walking, and arms long strained in rowing, lose their powers — that eyes become enfeebled by reading or writing without intermission — that concentrated attention unbroken by rest, so prostrates the brain as to incapacitate it for think¬ ing; are familiar truths. And though we have no direct evidence to this effect, there is little danger in concluding that muscles exercised until they ache or become stiff, and nerves of sense rendered weary or obtuse by work, are organs so much wasted by action as to be partially incompetent. Repair is everywhere and alwaj^s making up for waste Though the two processes vary in their relative rates, both 172 THE INDUCTIONS OF BIOLOGY are constantly going on. Though during the active, waking state of an animal, waste is in excess of repair, yet repair is in progress ; and though during sleep, repair is in excess of waste, yet some waste is necessitated by the carrying on of certain never-ceasing functions. The organs of these never- ceasing functions furnish, indeed, the most conclusive proofs of the simultaneity of repair and waste. Day and night the heart never stops beating, but only varies in the rapidity and vigour of its beats ; and hence the loss of substance which its contractions from moment to moment entail, must from moment to moment be made good. Day and night the lungs dilate and collapse ; and the muscles which make them do this, must therefore be ever kept in a state of integ¬ rity by a repair which keeps pace with waste, or which alternately falls behind and gets in advance of it to a very slight extent. On a survey of the facts, we see, as we might expect to see, that repair is most rapid when activity is most reduced. Assuming that the organs which absorb and circulate nutri¬ ment are in proper order, the restoration of the organism to a state of integrity, after the disintegration consequent on expenditure of force, is proportionate to the diminution in expenditure of force. Thus we all know that those who are in health, feel the greatest return of vigour after profound sleep — after complete cessation of motion. We know that a night during which the quiescence, bodily and mental, has been less decided, is usually not followed by that spontaneous overflow of energy that indicates a high state of efficiency throughout the organism. We know, again, that long- continued recumbency, even with wakeful¬ ness (providing the wakefulness is not the result of disorder), is followed by a certain renewal of strength; though a re¬ newal less than that which would have followed the greater inactivity of slumber. We know, too, that when exhausted by labour, sitting brings a partial return of vigour. And we also know that after the violent exertion of running. WASTE AND REPAIR 173 a lapse into the less violent exertion of walking, results in a gradual disappearance of that prostration which the run¬ ning produced. This series of illustrations conclusively proves that the rebuilding of the organism is ever making up for the pulling down of it caused by action ; and that the effect of this rebuilding becomes manifest, in proportion as the pulling down is less rapid. From each digested meal, there is every few hours absorbed into the mass of prepared nutriment circulating through the bodjq a fresh supply of the needful organic compounds ; and from the blood thus occasionally re-enriched, the organs through which it passes are ever taking up materials to replace the materials used up in the discharge of functions. During activity, the reinte¬ gration falls in arrear of the disintegration ; until, as a conse¬ quence, there presently comes a general state of functional languor ; ending, at length, in a quiescence which permits the reintegration to exceed the disintegration, and restore the parts to their state of integrity. Here, as wherever there are antagonistic actions, we see rhythmical divergences on opposite sides of the medium state — changes which equilibrate each other by their alternate excesses. (First Principles, §§ 96, 133.) Illustrations are not wanting of special repair, that is similarly ever in progress, and similarly has intervals during which it falls below waste and rises above it, Everv one knows that a muscle, or a set of muscles, continuously strain¬ ed, as by holding out a weignt at arm's length, soon loses its power ; and that it recovers its power more or less fully after a short rest. The several organs of special sensation yield us like experiences : strong tastes, powerful odours, and loud sounds, temporarily unfit the nerves impressed by them, for appreciating faint tastes, odours, or sounds ; but these inca¬ pacities are remedied by brief intervals of repose. Vision still better illustrates this simultaneity of waste and repair Looking at the sun so affects the eye that, for a short time, it cannot perceive the ordinary contrasts of light and shade. 174 THE INDUCTIONS OF BIOLOGY. After gazing at a bright light of a particular colour, we see on turning the eyes to adjacent objects, an image of the complementary colour; showing that the retina has, for the moment, lost the power to feel small amounts of those rays which have strongly affected it. Such inabilities disappear in a few seconds or a few minutes, according to circumstances. And here, indeed, we are introduced to a conclusive proof that special repair is ever neutralizing special waste. For the rapidity with which the eyes recover their sensitiveness, varies with the reparative power of the individual. In youth, the visual apparatus is so quickly restored to its state of in¬ tegrity, that many of these photogenes, as they are called, cannot be perceived. When sitting on the far side of a room, and gazing out of the window against a light sky, a person who is debilitated by disease or advancing years, perceives, on transferring the gaze to the adjacent wall, a momentary negative image of the window — the sash-bars appearing light and the squares dark ; but a young and healthy person has no such experience. With a rich blood and vigorous circu¬ lation, the repair of the visual nerves after impressions of moderate intensity, is nearly instantaneous. Function carried to excess, may produce waste so great, that repair cannot make up for it during the ordinary daily periods of rest ; and there may result incapacities of the overtaxed organs, lasting for considerable periods. We know that e}^es strained by long-continued minute work, lose their power for months or years : perhaps suffering an injury which they never wholly recover. Brains, too, are often so unduly worked that permanent relaxation fails to restore them to vigour. Even of the motor organs the like holds. The most frequent cause of what is called “ wasting palsy,” or atrophy of the muscles, is habitual excess of exertion : the proof being, that the disease occurs most frequently among those engaged in laborious handicrafts, and usually attacks first the muscles that have been most worked. There has yet to be noticed another kind of repair ; — that WASTE AND REPAIR. 1 7 o namely, by which injured or lost parts are restored. Among the Hyclrozoa it is common for any portion of the body to re¬ produce the rest ; even though the rest to be so reproduced is the greater part of the whole. In the more highly-organ¬ ized Actinozoa, the half of an individual will grow into a complete individual. Some of the lower Annelids, as the Nais , may be cut into thirty or forty pieces, and each piece will eventually become a perfect animal. As we ascend to higher forms, we find this reparative power much diminished, though still considerable. The reproduction of a lost claw by a lobster or crab, is a familiar instance. Some of the inferior Vcrtebrata also, as lizards, can develop new limbs or new tails, in place of those that have been cut off ; and can even do this several times over, though with decreasing complete¬ ness. The highest animals, however, thus repair themselves to but a very small extent. Mammals and birds do it only in the healing of wounds ; and very often but imperfectly even in this. For in muscular and glandular organs, the tissues destroyed are not properly reproduced, but are re¬ placed by tissue of an irregular kind, which serves to hold the parts together. So that the power of reproducing lost parts is greatest where the organization is lowest ; and almost dis¬ appears where the organization is highest. And though we cannot say that between these extremes there is a constant in¬ verse relation between reparative power and degree of organ¬ ization ; yet we may say that there is some approach to such a relation. § 63. There is a very obvious and complete harmony be¬ tween the first of the above inductions, and the deduction that follows immediately from first principles. TVe have already seen (§ 23) “ that whatever amount of power an organism expends in any shape, is the correlate and equi¬ valent of a power that was taken into it from without. ” Motion, sensible or insensible, generated by an organism, is insensible motion which was absorbed in producing certain 176 TI1E INDUCTIONS OF BIOLOGY. chemical compounds appropriated by the organism under the form of food. As much power as was required to raise the elements of these complex atoms to their state of unsta¬ ble equilibrium, is given out in their falls to a state of stable equilibrium ; and having fallen to a state of stable equilib¬ rium, they can give out no further power, but have to be got rid of as inert and useless. It is an inevitable corollary “ from the persistence of force, that each portion of mechanical or other energy which an organism exerts, implies the trans¬ formation of as much organic matter as contained this energy in a latent state ; ” and that this organic matter in yielding up its latent energy, loses its value for the purposes of life, and becomes wnste matter needing to be excreted. The loss of these complex unstable substances must hence be proportionate to the quantity of expended force. Here then is the rationale of certain general facts lately indicated. Plants do not waste to any considerable degree, for the obvi¬ ous reason that the sensible and insensible motions they generate are inconsiderable. Between the small waste, small activity, and low temperature of the inferior animals, the rela¬ tion is similarly one admitting of a priori establishment. Con¬ versely, the rapid waste of energetic, hot-blooded animals might be foreseen with equal certainty. And not less mani¬ festly necessary is the variation in waste which, in the same organism, attends the variation in the heat and mechanical motion produced. Between the activity of a special part and the waste of that part, a like relation may be deductively inferred ; though it cannot be inferred that this relation is equally defi¬ nite. Were the activity of every organ quite independent of the activities of other organs, we might expect to trace out this relation distinctly ; but since one part of the force which any organ expends, is derived from materials brought to it by the blood from moment to moment in quantities varying with the demand, and since another part of the force which such organ expends, comes to it in the shape of WASTE AND REPAIR. 177 nervous discharges from distant organs ; it is clear that spe¬ cial waste and general waste are too much entangled to admit of a definite relation being established between special waste and special activity. We may fairly say, however, that this relation is quite as manifest as we can reasonably anticipate. § G4. Deductive interpretation of the phenomena of Re¬ pair, is by no means so easy. The tendency displayed by an animal organism, as well as by each of its organs, to return to a state of integrity by the assimilation of new matter, when it has undergone the waste consequent on activity, is a tendency which is not manifestly deducible from first princi¬ ples ; though it appears to be in harmony with them. If in the blood there existed ready-formed units exactly like in kind to those of which each organ consists, the sorting of these units, ending in the union of each kind with already existing groups of the same kind, would be merely a good example of Differentiation and Integration [First Principles, § 123). It would be analogous to the process by which, from a mixed solution of salts, there are deposited segregated masses of these salts, in the shape of different crystals. Rut as already said (§ 54), though the selective assimilation by which the • repair of organs is effected, no doubt results in part from an action of this kind, which is consequent on the persistence of force ( First Principles, § 129), the facts cannot be thus wholly accounted for ; since organs are in part made up of units that do not exist as such in the circulating fluids. The pro¬ cess becomes comprehensible however, if it be shown that, as suggested in § 54, groups of compound units have a certain power of moulding adjacent fit materials into units of their own form. Let us see whether there is not reason to think such a power exists. “ The poison of small-pox or of scarlatina,” remarks Mr Paget, “ being once added to the blood, presently affects the composition of the whole : the disease pursues its course, 178 TI1E INDUCTIONS OF BIO LOG V. and, if recovery ensue, tlie blood will seem to have returned to its previous condition : yet it is not as it was before ; for now the same poison may be added to it with impunity.” * * # << The change once effected, may be maintained through life. And herein seems to be a proof of the assimil¬ ative force in the blood ; for there seems no other mode of explaining these cases than by admitting that the altered particles have the power of assimilating to themselves all those by which they are being replaced : in other words, all the blood that is formed after such a disease deviates from the natural composition, so far as to acquire the peculiarity engendered by the disease : it is formed according to the altered model.” Now if the compound molecules of the blood, or of an organism considered in the aggregate, have the power of moulding into their own type, the matters which they absorb as nutriment ; and if, as Mr Paget points out, they have the power when their type has been changed by disease, of moulding all materials afterwards received into the modified type ; may we not reasonably suspect that the more or less specialized molecules of each organ, have, in like manner, the power of moulding the materials which the blood brings to them, into similarly specialized molecules ? The one conclusion seems to be a corollary from the other. Such a power cannot be claimed for the component units of the blood, without being con¬ ceded to the component units of every tissue. Indeed the assertion of this power is little more than an assertion of the fact, that organs composed of specialized units are capable of resuming their structural integrity, after they have been wasted by function. For if they do this, they must do it by forming from the materials brought to them, certain special¬ ized units like in kind to those of which they are composed , and to say that they do this, is to say that their component units have the power of moulding fit materials into othei units of the same order. The repair of a wasted tissue may therefore be considered 179 WASTE AND RET/YIE. as due to forces analogous to those by which a crystal repro¬ duces its lost apex, when placed in a solution like that from which it was formed. In either case, a mass of units of a given kind, show's a powder of integrating with itself diffused units of the same kind : the only difference being, that the organic mass of units arranges the diffused units into special compound forms, before integrating them with itself. In the case of the crystal, this reintegration is ascribed to polarity — a power of whose nature we know nothing. What¬ ever be its nature, however, it appears probable that the pow'er by which organs repair themselves from the nutritive matters circulating through them, is of the same order. § 65. That other kind' of repair w'hich shows itself in the regeneration of lost members, is comprehensible only as an effect of actions like those just referred to. The ability of an organism to recomplete itself w'hen one of its parts has been cut off, is of the same order as the ability of an injured crystal to recomplete itself. In either case, the newly- assimi¬ lated matter is so deposited as to restore the original outline. And if in the case of the crystal, we say that the whole aggregate exerts over its parts, a force wThich constrains the newly-integrated atoms to take a certain definite form ; we must in the case of the organism, assume an analogous force. This is, in truth, not an hypothesis : it is nothing more than a generalized expression of the facts. If when the leg of a lizard has been amputated, there presently buds out the germ, of a new one, which, passing through phases of development like those of the original leg, eventually assumes a like shape and structure ; we assert nothing more than what we see, when we assert that the organism as a whole exercises such power over the newly- forming limb, as makes it a repetition of its predecessor. If a leg is reproduced where there was a leg, and a tail where there was a tail ; we iiave no alternative but to conclude that the aggregate forces of the body, con¬ trol the formative processes going on in each part. And on 180 THE INDUCTIONS OF BIOLOGY. contemplating these facts in connexion with various kindred ones, there is suggested the hypothesis, that the form of each species of organism is determined by a peculiarity in the con¬ stitution of its units — that these have a special structure in which they tend to arrange themselves ; just as have the simpler units of inorganic matter. Let us glance at the evi¬ dences which more especially thrust this conclusion upon us. A fragment of a Begonia-leaf, imbedded in fit soil and kept at an appropriate temperature, will develop a young Bego¬ nia ; and so small is the fragment which is thus capable of originating a complete plant, that something like a hundred plants might be produced from a single leaf. The friend to whom I owe this observation, tells me that various succulent plants have like powers of multiplication. Illustrating a similar power among animals, we have the often-cited exper¬ iments of Trembley on the common polype. Each of the four pieces into which one of these creatures was cut, grew into a perfect individual. In each of these again, bisection and tri- section effected a like result. And so wTith their segments, similarly produced, until as many as fifty polypes had resulted from the original one. Bodies wThen cut off regenerated heads ; heads regenerated bodies ; and when a polype had been divided into as many pieces as was practica¬ ble, nearly every piece survived and became a complete animal. What, now, is the implication ? We cannot say that in each portion of a Begonia-leaf, and in every fragment of a Hydra’s body, there exists a ready- formed model of the entire organism. Even wTere there warrant for the now abandoned doctrine, that the germ of every organism contains the perfect organism in miniature, it still could not be contended that each considerable part of the perfect organism resulting from such a germ, contains another such miniature. Indeed the one hypothesis obviously nega¬ tives the other. We have therefore no alternative but to say, that the living particles composing one of these frag¬ ments, have an innate tendency to arrange themselves into WASTE AND REPAIR. 181 the shape of the organism to which they belong. We must infer that a plant or animal of any species, is made up of special units, in all of which there dwells the intrinsic apti¬ tude to aggregate into the form of that species : just as in the atoms of a salt, there dwells the intrinsic aptitude to crystal¬ lize in a particular way. It seems difficult to conceive that this can be so ; but we see that it is so. Groups of units taken from an organism (providing they are of a certain bulk and not much differentiated into special structures) have this power of re-arranging themselves; and we are thus compelled to recognize the tendency to assume the specific form, as inherent in all parts of the organism. Mani¬ festly too, if we are thus to interpret the reproduction of an organism from one of its amorphous fragments, we must thus interpret the reproduction of any minor portion of an organism by the remainder. When in place of its lost claw, a lobster puts forth from the same spot a cellular mass, which, while increasing in bulk, assumes the form and structure of the original claw ; we can have no hesitation in ascribing this result to a play of forces like that which moulds the materials contained in a piece of Begonia-leaf into the shape of a young Begonia. In the one case as in the other, the vitalized molecules composing the tissues, show their proclivity towards a particular arrange¬ ment ; and whether such proclivity is exhibited in repro¬ ducing the entire form, or in completing it when rendered imperfect, matters not. For this property there is no fit term. If we accept the word polarity, as a name for the force by which inorganic units are aggregated into a form peculiar to them ; we may apply this word to the analogous force displayed by organic units. But, as above admitted, polarity, as ascribed to atoms, is but a name for something of which we are ignorant — a name for a hypothetical property which as much needs ex¬ planation as that which it is used to explain. Nevertheless, in default of another word, we must employ this : taking 182 TIIE INDUCTIONS OF BIOLOGY. care, however, to restrict its meaning. If we simply substi¬ tute the term polarity, for the circuitous expression — the power which certain units have of arranging themselves into a special form, we may, without assuming anything more than is proved, use the term organic polarity or po¬ larity of the organic units, to signify the proximate cause of the ability which organisms display of reproducing lost parts. § 66. As we shall have frequent occasion hereafter to refer to these units, which possess the property of arranging themselves into the special structures of the organisms to which they belong ; it will be well here to ask what these units are, and by what name they may be most fitly called. On the one hand, it cannot be in those proximate chemical compounds composing organic bodies, that this specific polar¬ ity dwells. It cannot be that the atoms of albumen, or fibrine, or gelatine, or the hypothetical protein-substance, possess this power of aggregating into specific shapes ; for in such case, there would be nothing to account for the unlike¬ nesses of different organisms. Millions of species of plants and animals, more or less contrasted in their structures, are all mainly built up of these complex atoms. But if the polarities of these atoms determined the forms of the or¬ ganisms they composed, the occurrence of such endlessly varied forms would be inexplicable. Hence, what we may call the chemical units, are clearly not the possessors of this property. On the other hand, this property cannot reside in what may be roughly distinguished as the morphological units. The germ of every organism is a microscopic cell. It is by multiplication of cells that all the early developmental changes are effected. The various tissues which successively arise in the unfolding organism, are primarily cellular ; and in many of them the formation of cells continues to be, through- WASTE AXD HEP ATE. 183 out life, the process by which repair is carried on. But though cells are so generally the ultimate visible components of organisms, that they may with some show of reason be called the morphological units ; yet, as they are not uni¬ versal, we cannot say that this tendency to aggregate into specified forms dwells in them. Finding that in many cases a fibrous tissue arises out of a structureless blastema, without cell- formation ; and finding that there are creatures, such as Bhizopods, which are not cellular, but nevertheless exhibit vital activities, and perpetuate in their progeny certain specific distinctions ; we are forbidden to ascribe to cells this peculiar power of arrangement. Nor, indeed, were cells universal, would such an hypothesis be acceptable ; since the formation of a cell is, to some extent a manifesta¬ tion of this same peculiar power. If, then, this organic polarity can be possessed neither by the chemical units nor the morphological units, we must conceive it as possessed by certain intermediate units, which we may term jjhysiologicaL There seems no alternative but to suppose, that the chemical units combine into units immensely more complex than themselves, complex as they are ; and that in each organism, the physiological units produced by this further compounding of highly compound atoms, have a more or less distinctive character. We must conclude that in each case, some slight difference of com¬ position in these units, leading to some slight difference in their mutual play of forces, produces a difference in the form which the* aggregato-of them assumes. The facts contained in this chapter, form but a small part of the evidence which thrusts this assumption upon us. We shall hereafter find various reasons for inferring that such physiological units exist, and that to their specific properties, more or less unlike in each plant and animal, various organio phenomena are due. CHAPTER, Y. ADAPTATION. § 67. In plants, waste and repair being scarcely appre¬ ciable, there are not likely to arise appreciable changes in the proportions of already- formed parts. The only divergences from the average structure of a species, which we may expect particular conditions to produce, are those producible by the action of these conditions on parts in course of formation ; and such divergences we do find. "We know that a tree which, standing alone in an exposed position, has a short and thick stem, has a tall and slender stem when it grows in a wood ; and that its branches then take a different inclin¬ ation. We know that potato-sprouts which, on reaching the light, develop into foliage, will, in the absence of light, grow to a length of several feet without foliage. And every in-door plant furnishes proof, that shoots and leaves, by habitually turning themselves* to the light, exhibit a certain adaptation — an adaptation due, as we must suppose, to the special effects of the special conditions on the still grow¬ ing parts. In animals, however, besides analogous structural changes wrought during the period of growth, by subjection to circumstances unlike the ordinary circum¬ stances ; there are structural changes similarly wrought, after maturity has been reached. Organs that have arrived at their full size, possess a certain modifiability ; BO that while the organism as a whole, retains pretty ADAPTATION. 185 nearly the same bulk, the proportions of its parts may be considerably varied. Their variations, here treated of under the title Adaptation, depend on specialities of individual action. We saw in the last chapter, that the actions of or¬ ganisms entail re-actions on them ; and that specialities of action entail specialities of re-action. Here it remains to be pointed out, that the special actions and re-actions do not end with temporary changes, but work permanent changes. If, in an adult animal, the waste and repair in all parts were exactly balanced — if each organ daily gained by nutrition, exactly as much as it lost daily by the discharge of its function — if excess of function were followed only by sucii excess of nutrition as balanced the extra waste ; it is clear that there would occur no change in the relative sizes of organs. Hut there is no such exact balance. If the excess of function, and consequent excess of waste, is moderate, it is not simply compensated by repair, but more than compensated — there is a certain increase of bulk. This is true to some degree of the organism as a whole, when the organism is framed for activity. A considerable waste giving considerable power of assimilation, is more favourable to accumulation of tissue, than is quiescence with its comparatively feeble assimi¬ lation : whence results a certain adaptation of the whole organism to its requirements. Hut it is more especially true of the parts of an organism in relation to each other. The illustrations fall into several groups. The growth of muscles exercised to an unusual degree, is a matter of com¬ mon observation. In the often-cited blacksmith’s arm, the dancer’s legs, and the jockey’s crural adductors, we have marked examples of a modifiability which almost every one has to some extent experienced. It is needless to multi¬ ply proofs. The occurrence of changes in the struc¬ ture of the skin, where the skin is exposed to a stress of function, is also familiar. That thickening of the epidermis on a labourer’s palm, results from continual pressure and friction, is certain : those who have not before exerted their 9 186 TirE INDUCTIONS OF BIOLOGY. hands, find that such an exercise as rowing, soon begins to produce a like thickening. This relation of cause and effect is still better shown by the marked indurations at the ends of a violinist’s fingers. Even in mucous membrane, which ordinarily is not subject to mechanical forces of any intensity, similar modifications are possible : witness the callosity of the gums which arises in those who have lost their teeth, and have to masticate without teeth. The vascular system furnishes good instances of the increased growth that follows increased function. When, because of some permanent obstruction to the circulation, the heart has to exert a greater contractile force on the mass of blood which it propels at each pulsation into the arteries, and when there re¬ sults the laboured action known as palpitation ; there usually occurs dilatation, or hypertrophy, or a mixture of the two : the dilatation, which is a yielding of the heart’s structure under the increased strain, implying a failure to meet the emergency ; but the hypertrophy, which consists in a thick¬ ening of the heart’s muscular walls, being an adaptation of it to the additional effort required. Again, when an aneurism in some considerable artery has been obliterated, either arti¬ ficially or by a natural inflammatory process ; and when this artery has consequently ceased to be a channel for the blood ; some of the adjacent arteries which anastomose with it, become enlarged, so as to carry the needful quantity of blood to the parts supplied. Though we have no direct proof of analogous modifications in nervous structures ; yet indirect proof is given by the greater efficiency that fol¬ lows greater activity. This is manifested alike in the senses and the intellect. The palate may be cultivated in¬ to extreme sensitiveness, as in professional tea-tasters. An orchestral conductor gains by continual practice, an unusually great ability to discriminate differences of sound. And in the finger-reading of the blind, we have evidence that the sense of touch may be brought by exercise to a far higher capability than is ordinary. The increase of power ivhich ADAPTATION. 187 habitual exertion gives to mental faculties, needs no illustra¬ tion : every person of education has personal experience of it. Even from the osseous structures, evidence may be drawn. The bones of men accustomed to great mus¬ cular action, are more massive and have more strongly marked processes for the attachment of muscles, than the bones of men who lead sedentary lives ; and a like contrast holds between the bones of wild and tame animals of the same species. Adaptations of another order, in which there is a qualitative rather than a quantitative modification, arise after certain accidents to which the skeleton is liable. When the hip-joint has been dislocated, and long delay has made it impossible to restore the parts to their proper places, the head of the thigh-bone, imbedded in the surrounding muscles, becomes fixed in its new position by attachments of fibrous tissue, wdiich afford support enough to permit a halting walk. But the most remarkable modification of this order occurs in ✓ ununited fractures. “ False joints ” are often formed — - joints which rudely simulate the hinge structure or the ball- and-socket structure, according as the muscles tend to pro¬ duce a motion of flexion and extension or a motion of rota¬ tion. In the one case, according to Rokitansky, the two ends of the broken bone become smooth and covered with perios¬ teum and fibrous tissue, and are attached by ligaments that allow a certain backward and forward motion; and in the other case, the ends, similarly clothed with the appropriate membranes, become the one convex and the other concave, are inclosed in a capsule, and are even occasionally supplied with synovial fluid ! The general truth that extra function is followed by extra growth, must be supplemented by the equally general truth, that beyond a limit, usually soon reached, very little, if any, further modification can be produced. The experiences from which we draw the one induction thrust the other upon us. After a time, no training makes the pugilist or the athlete any stronger. The adult gymnast at last acquires the power 188 THE INDUCTIONS OF BIOLOGY. to perform certain difficult feats ; but certain more difficult feats, no additional practice enables bim to perform. Years of discipline give tbe singer a particular loudness and range of voice, beyond which further discipline does not give greater loudness or wider range : on the contrary, increased vocal ex¬ ercise, causing a waste in excess of repair, is often followed by decrease of power. In the perceptions we see similar limits. The culture which exalts the susceptibility of the ear to the intervals and harmonies of notes, will not turn a bad ear into a good one. Life-long effort fails to make this artist a correct draftsman, or that a fine colourist : each does better than he did at first, but each falls short of the power attained by some other artists. Nor is this truth less clearly illustrated among the more complex mental powers. Each man has a mathematical faculty, a poetical faculty, or an oratorical faculty, which special educa¬ tion improves to a certain extent. But unless he is unusually endowed in one of these directions, no amount of education will make him a first-rate mathematician, a first-rate poet, or a first-rate orator. Thus the general fact appears to be, that while in each individual, certain changes in the proportions of parts, may be caused by variations of function, the congenital structure of each individual puts a limit to the modifiability of every part. Nor is this true of individuals only : it holds, in a sense, of species. Leaving open the question whether, in indefinite time, indefinite modi¬ fication may not be produced ; experience proves that within assigned times, the changes wrought in races of organisms by changes of conditions fall within narrow limits. "We see, for instance, that though by discipline, aided by selective breeding, one variety of horse has had its locomotive power increased considerably beyond the locomotive powers of other varieties ; yet that further increase takes place, if at all, at an inappreciable rate. The different kinds of dogs, too, in which different forms and capacities have been established, do not show aptitudes for diverging in the same directions at ADAPTATION. 189 considerable rates. In domestic animals generally, certain accessions of intelligence have been produced by culture ; but accessions beyond these are inconspicuous. It seems that in each species of organism, there is a margin for functional oscillations on all sides of a mean state, and a consequent margin of structural variations ; that it is possible rapidly to push functional and structural changes towards the extreme of this margin in any direction, both in an individual and in a race ; but that to push these changes further in any direction, and so to alter the organism as to bring its mean state up to the extreme of the margin in that direction, is a comparatively slow process.* We have also to note that the limited increase of size pro¬ duced in any organ by a limited increase of its function, is not maintained unless the increase of function is permanent. A mature man or other animal, led by circumstances into exerting particular members in unusual degrees, and acquir¬ ing extra size and power in these members, begins to lose such extra size and power on ceasing to exert these members ; and eventually lapses more or less nearly into the original state. Legs strengthened by a pedestrian tour, become weak again after a prolonged return to sedentary life. The acquired ability to perform feats of skill, disappears in course of time, if the performance of them is given up. For compara¬ tive failure in executing a piece of music, in playing a game at chess, or in anything requiring special culture, the being out of practice is a reason of which every one recognizes the validity. It is observable, too, that the rapidity and com¬ pleteness with which an artificial power is lost, is proportionate to the shortness of the cultivation which evoked it. One who has for many years persevered in habits which exercise special muscles or special faculties of mind, retains the extra * Here, as in sundry places throughout this chapter, the necessities of the argu¬ ment have obliged me to forestall myself, by assuming the conclusion reached in a subsequent chapter, that modifications of structure produced by modifications of function, are transmitted to offspring. 190 THE INDUCTIONS OF BIOLOGY-. capacity produced, to a very considerable degree, even after a long period of desistance ; but one wlio has persevered in such habits for but a short time, has, at the end of a like period, scarcely any of the facility he had gained. Here, loo, as before, successions of organisms present an analogous fact. A species in which domestication, continued through many generations, has organized certain peculiarities ; and which afterwards, escaping domestic discipline, returns to something like its original habits ; soon loses, in great mea¬ sure, such peculiarities. Though it is not true, as alleged, that it resumes completely the structure it had before domes¬ tication ; yet it approximates to that structure. The Dingo, or wild dog of Australia, is one of the instances given of this ; and the wild horse of South America is another. Mankind, too, supplies us with instances. In the Austra¬ lian bush, and in the backwoods of America, the Anglo- Saxon race, in which civilization has developed the higher feelings to a considerable degree, rapidly lapses into compara¬ tive barbarism : adopting the moral code, and sometimes the habits, of savages. § 68. It is important to reach, if possible, some rationale of these general truths — especially of the last two. A right understanding of these laws of organic modification, underlies a right understanding of the great question of species. While, as before hinted (§ 40), the action of structure on function, is one of the factors in that process of differentiation by which unlike forms of plants and animals are produced, the re-action of function on structure, is another factor. Hence, it is well worth while inquiring how far these induc¬ tions are deductively interpretable. The first of them is the most difficult to deal with. Why an organ exerted somewhat beyond its wont, should presently grow, and thus meet increase of demand by increase of sup¬ ply, is not obvious. We know, indeed, (First Principles, §§ 96, 133,) that of necessity, the rhythmical changes pro- ADAPTATION. 191 duced by antagonist organic actions, cannot any of them bo carried to an excess in one direction, without there being produced an equivalent excess in the opposite direction. It is a corollary from the persistence of force, that any deviation effected by a disturbing cause, acting on some member of a moving equilibrium, must (unless it altogether destroys the moving equilibrium) be eventually followed by a compensating deviation. Hence, that excess of repair should succeed ex¬ cess of waste, is to be expected. But how happens the mean state of the organ to be changed ? If daily extra waste naturally brings about daily extra repair, only to an equiva¬ lent extent, the mean state of the organ should remain con¬ stant. IIow then comes the organ to augment in size and power ? Such answer to this question as we may hope to find, must be looked for in the effects wrought on the organism as a whole, by increased function in one of its parts. Bor since the discharge of its function by any part, is possible only on condition that those various other functions on which its own is immediately dependent, are also discharged ; it follows that excess in its function presupposes some excess in their functions. Additional work given to a muscle, implies ad¬ ditional work given to the branch arteries which, bring it blood, and additional work, smaller in proportion, to the arteries from which these branch arteries come. Similarly, the smaller and larger veins which take away the blood, as well as the absorbents which carry off effete products, must have more to do. And yet further, on the nervous .centres which excite the muscle, a certain extra duty must fall. But excess of waste will entail excess of repair, in these parts as well as in the muscle. The several appliances by which the nutrition and excitation of an organ are carried on, must also be influenced by this rhythm of action and re-action ; and therefore, after losing more than usual by the destructive process, they must gain more than usual by the constructive process. But temporarily-increased efficiency in these ap- 192 THE INDUCTIONS OF BIOLOGY. pliances by which blood and nervous force are brought to an organ, will cause extra assimilation in the organ, beyond that required to balance its extra expenditure. Regarding the functions as constituting a moving equilibrium, we may say, that divergence of any function in the direction of in¬ crease, causes the functions with which it is bound up to diverge in the same direction ; that these again cause the functions which they are bound up with, also to diverge in the same direction ; and that these divergences of the con¬ nected functions, allow the specially-affected function to be carried further in this direction than it could otherwise be — further than the perturbing force could carry it if it had a fixed basis. It must be admitted that this is but a vague explanation. Among actions so involved as these, we can scarcely expect to do more than dimly discern a harmony with first princi¬ ples. That the facts are to be interpreted in some such way, may, however, be inferred from the circumstance that an extra supply of blood continues for some time to be sent to an organ that has been unusually exercised ; and that when unusual exercise is long continued, a permanent increase of vascularity results. § 69. Answers to the questions — Why do these adaptive modifications in an individual animal, soon reach a limit ? and why, in the descendants of such animal, similarly condi¬ tioned, is this limit very slowly extended P — are to be found in the same direction as was the answer to the last question. And here the connexion of cause and consequence is much more manifest. Since the function of any organ is dependent on the func¬ tions of the organs which supply it with materials and forces; and since the functions of these subsidiary organs are de¬ pendent on the functions of organs which supply them with materials and. forces ; it follows that before any great extra power of discharging its function, can be gained by a ADAPTATION. 193 specially- exercised organ, a considerable extra power must bo gained by a series of immediately-subservient organs, and some extra power by a secondary series of remotely-sub- servient organs. Thus there are required numerous and wide-spread modifications. Before the artery which feeds a hard-worked muscle, can permanently furnish a large ad¬ ditional quantity of blood, it must increase in diameter and contractile power ; and that its increase of diameter and con¬ tractile power may be of use, the main artery from which it diverges, must also be so far modified as to bring this addi¬ tional quantity of blood to the branch artery. Similarly with the veins ; similarly with the absorbents ; similarly with the nerves. And when we ask what these subsidiary changes imply, we are forced to conclude that there must be an analogous group of more numerous changes, ramifying throughout the system. The growth of the arteries prima¬ rily and secondarily implicated, cannot go to any extent, without growth in the minor blood-vessels on which their nutrition depends ; while their greater contractile power in¬ volves enlargement of the nerves which excite them, and some modification of that part of the spinal cord whence these nerves proceed. Thus, without tracing the like remote alterations implied by extra growth of the veins, absorbents, and other agencies, it is manifest that a large amount of re¬ building must be done throughout the organism, before any organ of importance can be permanently increased in size and power to a great extent. Hence, though such extra growth in any part as does not necessitate considerable changes throughout the rest of the organism, may rapidly take place ; a further growth in this part, requiring a re¬ modelling of numerous parts remotely and slightly affected, must take place but slowly. TTe have before found our conceptions of vital processes made clearer by studying analogous social processes. In societies there is a mutual dependence of functions, essentially like that which exists in organisms; and there is also an 194 TI1E INDUCTIONS OE BIOLOGY. essentially like re-action of functions on structures. From the laws of adaptive modification in societies, we may therefore hope to get a clue to the laws of adaptive modification in organisms. Let us suppose, then, that a society lias arrived at a state of equilibrium like that of a mature animal — a state not like our own, in which growth and structural de¬ velopment are rapidly going on ; but a state of settled balance among the functional powers of the various classes and industrial bodies, and a consequent fixity in the relative sizes of such classes and bodies. Further, let us suppose that in a society thus balanced, there occurs something which throws an unusual demand on some one industry — say an unusual demand for ships (which we will assume to be built of iron) in consequence of a competing mercantile nation having been prostrated by famine or pestilence. The imme¬ diate result of this additional demand for iron ships, is the employment of more workmen, and the purchase of more iron, by the ship-builders ; and when, presently, the demand con¬ tinuing, the builders find their premises and machinery in¬ sufficient, they enlarge them. If the extra requirement persists, the high interest and high wages bring such extra capital and labour into the business, as are needed for new ship-building establishments. But such extra capital and labour do not come quickly ; since, in a balanced communitjq not increasing in population and wealth, labour and capital have to be drawn from other industries, where they are already yielding the ordinary returns. Let us now go a step further. Suppose that this iron-ship-building industry, having enlarged as much as the available capital and labour permit, is still unequal to the demand ; what limits its im¬ mediate further growth ? The lack of iron. By the h}^po- thesis, the iron-producing industry, like all the other indus¬ tries throughout the community, yields only as much iron as is habitually required for all the purposes to which iron is applied : ship-building being only one. If, then, extra iron is required for ship-building, the first effect is to withdraw ADAPTATION ISO part of the iron habitually consumed for other purposes, and to raise the price of iron. Presently, the iron-makers feel this change, and their stocks dwindle. As, however, the quantity of iron required for ship-building, forms but a small part of the total quantity required for all purposes ; the ex¬ tra demand on the iron-makers, can. be nothing like so great in proportion as is the extra demand on the ship-builders. Whence it follows, that there will be much less tendency to an immediate enlargement of the iron-producing industry — the extra quantity will for some time be obtained by working extra hours. Nevertheless, if, as fast as more iron can be thus supplied, the ship- building industry goes on growing — if, consequently, the iron-makers experience a permanently- increased demand, and out of their greater profits get higher interest on capital, as well as pay higher wages ; there will eventually be an abstraction of capital and labour from other industries, to enlarge the iron-producing industry : new blast¬ furnaces, new rolling-mills, new cottages for workmen, will be erected. Put obviously, the inertia of capital and labour to be overcome, before the iron-producing industry can grow by a decrease of some other industries, will prevent its growth from taking place until long after the increased ship-build- . irig industry has demanded it ; and meanwhile, the growth of the ship-building industry must be limited by the deficiency of iron. A remoter restraint of the same nature, meets us if we go a step further — a restraint which can be overcome, only in a still longer time. For the manu¬ facture of iron depends on the supply of coal. The pro¬ duction of coal being previously in equilibrium with the consumption ; and the consumption of coal for the manu¬ facture of iron, being but a small part of the total con¬ sumption ; it follows that a considerable extension of the iron manufacture, when it at length takes place, will cause but a comparatively small additional demand on the coal-owners and coal-miners— a demand which will not, for a long period, suf¬ fice to cause enlargement of the coal- trade, by drawing capital 19 6 THE INDUCTIONS OF BIOLOGY. and labour from other investments and occupations. And until the permanent extra demand for coal, has become great enough to draw from other investments and occupations, suf¬ ficient capital and labour to sink new mines, the increasing production of iron must be restricted by the scarcity of coal ; and the multiplication of ship-yards and ship-builders, must be checked by the want of iron. Thus, in a com¬ munity which has reached a state of moving equilibrium, though any one industry directly affected by an additional demand, may rapidly undergo a small extra growth ; yet a growth beyond this, requiring, as it does, the build¬ ing-up of subservient industries, less directly and strongly affected, as well as the partial wwbuilding of other industries, can take place only with comparative slowness. And a still further growth, requiring structural modifications of industries still more distantly affected, must take place still more slowly. Returning from this analogy, we realize more clearly the truth, that any considerable member of an animal organism, cannot be greatly enlarged without some general re-organiza¬ tion. Besides a building-up of the primary, secondary, and tertiary groups of subservient parts, there must be an un- building of sundry non-subservient parts ; — or at any rate, . there must be permanently established, a lower nutrition of such non-subservient parts. For it must be remembered that in a mature animal, or one which has reached a balance between assimilation and expenditure, there cannot be an in¬ crease in the nutrition of some organs, without a decrease in the nutrition of others ; and an organic establishment of the increase, implies an organic establishment of the decrease — implies more or less change in the processes and structures throughout the entire system. And here, in¬ deed, is disclosed one reason why growing animals under¬ go adaptations so much more readily than adult ones. For while there is surplus nutrition, it is possible for specially-ex¬ ercised parts to be specially enlarged, without any positive AI)A PTAT10N. 197 deduction from other parts. There is required only that negative deduction, shown in the diminished growth of other parts. § 70. Pursuing the argument further, we reach an ex¬ planation of the third general truth ; namely, that organisms, and species of organisms, which, under new conditions, have undergone adaptive modifications, soon return to something like their original structures, when restored to their original conditions. Seeing, as we have done, how excess of action and excess of nutrition in any part of an organism, must affect action and nutrition in subservient parts, and these again in other parts, until the re-action has divided and subdivided itself throughout the organism, affecting in decreasing degrees the more and more numerous parts more and more remotely implicated ; we see that the consequent changes in the parts remotely implicated, consti¬ tuting the great mass of the organism, must be extremely slow. Hence, if the need for the adaptive modification ceases, before the great mass of the organism has been much altered in its structure by these ramified but minute re-ac¬ tions ; we shall have a condition in which the specially- modified part, is not in equilibrium with the rest. All the remotely- affected organs, as yet but little changed, will, in the absence of the perturbing cause, resume very nearly their previous actions. The parts that depend on them, will consequently by and by do the same. Until at length, by a reversal of the adaptive process, the organ at first affected will be brought back almost to its original state. Pecon- sidering the above-drawn analogy between an organism and society, will enable us better to realize this necessity. If, in the case supposed, the extra demand for iron ships, after causing the erection of some additional ship-yards and the drawing of iron from other manufactures, were to cease ; the old dimensions of the ship-building trade would be quickly returned to : discharged workmen would seek fresh 198 THE INDUCTIONS OF BIOLOGY. occupations, and tlie new yards would be devoted to othei uses. But if the increased need for ships lasted long enough, and became great enough, to cause a flow of capital and labour from other industries into the iron-manufacture, a falling off in the demand for ships, would much less rapidly entail a dwindling of the ship-building industry. For iron being now produced in greater quantity, a diminished con¬ sumption of it for ships, would cause a fall in its price, and a consequent fall in the cost of ships : thus enabling the ship-builders to meet the competition which we may sup¬ pose led to a decrease in the orders they received. And since, when new blast-furnaces and rolling-mills, &c., had been built with capital drawn from other industries, its transference back into other industries, would involve great loss ; the owners, rather than transfer it, would accept unusually low in¬ terest ; and an excess of iron would continue to be produced ; resulting in an undue cheapness of ships, and a maintenance of the ship-building industry at a size beyond the need. Eventually, however, if the number of ships required still diminished, the production of iron in excess would become very unremunerative : some of the blast-furnaces would be blown out ; and as much of the capital and labour as remained available, would be re-distributed among other occupations. Without repeating the steps of the argument, it will be clear that were the enlargement of the ship-building industry great enough, and did it last long enough, to cause an in¬ crease in the number of coal-mines ; the ship-building in¬ dustry would be still better able to maintain itself under •/ • adverse circumstances ; but that it would, though at a more distant period, end by sinking down to the needful dimensions. Thus our conclusions are : — First, that if the extra activity and growth of a particular industry, has lasted long enough only to remodel the proximately-affected industries ; it will dwindle away again after a moderate period, if the need for it disappears. Second, that an enormous period must be re¬ quired before the re-actions produced by an enlarged industry. ADAPTATION. 199 can cause a re- construction of the whole society, and before the countless re-distributions of capital and labour, can again reach a state of equilibrium. And third, that only when such a new state of equilibrium is eventually reached, can the adaptive modification become a permanent one. IIow, in animal organisms, the like argument will hold, needs not be pointed out. The reader will readily follow the parallel. That organic types should be comparatively stable, might be anticipated on the hypothesis of Evolution. If we assume, as we must according to this hypothesis, that the structure of any organism is a product of the almost infinite series of actions and re-actions to which all ancestral organisms have been exposed ; we shall see that any unusual actions and re¬ actions brought to bear on an individual, can have but an infinitesimal effect in permanently changing the structure of the organism as a whole. The new set of forces, com¬ pounded with all the antecedent sets of forces, can but inap¬ preciably modify that moving equilibrium of functions which all these antecedent sets of forces have established. Though there may result a considerable perturbation of certain func¬ tions — a considerable divergence from their ordinary rhythms; yet the general centre of equilibrium cannot be sensibly changed. On the removal of the perturbing cause, the pre¬ vious balance will be quickly restored : the effect of the new forces being almost obliterated by the enormous aggregate of forces which the previous balance expresses. § 71. As thus understood, the phenomena of adaptation fall into harmony with first principles. The inference that organic types are fixed, because the deviations from them which can be produced within assignable periods, are relatively small ; and because, when a force producing deviation ceases, there is a return to something like the original state ; proves to be an invalid inference. Without assuming fixity of species, we find good reasons for anticipating that kind and degree of stability which is observed. We find grounds for concluding, 200 THE INDUCTIONS OF BIOLOGY. a priori, that an adaptive change of structure, will soon reach a point beyond which further adaptation will be slow ; for concluding that when the modifying cause has been but a short time in action, the modification generated, will be evanescent ; for concluding that a modifying cause acting even for many generations, will do but little towards per¬ manently altering the organic equilibrium of a race ; and for concluding that on the cessations of such cause, its effects will become unapparent in the course of a few gener¬ ations. CHAPTER VL INDIVIDUALITY. § 72. What is an individual? is a question which many readers will think it easy to answer. Yet it is a question that has led to much controversy among Zoologists and Botanists ; and no quite satisfactory reply to it seems possi¬ ble. As applied to a man, or to any one of the higher animals, which are all sharply- defined and independent, the word individual has a clear meaning ; though even here, when we turn from average cases to exceptional cases — as a calf with two heads and two pairs of fore-limbs — we find ourselves in doubt whether to predicate one individuality or two. But when we extend our range of observation to the organic world at large, we find that difficulties allied to this exceptional one, meet us everywhere under every variety of form. Each uniaxial plant may perhaps fairly be regarded as a distinct individual ; though there are botanists wdio do not make even this admission. What, however, are we to say of a multiaxial plant ? It is, indeed, usual to speak of a tree with its many branches and shoots, as singular ; but strong reasons may be urged for considering it as plural. Every one of its axes has a more or less independent life, and when cut off and planted, may grow into the likeness of its parent ; or by grafting and budding, parts of this tree may be developed upon another tree, and there manifest their 202 THE INDUCTIONS OF BIOLOGY. specific peculiarities. Shall we regard all the growing axes thus resulting from slips and grafts and buds, as parts of one individual, or as distinct individuals P If a strawberry- plant sends out runners carrying buds at their ends, which strike root and grow into independent plants, that separate from the original one by decay of the runners, must we not say that they possess separate individualities ; and yet if we do this, are wTe not at a loss to say when their separate individu¬ alities were established, unless we admit that each bud was from the beginning an individual ? Commenting on such perplexities, Schleiden says — “ Much has been written and disputed concerning the conception of the individual, with¬ out, however, elucidating the subject, principally owing to the misconception that still exists as to the origin of the con¬ ception. Now the individual is no conception, but the mere subjective comprehension of an actual object, presented to us under some given specific conception, and on this latter it alone depends whether the object is or is not an individual. Under the specific conception of the solar system, ours is an individual : in relation to the specific conception of a planet¬ ary body, it is an aggregate of many individuals.” * * * “I think, however, that looking at the indubitable facts already mentioned, and the relations treated of in the course of these considerations, it will appear most advantageous and most useful, in a scientific point of view, to consider the vegetable cell as the general type of the plant (simple plant of the first order). Under this conception, Protococcus and other plants consisting of only one cell, and the spore and pollen- granule, will appear as individuals. Such individuals may, however, again, with a partial renunciation of their in¬ dividual independence, combine under definite laws into definite forms (somewhat as the individual animals do in the globe of the Volcox globator *). These again appear empiri¬ cally as individual beings, under a conception of a species * It is now generally agreed that the Volvox globator is a plant. INDIVIDUALITY. 203 (simple plants of tlie second order) derived from the form of the normal connexion of the elementary individuals. But we cannot stop here, since nature herself combines these in¬ dividuals, under a definite form, into larger associations, whence wTe draw the third conception of the plant, from a connexion, as it were, of the second power (compound plants • — plants of the third order). The simple plant proceeding from the combination of the elementary individuals is then termed a bud {gemma), in the composition of plants of the third order.” The animal kingdom presents still greater difficulties. When, from sundry points on the body of a common polype, there bud-out young polypes, which, after acquiring mouths and tentacles and closing up the communications between their stomachs and the stomach of the parent, finally separate from the parent ; we may with propriety regard them as dis¬ tinct individuals. But when, in the allied compound Hydro- zoa, we find that these }Toung polypes continue permanently connected with the parent ; and when, by this continuous budding-out, there is presently produced a tree-like aggre¬ gation, having a common alimentary canal into which the digestive cavity of each polype opens; it is no longer so clear that these little sacs furnished with mouths and tenta¬ cles, are severally to be regarded as distinct individuals. We cannot deny a certain individuality to the polypedom. And on discovering that some of the buds, instead of unfolding in the same manner as the rest, are transformed into capsules in which eggs are developed — on discovering that certain of the incipient polypes thus become wholly dependent on the aggregate for their nutrition, and discharge functions which have nothing to do with their own maintenance, wre have still clearer proof that the individualities of the members are partially merged in the individuality of the group. Other organisms belonging to the same order, display still more decidedly this transition from simple individualities to a com¬ plex individuality. In the Uipliyes there is a special modifi- 204 THE INDUCTIONS OF BIOLOGY. cation of one or more members of tbe polypedom into a swimming apparatus, which, by its rhythmical contractions, propels itself through the water, drawing the polypedom after it. And in the more differentiated Physalia , various organs result from the metamorphosis of parts that are the homo- logues of individual polypes. In this last instance, the in¬ dividuality of the aggregate is so predominant, that the individualities of the members are practically lost. This combination of individualities in such way as to produce a composite individual, meets us in other forms among the ascidian molluscs. While in some of these, as in the Clavelina, the animals associated are but little subordinated to the community they form ; in others, as in the Botryllidce , they are so fused into a rounded mass, as to present the appearance of a single animal with several mouths and stomachs. On the hypothesis of Evolution, perplexities of this nature are just such as we might anticijaate. If Life in general, com¬ menced with minute and simple forms, like those out of which all individual organisms, however complex, now originate ; and if the transitions from these primordial units to organisms made up of groups of such units, and to higher organisms made up of groups of such groups, took place by degrees ; it is clear that individualities of the first and simplest order, would merge gradually in those of a larger and more complex order, and these again in others of an order having still greater bulk and organization ; and that hence it would be impossible to say where the lower indivi¬ dualities ceased, and the higher individualities commenced. § 73. To meet these difficulties, it has been proposed that the whole product of a single fertilized germ, shall be re¬ garded as a single individual : whether such whole product be organized into one mass, or whether it be organized into many masses, that are partially or completely separate. It is urged that whether the development of the fertilized germ INDIVIDUALITY. 205 be continuous or discontinuous (§ 50) is a matter of secondary importance ; that the totality of living tissue to which the fertilized germ gives rise in any one case, is the equivalent of the totality to which it gives rise in any other case ; and that we must recognize this equivalence, whether such totality of living tissue takes a concrete or a discrete arrangement. In pursuance of this view, a zoological individual is consti¬ tuted either by any such single animal as a mammal or bird, which may properly claim the title of a zooii, or by any such group of animals as the numerous Medusa that have been developed from the same egg, which are to be severally dis¬ tinguished as zooids. Admitting it to be very desirable that there should be words for expressing these relations and this equivalence, it may still be objected, that to apply the word individual to a number of separate living bodies, is inconvenient : conflictingso much, as it does, with the ordinary conception which this word suggests. It seems a questionable use of language to say that the countless masses of Anacharis Alsinastrum , which, within these few years, have grown up in our rivers, canals, and ponds, are all parts of one individual ; and yet as this plant does not seed in England, these countless masses, having arisen by discontinuous development, must be so regarded, if we accept the above definition. It may be contended, too, that while it does violence to our established way of thinking, this mode of interpreting the facts is not without its difficulties — smaller, perhaps, than those it escapes, but still considerable. Something seems to be gained by restricting the application of the title individual, to organisms which, being in all respects fully developed, possess the power of producing their kind after the ordinary sexual method ; and denying this title to those incomplete organisms which have not this power. But the definition does not really establish this distinction for us. On the one hand, we have cases in which, as in the working bee, the wffiole of the germ-product is aggregated into a single 206 THE INDUCTIONS OF BIOLOGY. organism ; and yet, though, an individual according to the definition, this organism has no power of reproducing its kind. On the other hand, we have cases like that of the perfect Aphides, where the organism is hut an infinitesimal part of the germ-product ; and yet has that completeness required for sexual reproduction. Moreover, if we adopt the proposed view, we find ourselves committed to the anomalous position, that among many orders of animals, there are no concrete individuals at all. If the individual is consti¬ tuted by the whole germ-product, whether continuously or discontinuousiy developed, then, not only must individuality be denied to each of the imperfect Aphides, but nlso to each of the perfect males and females ; since no one of them is more than a minute fraction of the total germ- product. And yet further, it might he urged with some show of reason, that if the conception of individuality involves the conception of completeness ; then, an organism which possesses an independent power of reproducing itself, being more complete than an organism in which this power is dependent on the aid of another organism, is more in¬ dividual. § 74. There is, indeed, as already implied, no definition of individuality that is unobjectionable. All we can do is to make the best practicable compromise. As applied either to an animate or an inanimate object, the word individual ordinarily connotes union among the parts of the object, and separateness from other objects. This fundamental element in the conception of individuality, we cannot with propriety ignore in the biological application of the word. That which we call an individual plant or animal, must, therefore, be some concrete whole, and not a discrete whole. If, however, we say that each concrete living whole is to be regarded as an individual, we are still met by the question — What constitutes a concrete living whole P A young organism arising by internal or external INDIVIDUALITY. 207 gemmation from a parent organism, passes gradually from a state in which it is an indistinguishable part of the parent organism, to a state in which it is a separate organism of like structure with the parent. At what stage does it become an individual ? And if its individuality be conceded only when it completely separates from the parent, must we deny in¬ dividuality to all organisms thus produced, which permanently retain their connexions with their parents P Or again, what must wre say of the Hectocotylus , w7hich is an arm of the Cuttle-fish that undergoes a special development, and then detaching itself, lives independently for a considerable period? And wdiat must we say of that larval Echinus , which is left to move about awhile after being robbed of its viscera by the young Echinus developed within it ? To answer such questions, we must revert to the definition of Life. The distinction between individual in its biological sense, and individual in its more general sense, must consist in the manifestation of Life, properly so called. Life we have seen to be, “ the definite combination of heterogeneous changes, both simultaneous and successive, in correspondence with external co-existences and sequences.” Hence, a biolo¬ gical individual is any concrete whole having a structure which enables it, when placed in appropriate conditions, to continuously adjust its internal relations to external relations, so as to maintain the equilibrium of its functions. In pursuance of this conception, we must consider as individuals, all those wholly or partially independent organized masses, which arise by multicentral and multiaxial development that is either continuous or discontinuous (§ 50). We must accord the title to each separate aphis, each polype of a polypedom, each bud or shoot of a flowering plant, whether it detaches itself as a bulbil or remains attached as a branch. By thus interpreting the' facts, we do not, indeed, avoid all anomalies. While, among flowering plants, the power of in¬ dependent growth and development, is usually possessed only by shoots or axes ; yet, in some cases, as in that of the Begonia- 2 OS THE INDUCTIONS OF BIOLOGY. leaf awhile since mentioned, the appendage of an axis, or even a small fragment of such appendage, is capable of initiating and carrying on the functions of life ; and in other cases, as shown by M. Naudin in the Dr os era intermedia , young plants are occasionally developed from the surfaces of leaves, while still connected with the parent plant. Nor among forms like the compound Ili/drozoa , does the definition enable us to decide where the line is to be drawn between the individuality of the group and the individualities of the members — merging into each other, as these do, in different degrees. But, as before said, such difficulties must necessa¬ rily present themselves, if organic forms have arisen by in¬ sensible gradations. We must be content with a course which commits us to the smallest number of incongruities ; and this course is, to consider as an individual, any centre or axis that is capable of independently carrying on that con¬ tinuous adjustment of inner to outer relations which consti¬ tutes Life. i CHAPTER YIL GENESIS. § 75. IIa\ing concluded what constitutes an individual, we are in a position to deal with the multiplication of in¬ dividuals. For this, the title Genesis is here chosen, as being the most comprehensive title — the least specialized in its meaning. By some biologists, Generation has been used to signify one method of multiplication, and Reproduction to signify another method ; and each of these words has been thus rendered in some degree unfit to signify multiplication in general. Here the reader is indirectly introduced to the fact, that the production of new organisms is carried on in fundament¬ ally unlike ways. lip to quite recent times, it was believed, even by naturalists, that all the various processes of multipli¬ cation observable in different kinds of organisms, have one essential character in common : it was supposed that in every species, the successive generations are alike. It has now been proved, however, that in plants, and in numerous animals, the successive generations are not alike ; that from one generation there proceeds another whose members differ more or less in structure from their parents ; that these produce others like themselves, or like their parents, or like neither ; but that eventually, the original form re- appears. Instead of there being, as in the cases most familiar to us, a constant recurrence of the same form, there is a cyclical recurrence of 10 210 THE INDUCTIONS OF BIOLOGY. the same form. These two distinct processes of multiplication, may be aptty termed homogenesis and heterogenesis * Under these heads let us consider them more closely. The kind of genesis, once supposed to be universal, in which the successive generations are alike, is always sexual genesis ; or, as it has been otherwise called — gamogenesis. In every species of organism which multiplies by homogenesis, each generation consists of males and females ; and from the fertilized germs they produce, the next generation of similar males and females arises. This method of propagation is further distinguished by the peculiarity, that each fertilized germ gives rise to but one individual — the product of de¬ velopment is always organized round one axis, and not round several axes. Between the different kinds of homo¬ genesis, the most marked contrast, and the only one which need here detain us, is that between the oviparous and the viviparous. The oviparous kind is that in which the fertil¬ ized germ is detached from the parent, before it has undergone any considerable development. The viviparous kind is that in which development is considerably advanced, or almost completed, before final detachment takes place. This distinction is, however, not a sharply- defined one : there are transitions between the oviparous and the viviparous processes. In ovo-viviparous genesis, there is an internal incubation ; and though the young are in this case finally detached from the parent in the shape of eggs, they do not leave the parent’s body until after they have assumed something like the parental form. Looking around, we find that homogenesis is universal among the Vertebrata : there is no known vertebrate animal but what arises from a fertilized germ, and unites into its single indi¬ viduality the whole products of this fertilized germ. In * Unfortunately the word heterogenesis, has been already used as a synonyme tor ‘ spontaneous generation.” Save by those few who believe in “ spontaneous generation,” however, little objection will be felt to using the word in a sense that seems much more appropriate. GENESIS. 211 the mammals or highest Vertebrata , this homogenesis is in every case viviparous ; in birds it is uniformly oviparous ; and in reptiles and fishes, it is always essentially oviparous, though there are cases, of the kind above referred to, in which viviparity is simulated. Passing to the Invertebrata , we find oviparous homogenesis universal among the Aracli- mda (except the Scorpions, which are ovo- viviparous) ; universal among the higher Crustacea, but not among the lower; extremely general, though not universal, among Insects ; and universal among the higher Mollusca , though not among the lower. Along with extreme inferiority among animals, we find homogenesis to be the exception rather than the rule ; and in the vegetal kingdom, there appear to be no cases, save those of a few aberrant parasites like the Raffle siacece, in which the centre or axis which arises from a fertilized germ, becomes the immediate producer of fertilized germs. Where propagation is carried on by heterogenesis, or is characterized by unlikeness of the successive generations, there is always asexual genesis with occasionally-recurring sexual genesis ; in other words — agamogenesis interrupted more or less frequently by gamogenesis. If we set out with a generation of perfect males and females ; then, from their ova or seeds, there arise individuals that are neither males nor females, but that produce the next generation from buds. By this method of multiplication, many individuals originate from a single fertilized germ : the product of development is organized round more than one centre or axis* The simplest form of heterogenesis is that seen in uniaxial plants. If, as we find ourselves obliged to do, wre regard each separate shoot or axis of growth, as a dis¬ tinct individual ; then, in uniaxial plants, the successive in¬ dividuals are not represented by the series A, A, A, A, &c., like those resulting from homogenesis ; but they are repre¬ sented by the series A, B, A, B, A, B, &c. For in plants which were before classed as uniaxial (§ 50), and which may 212 THE INDUCTIONS OF BIOLOGY. be conveniently so distinguished from other plants, the axis which shoots up from the seed, and substantially constitutes the plant, does not itself flower and bear seed; but gives lateral origin to flowering, or seed-bearing, axes. Though in uni¬ axial plants, the fructifying apparatus appears to be at the end of the primary, vertical axis ; yet dissection shows that, morphologically considered, each fructifying axis is usually an offspring from the primary axis. There arises from the seed, a sexless individual, from which spring by gemmation, in¬ dividuals having reproductive organs ; and from these there result fertilized germs or seeds, that give rise to sexless individuals. That is to say, gamogenesis and agamogenesis alternate : the peculiarity being, that the sexual individu¬ als arise from the sexless ones by continuous development. The Salpce show us an allied form of heterogenesis in the animal kingdom. Individuals developed from fertilized ova, instead of themselves producing fertilized ova, produce, by gemmation, strings of individuals ; from which fertilized ova again originate. In multiaxial plants, we have a succession of generations represented by the series A, B, B, B, &c., A, B, B, B, &c. Supposing A to be a flowering axis, or sexual individual ; then, from any fertilized germ it casts off, there grows up a sexless individual, B ; from this there bud-out other sexless individuals, B ; and so on for generations more or less numerous ; until at length, from some of these sexless individuals, there bud- out seed-bearing individuals of the original form A. Branched herbs, shrubs, and trees, exhibit this form of hetero genesis ; the successive generations of sexless individuals thus produced, being in most cases continuously developed, or aggregated into a compound individual ; but being in some cases dis- continuously developed. Among animals, a kind of hetero¬ genesis represented by the same succession of letters, occurs in such compound polypes as the Sertularia ; and in those of the Hydrozoa which assume alternately the poly¬ poid form, and the form of the Medusa : the chief differences GENESIS. 213 presented by these groups, arising from the fact that the successive generations of sexless individuals produced by budding, are in some cases continuously developed, and in others discontinuously developed ; and from the fact that, in some cases, the sexual individuals give off their fertilized germs while still growing on the parent-polypedom, but in other cases, not until after leaving the parent-polypedom and undergoing further development. Where, as in all the foregoing kinds of agamogenesis, the new individuals bud-out, not from any specialized reproductive organs, but from unspecialized parts of the parent ; the process has been named, by Prof. Owen, metagenesis. In most instances, the individuals thus produced, grow from the outsides of the parents— the metagenesis is external. But there is also a kind of metagenesis which we may distinguish as internal. Certain entozoa of the genus Distoma, exhibit it. From the egg of a Distoma , there results a rudely-formed creature known to naturalists as the “ King’s-yellow worm.” Gradu¬ ally as this increases in size, the greater part of its inner substance is transformed into young animals called Cercarice (which are the larvae of Distomata ) ; until at length, it becomes little more than a living sac, full of living offspring*. In the Distoma pacifica, the brood of young animals thus arising by internal gemmation, are not Cercarice , but are of the same form as their parent : themselves becoming the producers of Cercarice after the same manner, at a subsequent period. So that sometimes the succession of forms is repre¬ sented by the series A, B, A, B, &c. ; and sometimes by the series A, B, B, A, B, B, &c. Both cases, however, exemplify internal metagenesis, in contrast with the several kinds of external metagenesis described above. That agamo¬ genesis which is carried on in a reproductive organ — either a true ovarium, or the homologue of one — has been called, by Prof. Owen, parthenogenesis. In his work published under this title, he embraced those cases in which the buds arising in the pseud- ovarium, are not ova in the full sense of the 214 THE INDUCTIONS OF BIOLOGY. word ; but rather, as they have since been called by Prof. Huxley, pseud-ova. Yon Siebold and other naturalists, have hence applied the term parthenogenesis to a narrower class of cases. Perhaps it would be best to distinguish this process, which is intermediate between metagenesis and parthenogenesis, by the term pseudo-parthenogenesis. It is the process familiarly exemplified in the Aphides. Here, from the fertilized eggs laid by perfect females, there grow up imperfect females, in the pseud-ovaria of which there are developed pseud-ova ; and these, rapidly assuming the organization of other imperfect females, are born vivi- parously. From this second generation of imperfect females, there by and by arises, in the same manner, a third genera¬ tion, of the same kind ; and so on for many generations : the series being thus sjunbolized by the letters A, B, B, B, B, B, &c., A. [Respecting this kind of heterogenesis, it should be added, that in animals, as in plants, the number of genera¬ tions of sexless individuals produced before the re-appearance of sexual ones, is indefinite ; both in the sense that in the same species it may go on to a greater or less extent accord¬ ing to circumstances, and in the sense that among the genera¬ tions of individuals proceeding from the same fertilized germ, a recurrence of sexual individuals takes place earlier in some of the diverging lines of multiplication than in others. In trees we see that on some branches, flower-bearing axes arise while other branches are still producing only leaf-bearing axes ; and in the successive generations of Aphides , a parallel truth has been observed. Lastly has to be set down, that form of heterogenesis in which, along with gamogenesis, there occurs a form of agamogenesis exactly like it, save in the absence of fecundation. This is called true parthenogenesis — reproduction carried on by virgin mothers, which are in all respects like other mothers. In the silk-worm-moths this parthenogenesis is exceptional, rather than ordinary : usually the eggs of these insects are fertilized ; but if they are not, they are still laid, and some of them produce larvae. In certain Lepidoptem, however, of the groups Psychidee and GENESIS. 215 Tincidce, parthenogenesis appears to bo & normal process _ - indeed, so far as is known, the only process ; for of some species the males have never been found. A general conception of the relations among the different modes of Genesis, thus briefly described, will be best given by the following tabular statement. Ilomogenesis, which is Gamogenesis Genesis is or ' Oviparous or < Ovo-viviparous or „ Viviparous ” Gamoo-enesis . IIeterogenesis,whichis- alternating f Parthenogenesis with 1 & . I. or . Agamogenesis *; Pseudo-parthenogenesis or f Internal _ Metagenesis or External This, like all other classifications of such phenomena, pre¬ sents anomalies. It may be justly objected, that the processes here grouped under the head agamogenesis, are the same as those before grouped under the head of discontinuous develop¬ ment (§ 50) : thus making development and genesis partially coincident. Doubtless it seems awkward that what are from one point of view considered as structural changes, are from another point of view considered as modes of multiplication.* * Prof. Iluxley avoids this difficulty by making every kind of Genesis a mode of development. Ilis classification, which suggested the one given above, is as follows : — Tontinuous Development* ( Growth ( Metamorphosis {Agamogenesis Gamogenesis {Metagenesis Parthenogenesis 216 THE INDUCTIONS OF BIOLOGY. There is, however, nothing for us hut a choice of imperfec¬ tions. We cannot by any logical dichotomies, accurately express relations which, in Nature, graduate into each other insensibly. Neither the above, nor any other scheme, can do more than give an approximate idea of the truth. § 76. Genesis under every form, is a process of negative or positive disintegration ; and is thus essentially opposed to that process of integration, which is one element of individual evolution. Negative disintegration occurs in those cases where, as among the compound Hydrozoa , there is a con¬ tinuous development of new individuals by budding from the bodies of older individuals ; and where the older individuals are thus prevented from growing to a greater size, or reach¬ ing a higher degree of integration. Positive disintegration occurs in those cases of agamogenesis where the formation of new individuals is discontinuous, and in all cases of gamo- genesis. The degrees of disintegration are various. At the one extreme, the parent organism is completely broken up, or dissolved into new individuals ; and at the other extreme, the new individual forms but a small deduction from the parent organism. Protozoa and Protophyta, show us that form of disintegration called spontaneous fission : two or four individuals being produced by the splitting-up of the original one. The Pol vox and the Hydrodictyon, are plants which, having developed broods of young plants within themselves, give them exit by bursting ; and among animals, the one lately referred to, which arises from the Distoma egg, entirely loses its individuality in the individ¬ ualities of the numerous DistomaAvuvxdd with which it be¬ comes filled. Speaking generally, the degree ol disintegration becomes less marked, as we approach the higher organic forms. Plants of advanced types throw off from themselves, whether by gamogenesis or agamogenesis, parts that are relatively small ; and among the higher animals, there is no case in which the parent individuality is habitually GENESIS. 217 lost, in tlie production of new individualities. To the last, however, there is of necessity a greater or less disinte¬ gration. The seeds and pollen-grains of a flowering plant, are disintegrated portions of tissue ; as are also the ova and spermatozoa of animals. And whether the fertilized germs carry away from their parents small or large quantities of nutriment, these quantities of nutriment in all cases involve further negative or positive disintegrations of the parents. New individuals that result from agamogenesis, usually do not separate from the parent-individuals, until they have undergone considerable development, if not complete develop¬ ment. The agamogenetic offspring of those lowest organisms which develop centrally, do not, of course, pass beyond cen¬ tral structure ; but the agamogenetic offspring of organisms that develop axially, commonly assume an axial structure before they become independent. The vegetal kingdom shows us this in the advanced organization of detached bulbils, and of buds that root themselves before separating. Of animals, the ' Hydrozoa, the Trematoda , the Salpce, and the Aphides, present us with different kinds of agamogenesis, in all of which the new individuals are organized to a considerable extent before being cast off. This rule is not without excep¬ tions, however. The winter-eggs of the Plumotella, developed in an unspecialized part of the body, present us with a case of metagenesis, in which centres of development, instead ol axes, are detached ; and in the above-described parthenogene¬ sis of moths and bees, such centres are detached from an ovarium. When produced by gamogenesis, the new individuals be¬ come independent of the parents while in the shape of centres of development, rather than axes of development ; and this even where the reverse is apparently the case. The fertilized germs of those inferior plants which are central, or multicen¬ tral, in their development, are of course thrown off as centres. In the higher plants, of the two elements that go to the form¬ ation of the fertilized germ, the pollen-cell is absolutely / 218 THE INDUCTIONS OF BIOLOGY. separated from the parent-plant under the shape of a centre J and the embryo -cell, though not absolutely separated from the parent, is still no longer subordinate to the organizing forces of the parent. So that when, the embryo- cell having been fertilized by matter from the pollen-tube, the develop¬ ment commences, it proceeds without parental control : the new individual, though remaining physically united with the old individual, becomes structurally and functionally separate while still only a centre of development ; and takes on its axial form by processes of its own — the old individual doing no more than supply materials. Through¬ out the animal kingdom, the new individuals produced by gamogenesis, are obviously separated in the shape of centres of development wherever the reproduction is oviparous . the only conspicuous variation being in the quantity of nutritive matter bequeathed by the parent to the new centre of de¬ velopment, at the time of its separation. And though, where the reproduction is viviparous, the process appears to be different, and in one sense is so; yet, intrinsically, it is' the same. For in these cases, the new individual really detaches itself from the parent while still only a centre of develop¬ ment ; but instead of being finally cast off in this state, it is re-attached, and supplied with nutriment until it assumes a more or less complete axial structure. § 77. Under all its various forms, the essential act in gamo¬ genesis, is the union of two centres or cells, produced by different parent organisms : the sperm-cell being the male product, and the germ-cell the female. There are very many modes and modifications of modes in which these cells are produced ; very many modes and modifications of modes by which they are brought into contact ; and very many modes and modifications of modes by which the result¬ ing fertilized germs have secured to them the fit conditions for their development. But passing over these many diver¬ gent and re-divergent kinds of sexual multiplication, which GENESIS. 219 t would take too muck space liere to specify, the one uni¬ versal peculiarity which it concerns us to remark, is, this co¬ alescence of a detached portion of one organism, with a more or less detached portion of another. Such protophytes as the PalmeUce and the Desmidiece, which are sometimes distinguished as unicellular plants, show us a coalescence, not of detached portions of two organisms, but of two entire organisms : in the PalmeUce , conjugation is a complete fusion of the individuals ; and in the Desmidiece , the entire contents of the individuals unite to form the germ- mass. Where, as among the Confervce, we have aggregated cells whose individualities are scarcely at all subordinate to that of the aggregate, the gamogenetic act is effected by the union of the contained granules of two adjacent cells. In Spirogyra, it is not adjacent cells in the same thread which thus combine ; but cells of one thread with those of another. As we ascend to plants of high organization, we find that the two reproductive elements become quite distinct in their characters ; and further, that they arise in different organs set apart for their production : the arrangements being such, that the sperm-cells of one plant combine with the germ-cells of another. There is reason to think that, among the lowest Protozoa , a fusion of two individualities, analogous to that which occurs in the conjugation of certain Algce , is the process from which results the germ of a new series of individuals. But in animals formed by the aggregation of units that are homolo¬ gous with Protozoa, the sperm-cells and germ-cells are differ¬ entiated. And even in these humble forms, where there is no differentiation of sexes, we have good evidence that, as in all higher forms, the union is not between sperm-cells and germ- cells that have arisen in the same individual ; but between those that have arisen in different individuals. The marvellous phenomena initiated by the meeting of sperm-cell and germ-cell, naturally suggest the conception of some quite special and peculiar properties possessed by these 220 THE INDUCTIONS OF BIOLOGY. cells. It seems obvious that this mysterious power which they display, of originating a new and complex organism, distinguishes them in the broadest "way from portions of organic substance in general. Nevertheless, the more we study the evidence, the more is this assumption shaken — the more are we led towards the conclusion, that these cells have not been made by some unusual elaboration, fundamentally different from all other cells. The first fact which points to this modified conclusion, is the fact recently dwelt upon (§ 63), that in many plants and inferior animals, a small fragment of tissue that is but little differentiated, is capable of developing into the form of the organism from which it was taken. Conclusive proof obliged us to admit, that the component units of organisms, have inherent powers of arranging themselves into the forms of the organisms to which they belong. And if to these component units, which we distinguished as physiological, such powers must be con¬ ceded — if, under fit conditions, and when not much specialized, they manifest such powers in a way as marked as that in which the contents of sperm-cells and germ-cells manifest them ; then, it becomes clear that the properties of sperm- cells and germ-cells are not so peculiar as we are apt to assume. Again, the organs for preparing sperm- cells and germ-cells, have none of the speciality of struc¬ ture which might be looked for, did sperm-cells and germ- cells need endowing with properties essentially unlike those of all other organic agents. On the contrary, these reproductive centres proceed from tissues that are character¬ ized by their low organization. In plants, for example, it is not appendages that have acquired considerable structure, which produce the fructifying particles : these arise at the extremities of the axes, where the degree of structure is the least. The embryo-cells are formed in the undifferentiated part of the cambium-layer ; the pollen-grains are formed at the little-differentiated extremities of the stamens ; and both are homologous with simple epithelium-cells. Among many GENESIS. 221 inferior animals devoid of special reproductive organs, such as the Hydra , the ova and spermatozoa originate in the layer of indifferent tissue that lies between the endoderm and the ectoderm ; that is, they consist of portions of the least specialized substance. And in the higher animals, these same generative agents appear to be merely modified epithelium- cells — cells not remarkable for their complexity of structure, but rather for their simplicity. If, by way of demurrer to this view, it is asked why other epithe¬ lium-cells do not exhibit like properties ; there are two replies. The first is, that other epithelium-cells are usually so far changed to fit them to their special functions, that they are unfitted for assuming the reproductive function. The second reply is, that in some cases, where the epithelium- cells are but very little specialized, they do exhibit the like properties: not, indeed, by uniting with other epithelium- cells to produce new germs, but by producing new germs without such union. I learn from Dr Hooker, that the Begonia phyllomaniaca habitually develops young plants from the scales of its stem and leaves — nay, that many young plants are developed by a single scale. The epithelium- cells composing one of these scales, swell, here and there, into large globular cells ; form chlorophyll in their interiors ; shoot out rudimentary axes ; and then, by spontaneous constrictions, cut themselves off; drop to the ground ; and grow into Begonias. It appears, too, that in a succulent English plant, the Malaxis paludosa, a like process occurs : the self-detached cells being, in this case, produced by the surfaces of the leaves. Thus, there is no warrant for the assumption that sperm-cells and germ- cells possess powers fundamentally unlike those of other cells. The inference to which the facts point, is, that they differ from the rest, mainly in not having undergone modifications such as those by which the rest are adapted to particular functions. They are cells that have departed but little from the original and most general type. Or, in the words suggested by a friend, it is not that they are peculiarly 222 THE INDUCTIONS OF BIOLOGY. specialized, but rather that they are unspecialized : such specializations as some of them exhibit in the shape of loco¬ motive appliances, &c., being interpretable not as intrinsic, but as extrinsic, modifications, that have reference to nothing beyond certain mechanical requirements. Sundry facts tend likewise to show, that there does not exist the pro¬ found distinction which we are apt to assume, between the male and female reproductive elements. In the common polype, sperm-cells and germ-cells are developed in the same layer of indifferent tissue ; and in Tethya , one of the sponges, Prof. Iluxley has observed that they occur mingled together in the general parenchyma. The pollen-grains and embryo- cells of plants, arise in adjacent parts of the cambium-layer ; and from a description of a monstrosity in the Passion-flower, recently given by Mr Salter to the Linnsean Society, it ap¬ pears, both that ovules may, in their general structure, graduate into anthers, and that they may produce pollen in their interiors. All which evidence is in perfect harmony with the foregoing conclusion ; since, if sperm-cells and germ-cells have natures not essentially unlike those of un¬ specialized cells in general, their natures cannot be essen¬ tially unlike each other. The next general fact to be noted, is, that these cells whose union constitutes the essential act of gamogenesis, are cells in which the developmental changes have come to a close — cells which, however favourably circumstanced in respect of nutrition, are incapable of further evolution. Though they are not, as many cells are, unfitted for growth and metamorphosis by being highly specialized ; yet they have lost the power of growth and metamorphosis. They have severally reached a state of equilibrium. And while the internal balance of forces prevents a continuance of con¬ structive changes, it is readily overthrown by external destructive forces, bor it uniformly happens that sperm- cells and germ- cells which are not brought in contact, disap¬ pear. In a plant, the embryo-cell, if not fertilized, is GENESIS. 223 absorbed or dissipated, while the ovule aborts ; and the un- impregnated ovum eventually decomposes. Such being the characters of these cells, and such being their fates if kept apart, we have now to observe what hap¬ pens when they are united. For a long time, the immediate sequence of their contact was not ascertained. This is at length, however, decided. It has been shown that in plants, the extremity of the elongated pollen-cell applies itself to the surface of the embryo-sac, but does not enter the embryo- sac. In animals, however, the process is different. Careful observers agree, that the spermatozoon passes through the limiting membrane of the ovum. The result in both cases is presumed to be a mixture of the contents of the two cells. The evidence goes to show that in plants, matter passes by osmose from the pollen-cell into the embryo¬ cell ; and that in animals, the substance contained in the spermatozoon becomes mingled with the substance contained in the ovum, either by simple diffusion or by cell-multiplica¬ tion. But the important fact which it chiefly con¬ cerns us to notice, is, that on the union of these reproductive elements, there begins, either at once or on the return of favourable conditions, a new series of developmental changes. The state of equilibrium at which each of them had arrived, is destroyed by their mutual influence ; and the constructive changes which had come to a close, recommence : a process of cell-multiplication is set up ; and the resulting cells pre¬ sently begin to aggregate into the rudiment of a new organism. Thus, passing over the variable concomitants of gamo- genesis, and confining our attention to what is constant in it, we see : — that there is habitually, if not universally, a fusion of two portions of organic substance, which are either them¬ selves distinct individuals, or are thrown off by distinct individuals ; that these portions of organic substance, which are severally distinguished by their low degree of special¬ ization, have arrived at states of structural quiescence or 224 THE INDUCTIONS OF BIOLOGY. equilibrium ; that if they are not united, this equilibrium ends in dissolution ; but that by the mixture of them, this equilibrium is destroyed, and a new evolution initiated. § 78. What are the conditions under which Genesis takes place ? IIow does it happen that some organisms multiply by homogenesis, and others by heterogenesis ? Why is it that where agamogenesis prevails, it is usually from time to time interrupted by gamogenesis P These are questions of extreme interest ; but questions to which decisive answers cannot yet be given. In the existing state of Biology, we must be content if we can learn the direction in which answers lie. A survey of the facts, discloses certain correla¬ tions which, if not universal, are too general to be without significance. Where the multiplication of individuals is carried on by heterogenesis, we find, in numerous cases, that agamogenesis continues as long as the forces which result in growth, are greatly in excess of the antagonistic forces. While conversely, we find that the recurrence of gamogenesis, takes place when the conditions are no longer so favourable to growth. In like manner, where there is homogenetic multiplication, new individuals are usually not formed while the preceding in¬ dividuals are still rapidly growing — that is, while the forces producing growth exceed the opposing forces to a great extent; but the formation of new individuals begins when nutrition is nearly equalled by expenditure. To specify all the facts that seem to warrant these inductions, would take more space than can be here spared. A few of them must suffice. fihe relation between fructification and innutrition, amouPe> is 80 well established by infinite illustrations, as to have assumed the character of an axiom ; it is not universally admitted that non-typical peculiarities are inherited. While the botanist would be so incredulous if told that a plant of one class had produced a plant of another class, or that from seeds belonging to one order individuals belonging to another order had grown, that he would deem it needless to examine the evidence ; and while the zoologist would treat with con¬ tempt the assertion, that from the egg of a fish a reptile had arisen, or that an implacental mammal had borne a pla¬ cental mammal, or that an unguiculate quadruped had sprung from an ungulate quadruped, or even that from individuals of one species offspring of an allied species had proceeded ; yet there are botanists and zoologists who do not consider it certain, that the minor specialities of organization are trans¬ mitted fiom one generation to another. Some naturalists seem to entertain a vague belief, that the law of Heredity applies only to main characters of structure, and not to de¬ tails ; or, at any rate, that though it applies to such details as constitute differences of species, it does not apply to smaller details. The circumstance that the tendency to re¬ petition, is in a slight degree qualified by the tendency to variation (which, as we shall hereafter see, is but an indirect result of the tendency to repetition), leads some to doubt whether Heredity is unlimited. A careful weighing of the evidence, however, and a due allowance for the influences by which the minuter manifestations of Heredity are obscured will remove the grounds for this scepticism. First in order of importance, comes the fact, that not only are there uniformly transmitted from an organism to its offspring, those traits of structure which distinguish the class, order, genus, and species ; but also those which distinguish the variety. W e have numerous cases, among both plants and animals, where, by natural or artificial conditions, there 240 TELE INDUCTIONS OF BIOLOGY. have been produced divergent modifications of the same species ; and abundant proof exists that the members of any one sub-species, habitually transmit their distinctive pecu¬ liarities to their descendants. Agriculturists and gardeners can furnish unquestionable illustrations. Several varieties of wheat are known ; of which each reproduces itself. Since its introduction into England, there have been formed from the potato, a number of sub-species: some of them differing greatly in their forms, sizes, qualities, and periods of ripening. Of peas, also, the like may be said. And the case of the cabbage-tribe, is often cited as showing the per¬ manent establishment of races that have diverged widely from a common stock. Among fruits and flowers, the multi¬ plication of kinds, and the continuance of each kind with certainty by agamogenesis, and to some extent by gamo- genesis, might be exemplified without end. From all sides evidence may be gathered showing a like persistence of varieties in each species of animal. TTe have our distinct breeds of sheep, our distinct breeds of cattle, our distinct breeds of horses : each breed maintaining its characteristics. The several sorts of dogs, which, if we accept the physiolo gical test, we must consider as all of one species, show us in a marked manner the hereditary transmission of small differ¬ ences — each sort, when kept pure, reproducing itself not only in size, form, colour, and quality of hair, but also in disposition and speciality of intelligence. Babbits, too, have their permanently-established races. And in the Isle of Man, we have a tail-less kind of cat. Even in the absence of other evidence, that which ethnology furnishes would suffice. Grant them to be derived from one stock, and the varieties of man yield proof upon proof that non-specific traits of structure are bequeathed from generation to gener¬ ation. Or grant only that there is evidence of their deriva¬ tion from several stocks, and we still have, between races de¬ scended from a common stock, distinctions which prove the inheritance of minor peculiarities. Besides seeing that HEREDITY. 241 neg.roes continue to produce negroes, copper-coloured men to produce men of a copper colour, and the fair-skinned races to perpetuate their fair skins — besides seeing that the broad¬ faced and flat-nosed Calmuck begets children with broad faces and flat noses, while the Jew bequeaths to his offspring the features which have so long characterized Jews ; we see that those small unlikenesses which distinguish more nearly-allied varieties of men, are maintained from generation to generation. In Germany, the ordinary shape of skull is appreciably differ¬ ent from that common in Britain : near akin though the Germans are to the British. The average Italian face con¬ tinues to be unlike the faces of northern nations. The French character is now, as it was centuries ago, contrasted in sundry respects with the characters of neighbouring peoples. Hay, even between races so closely allied as the Scotch Celts, the Welch Celts, and the Irish Celts, appreciable differences of form and nature have become established. That sub-species and sub-sub-species, thus exemplify that same general law of inheritance which shows itself in the per¬ petuation of ordinal, generic, and specific peculiarities ; is strong reason for the belief that this general law is unlimited in its application. In addition to the warrant which this be¬ lief derives from evidence of this kind, it has also the support of still more special evidence. Numerous illustrations of He¬ redity are yielded by experiment, and by direct observation of successive generations. They are divisible into two classes. In the one class come cases where congenital peculiarities, not traceable to any obvious causes, are bequeathed to de¬ scendants. In the other class come cases where the peculiar¬ ities thus bequeathed are not congenital, but have resulted from changes of functions during the lives of the individuals bequeathing them. We will consider first the cases that come in the first class. § 81. Note at the outset the character of the chief testi¬ mony. Excluding those inductions that have been so fully 242 THE INDUCTIONS OF BIOLOGY. verified as to rank with, exact science, there are no inductions so trustworthy as those which have undergone the mercantile test. When we have thousands of men whose profit or los3 depends on the truth of the inferences they draw from simple and perpetually-repeated observations ; and when we find that the inference arrived at, and handed down from genera¬ tion to generation of these deeply-interested observers, has become an unshakable conviction ; we may accept it without hesitation. In breeders of animals we have such a class, led by such experiences, and entertaining such a conviction — the conviction that minor peculiarities of organization are in¬ herited as well as major peculiarities. Hence the immense prices given for successful racers, bulls of superior forms, sheep that have certain desired peculiarities. Hence the careful record of pedigrees of high-bred horses and sporting doers. Hence the care taken to avoid intermixture with in- o ferior stocks. Citing the highest authorities respecting the effects of breeding from animals having certain superiorities, with the view of propagating those superiorities, Mr Darwin writes : — “ Youatt, who was probably better acquainted with the works of agriculturists than almost any other individual, and who was himself a very good judge of an animal, speaks of the principle of selection as t that which enables the agri¬ culturist not only to modify the character of his flock, but to change it altogether. It is the magician’s wand, by means of which he may summon into life whatever form and mould he pleases.’ ” Lord Somerville, speaking of what breeders have done for sheep, says : — “ It would seem that they had chalked upon a wall a form perfect in itself and then given it exist¬ ence.” That most skilful breeder, Sir John Sebright, used to say, with respect to pigeons, that “ he would produce any given feather in three years, but it would take him six years to obtain head and beak.” In all which statements the tacit assertion is, that individual traits are bequeathed from generation to generation ; and that when they are not brought into conflict with opposite traits, they may be HEREDITY. 243 iso perpetuated and increased as to become permanent dis¬ tinctions. Of special instances, tlierc are many besides that of the oft en-cited Otter-breed of sheep, descended from a single short legged lamb, and that of the six-fingered Gratio Kelleia, who transmitted his peculiarity in different degrees, to several of his children and to some of his grandchildren. In a paper con¬ tributed to the Edinburgh New Philosophical Journal for July 1863, Dr Struthers gives several cases of hereditary digital variations. Esther P — , who had six fingers on one hand, be¬ queathed this malformation, along some lines of her descend¬ ants, for two, three, and four generations. A — S — inherited an extra digit on each hand and each foot from his father ; and C— G — , wrho also had six fingers and six toes, had an aunt and a grandmother similarly formed. A collection of evidence has been made by Mr Sedgwick, and published by him in the Medico- Chirurgical Review for April and for July 1863, in two articles on “ The Influence of Sex in limiting Hereditary Transmission. ” From these articles are selected the following cases and authorities : — Augustin Duforet, a pastry-cook of Douai, who had but two instead of three phalanges to all his fingers and toes, inherited this malformation from his grand¬ father and father, and had it in common with an uncle and numerous cousins. An account has been given by Dr Lepine, of a man with only three fingers on each hand and four toes on each foot, and whose grandfather and son exhibited the like anomaly. Bechet describes Yictoire Barre as a woman who, like her father and sister, had but one developed finger on each hand, and but two toes on each foot, and whose mon¬ strosity re-appeared in two daughters. And there is a case where the absence of two distal phalanges on the hands was traced for two generations. The various recorded instances in which there has been transmission from one generation to another, of webbed-fingers, of webbed-toes, of hare-lip, of congenital luxation of the thigh, of absent patella?, of club-foot, &c., would occupy more space than can here be 244 THE INDUCTIONS OF BIOLOGY. spared. Defects in the organs of sense are also not unfrequently inherited. Four sisters, their mother, and grandmother, are described by Duval as similarly affected by cataract. Prosper Lucas details an example of hereditary am¬ aurosis affecting the females of a family for three generations. Duval, Graffe, Dufon, and others testify to like cases coming under their observation.* Deafness, too, is occasionally trans¬ mitted from parent to child. There are deaf-mutes whose imperfections have been derived from ancestors ; and mal¬ formations of the external ears have also been perpetuated in offspring. Of transmitted peculiarities of the skin and its appendages, many illustrations have been noted. One is that of a family remarkable for enormous black eyebrows ; another that of a family in which every member had a lock of hair of a lighter colour than the rest on the top of the head ; and there are also instances of congenital baldness being hereditary. Entire absence of teeth, absence of particular teeth, and anomalous arrangements of teeth, are recorded as traits that have descended to children. And we have evidence that soundness and unsoundness of teeth are transmissible. The inheritance of such diseases as gout, consumption, and insanity, is universally admitted. Among the less-common diseases of which the descent from one generation to another has been observed, are, ichthyosis, leprosy, pityriasis, &ebace- ous tumours, plica polonica, dipsomania, somnambulism, cata¬ lepsy, epilepsy, asthma, apoplexy, elephantiasis. General nervousness displayed by parents, almost always re-appears in their children. Even a bias towards suicide appears to be sometimes hereditary. § 82. To prove the transmission of those structural pecu¬ liarities that have resulted from functional peculiarities, is, * "While this chapter is passing through the press, I learn from Mr White Cooper, that not only are near sight, long sight, dull sight, and squinting, here¬ ditary ; hut that a peculiarity of vision confined to one eye, is frequently trans¬ mitted — re-appearing in the same eye in offspring. HEREDITY. 245 for several reasons, comparatively difficult. Changes pro¬ duced in the sizes of parts by changes in their amounts of action, are mostly unobtrusive. A muscle that has increased in bulk, is so obscured by natural or artificial clothing, that un¬ less the alteration is extreme it passes without remark. Such nervous developments as are possible in the course of a single life, cannot be seen externally. Visceral modifications of a normal kind, are observable but obscurely, or not at all. And if the changes of structure worked in individuals by changes m their habits, are thus difficult to trace ; still more difficult to trace must be the transmission of them — further hidden, as this is, by the influence of other individuals that are often otherwise modified by other habits. Moreover, such special¬ ities of structure as are due to specialities of function, are usually entangled with specialities of structure that are, or may be, due to selection, natural or artificial. In the majority of cases, it is impossible to say that a structural peculiarity which seems to have arisen in offspring from a functional peculiarity in the parent, is wholly independent of some congenital peculiarity of structure in the parent, which in¬ duced this functional peculiarity. We are restricted to cases with which natural or artificial selection can have had nothing to do ; and such cases are difficult to find. Some, however, may here be noted. A species of plant that has been transferred from one soil or climate to another, frequently undergoes what botanists call “ a change of habit ” — a change which, without affectum its specific characters, is yet conspicuous. In its new locality" the species is distinguished by leaves that are much larger' or much smaller, or differently shaped, or more fleshy " or instead of being, as before, comparatively smooth, it becomes hairy; or its stem becomes woody instead of being herbaceous; or its branches, no longer growing upwards, assume a droop¬ ing character. Vow these “ changes of habit” are clearly de¬ termined by functional changes. Occurring, as they do, in many individuals that have undergone the same transportation, 246 THE INDUCTIONS OF BIOLOGY. they cannot bo classed as “ spontaneous variations.” They are modifications of structure, consequent on modifications of function, that have been produced by modifications in tho actions of external forces. And as these modifications re-ap- pear in succeeding generations, we have, in them, examples of functionalty-established variations that are hereditarily transmitted. Further evidence is supplied by wliat are called “sports ” in plants. These are of two kinds — the gamogenetic and the agamogenetic. The gamogenetic may be ascribed wholly to “ spontaneous variations or if they are partly due to the inheritance of structural changes that are produced by functional changes, this cannot be proved. But where the individuals displaying the variations arise by agamogenesis, the reverse is the case : spontaneous variation is out of the question ; and the only possible interpretation is deviation of structure caused by deviation of function. A new axis which buds out from a parent-axis, assumes an un¬ like character — gives off lobed leaves in place of single leaves, or has an otherwise different mode of growth. This change of structure implies change in the developmental actions which produced the new bud — change, that is, in the actions going on in the parent shoot — functional change. And since the modified structure thus impressed on the new shoot by modified function, is transmitted by it to all the shoots it bears ; we are obliged to regard the case as one of acquired modification that has become hereditary. Evidence of analogous changes in animals, is difficult to disentangle. Only among domesticated animals, have we any opportunity of tracing the effects of altered habits ; and here, in nearly all cases, artificial selection has obscured the results. Still, there are some facts which seem to the point. Mr Darwin, while ascribing almost wholly to “natural selection ” the production of those modifications which eventuate in differences of species, nevertheless admits the effects of use and disuse. lie says — “ I find in the domestic duck that the bones of the wing weigh less and the bones of the leg more, in pro- heredity. 247 poraon to tlxe whole skeleton, than do the same bones in the .. “‘k*’ a“d jPresume that tllis change may bo safely attributed to the domestic duck flying much less, and walking moie, than its wild parent. The great and inherited develop ■ nt of the udders m cows and goats in countries where they ie abitually milked, in comparison with the state of these organs m other countries, is another instance of the effect of use._ hot a single domestic animal can be named which has not in some country drooping ears ; and the view suggested by some authors, that the drooping is due to the disuse of the muscles of the ear, from the animals not being much alarmed of S0ZSh’ S6emS Pmh'dhle-” ASain~“ The eyes of moles and of some burrowing rodents are rudimentary in size, and in some cases are quite covered up by skin and fur. This state but idT 1S1P1'0b“Wy due t0 8™*"* eduction from disuse, but aided perhaps by natural selection.” * * * « T, _p]l . nown that several animals, belonging 'to the most differ¬ ent classes, which inhabit the cayes of Styria and of Kentucky, a. e_ blind. In some of the crabs the footstalk of the eye re¬ mains hough the eye is gone; the stand for the telescope is t mre tliough the telescope with its glasses has been lost. As „nv T EcUl.t ? lmaSlne that eyes, though useless, could be in any way injurious to animals living in darkness, I attribute tueir oss wholly to disuse.” The direct inheritance of an ac- qun ed peculiarity is sometimes observable. Mr Lewes o-ives tad, a PUPPy taken from its “otier at six weeks old, who although never taught, • to beg’ (an accomplishment , m°th“.: ha\ been taught), spontaneously took to beo-dno for everything he wanted when about seven or efokt months Ok : he would beg for food, beg to be let out of tL room aiK one^ day was found opposite a rabbit hutch bego-i„„. for i a obits. Instances are on record, too, of sporting do-s which spontaneously adopted in the field, certain modes of bdi v four which their parents had learnt. But Lie best examples of inherited modifications produced by modifications of function, occur in the human race. To no 248 THE INDUCTIONS OF BIOLOGY. other cause can be ascribed the rapid metamorphoses under¬ gone by the British races when placed in new conditions. It is notorious that, in the United States, the descendants of the immigrant Irish lose their Celtic aspect, and become Ameri¬ canized. This cannot be ascribed to intermarriage with Americans ; since the feeling with which Irish are regard¬ ed by Americans, prevents any considerable amount of inter¬ marriage. Equally marked is the case of the immigrant Germans, who, though they keep themselves very much apart, rapidly assume the prevailing type. To say that “ spontaneous variation ” increased by natural selection, can have produced this effect, is going too far. Itaces so numer¬ ous, cannot have been supplanted in the course of two or three generations by varieties springing from them. Hence there is no escape from the conclusion, that physical and so¬ cial conditions have here wrought modifications of function and structure, which offspring have inherited and increased. Similarly with special cases. In the Cyclopcedia of Practical Medicine, Yol. II. p. 419, Dr Brown states that he “has in many instances observed in the case of individuals whose complexion and general appearance has been modified by re¬ sidence in hot climates, that children born to them subse¬ quently to such residence, have resembled them rather in their acquired than primary mien.” Some special modifications of organs caused by special changes in their functions, may also be noted. That large hands are inherited by men and women whose ancestors led laborious lives ; and that men and women whose descent, for many generations, has been from those unused to manual labour, commonly have small hands ; are established opinions. It seems very unlikely that in the absence of any such con¬ nexion, the size of the hand should thus have come to be generally regarded as some index of extraction. That there exists a like relation between habitual use of the feet and large¬ ness of the feet, we have strong evidence in the customs of the Chinese. The torturing practice of artificially arresting the HEREDITY. 249 growth of the feet, could never have become established among the ladies of China, had they not found abundant proof that a small foot was significant of superior rank — that is of a luxurious life — that is of a life without bodily labour. There is some evidence, too, that modifica¬ tions of the eyes, caused by particular uses of the eyes, are inherited. Short sight appears to be uncommon in rural populations ; but it is frequent among classes of people who use their eyes much for reading and writing ; and in these classes, short sight is often congenital. Still more marked is this relation in Germany. There, the educated classes are no¬ toriously studious ; and judging from the numbers of young Germans who wear spectacles, there is reason to think that congenital myopia is very frequent among them. Some of the best illustrations of functional heredity, are furnished by the mental characteristics of human races. Cer¬ tain powers which mankind have gained in the course of civil¬ ization, cannot, I think, be accounted for, without admitting the inheritance of acquired modifications. The musical faculty is one of these. To say that “ natural selection’’ has developed it, by preserving the most musically endowed, seems an in¬ adequate explanation. Even now that the development and prevalence of the faculty have made music an occupation by which the most musical can get sustenance and bring up families ; it is very questionable whether, taking the musical life as a whole, it has any advantage over others in the struggle for existence and multiplication. Still more if we look back to those early stages through which the faculty must have passed, before definite perception of melody was arrived at, we fail to see how those possessing the rudimental faculty in a somewhat greater degree than the rest, would thereby be enabled the better to maintain themselves and their children. If so, there is no explanation but that the habitual association of certain cadences of human speech with certain emotions, has slowly established in the race an organized and inherited connexion between such cadences and such emotions ; that the 250 THE INDUCTIONS OF BIOLOGY. combination of sncb cadences, more or less idealized, which constitutes melody, has all along had a meaning in the average mind, only because of the meaning which cadences had acquired in the average mind ; and that by the continual hearing and practice of melody, there has been gained and transmitted an increasing musical sensibility. Confirmation of this view may be drawn from individual cases. Grant that among a people endowed with musical faculty to a certain degree, spontaneous variation will occasionally produce men possessing it in a higher degree ; it cannot be granted that spontaneous variation accounts for the frequent production, by such highly- endowed men, of men still more highly endowed. On the average, the offspring of marriage with others not similarly endowed, will be less distinguished rather than more distin¬ guished. The most that can be expected is, that this unusual amount of faculty shall re-appear in the next generation undi- minislied. IIow then shall we explain cases like those of Bach, Mozart, and Beethoven, who were all sons of men having un¬ usual musical powers, but wdro greatly excelled their fathers in their musical powers ? What shall we say to the facts, that Haydn was the son of the organist, that Hummel was born to a music master, and that Weber’s father was a dis¬ tinguished violinist ? The occurrence of so many cases in one nation, within a short period of time, cannot rationally be ascribed to the coincidence of “ spontaneous variations.” It can be ascribed to nothing but inherited developments of structure, caused by augmentations of function. But the clearest proof that structural alterations caused by alterations of function, are inherited, occurs when the alter¬ ations are morbid. “ Certain modes of living engender gout ; ” and gout is transmissible. It is well known that in persons pre¬ viously healthy, consumption may be produced by unfavourable conditions of life — by bad and insufficient food ; by foul, damp, unventilated habitations ; and even by long- continued anxiety. It is still more notorious that the consumptive diathesis is conveyed from parent to child. Unless, then, a distinction HEREDITY. 251 be assumed between constitutional consumption and con¬ sumption induced by unwholesome conditions — unices it be asserted that consumption of unknown origin is transmiss- ible_, while functionally-produced consumption is not ; it must be admitted that those changes of structure from which the consumptive diathesis results, may be caused in parents by changes of function, and may be inherited by their chil- ^rcn* . Most striking of all, however, is the fact lately brought to light, that functional disorders artificially estab¬ lished, may be conveyed to offspring. Some few years since M. Lrown-Sequard, in the course of inquiries into the nature and causes of epilepsy, hit on a method by which epilepsy could be originated. Guinea-pigs were the creatures on which, chiefly, he experimented ; and eventually, he disco¬ vered the remarkable fact, that the young of these epileptic guinea-pigs were epileptic: the functionally-established epilepsy in the parents, became constitutional epilepsy in the offspring. Ifere we have an instance which, standing even alone, decides the question. We have a special form of nervous action, not caused by any natural variation of structure that had arisen spontaneously in the organism, but one caused by a certain incidence of external forces. We have this special form of nervous action becoming confirmed by re¬ petition : the fits are more and more easily induced — there is established the epileptic habit. That is to say, the connected nervous actions constituting a fit, produce in the nervous system such changes of structure, that subsequent connected nervous actions of like kind, follow one another with increased readiness. And that this epileptic habit is inherited, proves conclusively that these structural modifications worked by functional modifications, are impressed on the whole organism m such way as to affect the reproductive centres, and cause them to unfold into organisms that exhibit like modifications. Evidence nearly allied to this, and scarcely less significant, is furnished by that transmission of general nervousness, no¬ ticed in the last section. Nervousness is especially common 252 THE INDUCTIONS OF BIOLOGY, among classes of people who tax their brains much. Among tliese classes, we daily see tliis constitutional modification produced by excess of function, in men wliose progenitors were not nervous ; and the children of sucb men habitually inherit more or less of the modification. § 83. Two modified manifestations of Heredity remain to be noticed. The one is the re-appearance in offspring, of traits not borne by the parents, but borne by the grandparents or by remoter ancestors. The other is the limitation of Heredity by sex — the restriction of certain transmitted peculiarities to offspring of the same sex as the parent possessing these peculiarities. Atavism, which is the name given to the recurrence of ancestral traits, is proved by many and varied facts. In the picture-galleries of old families, and on the monumental brasses in the adjacent churches, are often seen types of feature that are still, from time to time, repeated in members of these families. It is matter of common remark that some con¬ stitutional diseases, such as gout and insanity, after missing a generation, will show themselves in the next. Dr Struthers, in his above-quoted paper on “ Variation in the Humber of Fingers and Toes, and of the Phalanges, in Man,” gives cases of malformations that were common to grandparent and grandchild, but of which the parent had no trace. M. Girou (as quoted by Mr Sedgwick) says — “ One is often surprised to see lambs black, or spotted with black, born of ewes and rams with white wool, but if one takes the trouble to go back to the origin of this phenomenon, it is found in the an¬ cestors.” Instances still more remarkable, in which the re¬ moteness of the ancestors copied is very great, are given by Mr Darwin. lie points out that in crosses between varieties of the pigeon, there will sometimes re-appear the plumage of the original rock-pigeon, from which these varieties descend¬ ed; and he instances the faint zebra-like markings occasion¬ ally traceable in horses, as having probably a like meaning. HEREDITY. 253 The limitation of Heredity by sex, cannot yet bo regarded as established. While in many cases it seems clearly mani¬ fested ; it is in other cases manifested to a very small degree, if at all. In Mr Sedgwick’s essays, already named, will be found evidence implying that there exists some such tendency to limitation, which does or does not show itself distinctly, according to the nature of the organic modification to bo conveyed. But more facts must be collected before any positive conclusion can be reached. ^ ol. A positive explanation of Heredity is not to be expected in the present state of Biology. We can look for nothing beyond a simplification of the problem ; and a reduction of it to the same category with certain other problems which also admit of hypothetical solution only. If an hypothesis which certain other wide-spread phenomena have already thrust upon us, can be shown to render the phenomena of Heredity more in¬ telligible than they at present seem, we shall have reason to enter tain it. The applicability of any method of interpreta¬ tion to two different but allied classes of facts, is evidence of its truth. The power which organisms display of reproducing lost parts, we saw to be inexplicable except on the assumption that the units of which any organism is built have an innate tendency to arrange themselves into the shape of that organ¬ ism (§ 65). We inferred that these units must be the pos¬ sessors of special polarities, resulting from their special struc¬ tures ; and that by tne mutual play of their polarities they are compelled to take the form of the species to which they belong. And the instance of the Begonia phyllomcmiaca left us no escape from the admission that the ability thus to arrange themseives, is latent in the units contained in every undiffer¬ entiated cell. Quite in harmony with this conclusion, are certain implications since noticed, respecting the characters oi sperm- cel Is and germ- cells. We saw sundry reasons for rejecting the supposition that these are highly-specialized cells 254 TIIE INDUCTIONS OF BIOLOGY. and for accepting the opposite supposition, that they are cells differing from others rather in being unspecialized. And here the assumption to which we seem driven by the ensemble of the evidence, is, that sperm-cells and germ- cells are essentially nothing more than vehicles, in which are contained small groups of the physiological units in a fit state for obeying their proclivity towards the structural arrangement of the species they belong to. Thus the phenomena of Heredity are seen to assimilate with other phenomena ; and the assumption which these other phenomena thrust on us, appears to be equally thrust on us by the phenomena of Heredity. We must con¬ clude that the likeness of any organism to either parent, is conveyed by the special tendencies of the physiological units derived from that parent. In the fertilized germ we have two groups of physiological units, slightly different in their structures. These slightly-different units, severally multiply at the expense of the nutriment supplied to the unfolding germ — each kind moulding this nutriment into units of its own type. Throughout the process of evolution, the two kinds of units, mainly agreeing in their polarities and in the form which they tend to build themselves into, but having minor differences, work in unison to produce an organism of the species from which they were derived, but work in antagonism to produce copies of their respective parent- organisms. And hence ultimately results, an organism in which traits of the one are mixed with traits of the other. If the likeness of offspring to parents is thus determined, it becomes manifest, a priori , that besides the transmission of generic and specific peculiarities, there will be a transmis¬ sion of those individual peculiarities which, arising without assignable causes, are classed as “ spontaneous.” For if the assumption of a special arrangement of parts by an organism, is due to the proclivity of its physiological units towrard3 that arrangement ; then the assumption of an arrangement of parts slightly different from that of the species, implies HEREDITY. 255 physiological units slightly unlike those of the species ; and these slightly- unlike physiological units, communicated through the medium of sperm-cell or germ-cell, will tend, in the offspring, to build themselves into a structure similarly diverging from the average of the species. It is not equally manifest, a 'priori , however, that on this hy¬ pothesis, alterations of structure caused by alterations of func¬ tion, must be transmitted to offspring. It is not obvious that change in the form of a part, caused by changed action, in¬ volves such change in the physiological units throughout the organism, that these, when groups of them are thrown off in the shape of reproductive centres, will unfold into organisms that have this part similarly changed in form. Indeed, when treating of Adaptation (§ 69), we saw that an organ modified by increase or decrease of function, can but slowly so re-act on the system at large, ‘as to bring about those correlative changes required to produce a new equilibrium ; and yet only when such new equilibrium has been established, can we ex¬ pect it to be fully expressed in the modified physiological units of which the organism is built — only then can we count on a complete transfer of the modification to descendants. Nevertheless, that changes of structure caused bv changes of action, must also be transmitted, however obscurety, from one generation to another, appears to be a deduction from first principles — or if not a specific deduction, still, a general implication. For if an organism A, has, by any peculiar habit or condition of life, been modified into the form A', it follows inevitably, that all the functions of A, reproductive function included, must be in some degree different from the functions of A. An organism being a combination of rhythmically-acting parts in moving equilibrium, it is im¬ possible to alter the action and structure of any one part, without causing alterations of action and structure in all the rest; just as no member of the Solar System could be modi¬ fied in motion or mass, without producing re-arrangements throughout the whole Solar System. And if the organism A 25G Till: INDUCTIONS OF BIOLOGY. when changed to A', must be changed in all its functions : then the offspring of A cannot be the same as they would have been had it retained the form A. It involves a denial of the persistence of force to say that A may be changed into A, and may yet beget offspring exactly like those it would have begotten had it not been so changed. That the change in the offspring must, other things equal, be in the same direction as the change in the parent, wt) may dimly see is implied by the fact, that the change propagated throughout the parental system is a change towards a new state of equilibrium — a change tending to bring the actions of all organs, reproductive included, into harmony with these new actions. Or, bringing the question to its ultimate and simplest form, we may say that as, on the one hand, phy¬ siological units will, because of their special polarities, build themselves into an organism of a special structure ; so, on the other hand, if the structure of this organism is modified by modified function, it will impress some corresponding modification on the structures and polarities of its units. The units and the aggregate must act and re-act on each other. The forces exercised by each unit on the aggregate and by the aggregate on each unit, must ever tend towards a balance. If nothing prevents, the units will mould the aggregate into a form in equilibrium with their pre-existing polarities. If. contrariwise, the aggregate is made by incident actions to take a new form, its forces must tend to re-mould the units into harmony with this new form. And to say that the physiological units are in any degree so re- moulded as to bring their polar forces towards equilibrium with the forces of the modified aggregate, is to say that when separated in the shape of reproductive centres, these units will tend to build themselves up into an aggregate modified in the same di¬ rection. CHAPTER IX. VARIATION. § 85. Equally conspicuous with the truth that every organ¬ ism hears a general likeness to its parents, is the truth that no organism is exactly like either parent. Though similar to both in generic and specific traits, and usually, too, in those traits which distinguish the variety, it diverges in^ numer¬ ous traits of minor importance. Ho two plants are indistin¬ guishable ; and no two animals are without differences. Variation is co-extensive with Heredity. The degrees of variation have a wide range. There are deviations so small as to be not easily detected ; and theie are deviations great enough to be called monstrosities. In plants, we may pass from cases of slight alteration in the shape or texture of a leaf, to cases where, instead of a flower with its cal\rx above the seed-vessel, there is produced a flower with its calyx below the seed-vessel ; and while in one animal, there arises a scarcely noticeable unlikeness in the length or colour of the hair, in another, an organ is absent, or a supernumerary organ appears. Though small variations are by far the most general, yet variations of considerable magnitude are not uncommon ; and even those variations constituted by additions or suppressions of parts, are not so rare as to be excluded from the list of causes by which organic forms are changed. Cattle without horns are fre- quent. Of sheep there are horned breeds and breeds that 12 258 THE INDUCTIONS OF BIOLOGY. have lost their horns. At one time, there existed in Scot¬ land a race of pigs with solid feet instead of cleft feet. In pigeons, according to Mr Darwin, “ the number of the cau¬ dal and sacral vertebrae vary ; as does the number of the ribs, together with their relative breadth and the presence of processes.” That variations both small and large which arise without any specific assignable cause, tend to become hereditary, was shown in the last chapter. Indeed the evidence which proves Heredity in its smaller manifestations, is the same evidence which proves Variation ; since it is only when there occur vari¬ ations, that the inheritance of anything beyond the structural peculiarities of the species, can be proved. It remains here, however, to be observed, that the transmission of variations is itself variable ; and that it varies both in the direction of decrease and in the direction of increase. An individual trait of one parent, may be so counteracted by the influence of the other parent, that it may not appear in the offspring ; or not being so counteracted, the offspring may possess it, perhaps in an equal degree or perhaps in a less degree ; or the off¬ spring may exhibit the trait in even a still higher degree. Of the illustrations of this, one must suffice. I quote it from the essay by Dr Struthers, referred to in the last chapter. “ The great-great-grandmother, Esther P - (who mar¬ ried A - L - ), had a sixth little finger on one hand. Of their eighteen children (twelve daughters and six sons), only one (Charles) is known to have had digital variety. We have the history of the descendants of three of the sons, Andrew, Charles, and James. “ (1.) Andrew L - had two sons, Thomas and Andrew ; and Thomas had two sons all without digital variety. Here we have three successive generations without the variety possessed by the great-grandmother showing itself. “(2.) James L - , who was normal, had two sons and seven daughters, also normal. One of the daughters became Mrs J - (one of the informants), and had three daughters variation. 259 • and five sons, all normal except one of the sons, James J , now set. 17, who had six fingers on each hand. * * * _ “ In this branch of the descendants of Esther, we see it passing over two generations and reappearing in one member of the third generation, and now on both hands. “ (3.) Charles L - , the only child of Esther who had digital variety, had six fingers on each hand. He had three sons, James, Thomas, and John, all of whom were born with six fingers on each hand, while John has also a sixth toe on one foot. lie had also five other sons and four daughters, all of whom were normal. “ (rt.) Of the normal children of this, the third generation, the five sons had twelve sons and twelve daughters, and the four daughters have had four sons and four daughters, being the fourth generation, all of whom were normal. A fifth generation in this sub-group consists as yet of only two boys and two girls, who are also normal. “ In this sub-branch, we see the variety of the first gener¬ ation present in the second, passing over the third and fourth, and also the fifth as far as it has yet gone. « ,7, j James had three sons and two daughters, who are normal. “ (9 § 90. Still there remains a difficulty. It may be said that admitting functional change to be the initiator of variation — granting that the physiological units of an organism, modified by long subjection to new conditions, will tend to be¬ come modified in such way as to cause change of structure m offspring; yet there will still be no cause of the supposed heterogeneity among the physiological units of different in¬ dividuals. There seems validity in the objection, that as all the members of a species whose circumstances have been al¬ tered, will be affected in the same manner, the results, when they begin to show themselves in descendants, will show them¬ selves in the same manner : not multiform variations will arise, but deviations all in one direction. The reply is simple. The members of a species thus cir¬ cumstanced, will not be similarly affected. In the absence of absolute uniformity among them, the functional changes caused in them will be more or less dissimilar. J ast as men of slightly-unlike dispositions behave in quite opposite ways under the same circumstances ; or just as men of slightly- unlike constitutions get diverse disorders from the same cause, and are diversely acted on by the same medicine ; so, the insensibly-differentiated members of a species whose con¬ ditions have been changed, may at once begin to undergo various kinds of functional changes. As we have already seen, small initial contrasts may lead to large terminal con¬ trasts. The intenser cold of the climate into which a species has migrated, may cause in one individual increased con¬ sumption of food, to balance the greater loss of heat ; while in another individual, the new requirement may be met by a thicker growth of fur. Or, when meeting with the new foods which the new region furnishes, mere accident may deter¬ mine one member of the species to begin with one kind and another member with another kind ; and hence may arise established habits in these respective members and their descendants. Now when the functional divergences thus set up in sundry families of a species, have lasted long enough 270 THE INDUCTIONS OF BIOLOGY. to affect tlieir constitutions profoundly, and to modify some¬ what the physiological units thrown off in their reproductive cells, the divergences produced by these in offspring, will be of diverse kinds. And the original homogeneity of constitu¬ tion having been thus destroyed, variation may go on with increasing facility. There will result a heterogeneous mix¬ ture of modifications of structure, caused by modifications of function ; and of still more numerous correlated modifica¬ tions, indirectly so caused. By natural selection of the most divergent forms, the unlikenesses of parents will grow more marked, and the limits of variation wider. TJntil at length the divergences of constitutions and modes of life, become great enough to lead to segregation of the varieties. § 91. That variations must occur, and that they must ever tend, both directly and indirectly, towards adaptive modifica¬ tions, are conclusions deducible from first principles; apart from any detailed interpretations like the above. That the state of homogeneity is an unstable state, we have found to be a universal truth. Each species must pass from the uni¬ form into the more or less multiform, unless the incidence of external forces is exactly the same for all its members ; which it never can be. Through the process of differentiation and integration, which of necessity brings together, or keeps to¬ gether, like individuals, and separates unlike ones from them, there must nevertheless be maintained a tolerably uniform species ; so long as there continues a tolerably uniform set of conditions in which it may exist. But if the conditions change, either absolutely by some disturbance of the habitat, or relatively by spread of the species into other habitats, then the divergent individuals that result, must be segregated by the divergent sets of conditions into distinct varieties (First Principles, § 126). When, instead of contemplating a species in the aggregate, we confine our attention to a single member and its descendants, we see it to be a corollary from the general law of equilibration, that the moving equili- VARIATION. 271 brium constituted by the vital actions in each member of this family, must remain constant so long as the external ac¬ tions to which they correspond remain constant ; and that if the external actions are changed, the disturbed balance of internal changes, if not overthrown, cannot cease undergoing modification until the internal changes are again in equili¬ brium with the external actions: corresponding structural alterations having arisen. Or passing from these derivative laws to the ultimate law, we see that Variation is necessitated by the persistence of force. The members of a species inhabiting any area, cannot be subject to like aggregates of forces over the whole of that area. And if, in different parts of the area, different kinds or amounts or combinations of forces act on them, they cannot but become different in themselves and in their progeny. To say otherwise, is to say that differences in the forces will not produce differ¬ ences in the effects ; which is to deny the persistence of force. Whence it is also manifest, that there can be no variation of structure, but what is directly or indirectly consequent on variation of function. On the one hand, organisms in com¬ plete equilibrium with their conditions, cannot be changed except by change in their conditions ; since, to assert other¬ wise, is to assert that there can be an effect without a cause ; which is to deny the persistence of force. On the other hand, any change of conditions can affect an organism only by changing the actions going on in it — -only by altering its func¬ tions. The alterations of functions being necessarily towards a re-establishment of the equilibrium, (for if not, the equili¬ brium must be destroyed and the life cease, either in the in¬ dividual or in descendants,) it follows that the structural alter¬ ations directly caused, are adaptations ; and that the correlated structural alterations indirectly caused, are the concomitants of adaptations. Hence, though, by the intercourse of organisms that have been functionally and structurally modified in dif¬ ferent directions, there may result organisms that deviate in compound ways which appear unrelated to external condi- 272 TIIE INDUCTIONS OF BIOLOGY. tions, tlie deviations of sncli organisms must still be regarded as indirect results of functional adaptations. We must say that in all cases, adaptive change of function is the primary and ever-acting cause of that change of structure which con¬ stitutes variation ; and that the variation which appears to be “ spontaneous,” is derivative and secondary. CHAPTER X. GENESIS, HEREDITY; AND VARIATION. § 92. A question raised, and hypothetically answered, in §§ 78 and 79, was there postponed until we had dealt with the topics of Heredity and Variation. Let us now resume the consideration of this question, in connexion with sundry others which the facts suggest. After contemplating the several methods by which the multiplication of organisms is carried on — after ranging ihem under the two heads of Ilomogenesis, in which the suc¬ cessive generations are similarly produced, and Ileterogenesis, in which they are dissimilarly produced — after observing that Ilomogenesis is always sexual genesis, while Hetcroge- nesis is asexual genesis with occasionally-recurring sexual genesis ; we came to the questions — why is it that some or¬ ganisms multiply in the one way, and some in the other ? and why is it that where agamogenesis prevails, it is usually, from time to time, interrupted by gamogenesis P In seeking an answer to this question, we inquired whether there are, common to both Ilomogenesis and Heterogenesis, any condi¬ tions under which alone sperm-cells and germ-cells arise and are united, for the production of new organisms ; and we reached the conclusion that, in all cases, they arise only when there is an approach to equilibrium between the forces which produce growth and the forces which oppose growth. This answer to the question — when does gamogenesis recur ? 274 THE INDUCTIONS OF BIOLOGY. still left unanswered the question — why does gamogenesia recur ? And to this the reply suggested was, that the ap¬ proach towards general equilibrium, in organisms, “ is ac¬ companied by an approach towards molecular equilibrium in them ; and that the need for this union of sperm-cell and germ-cell, is the need for overthrowing this equilibrium, and re-establishing active molecular change in the detached germ — a result which is probably effected by mixing the slightly - different physiological units of slightly- different individuals.’ ’ This is the hypothesis which we have now to consider. Let us first look at the evidences which certain inorganic pheno¬ mena furnish. The molecules of any aggregate which have not a balanced arrangement, inevitably tend towards a balanced arrangement. As before mentioned (First Principles, § 103) amorphous wrought iron, when subject to continuous jar, begins to arrange itself into crystals — its atoms assume a condition of polai equilibrium. The particles of unannealed glass, which are so unstably arranged that slight disturbing forces make them separate into small groups, take advantage of that greater ’ freedom of movement given by a raised temperature, to ad¬ just themselves into a state of relative rest. During any such re-arrangement, the aggregate exercises a coercive force over its units. Just as in a growing crystal, the atoms suc- cessivelv assimilated from the solution, are made by the al- ready- crystallized atoms to take a certain form, and even to re-complete that form when it is broken ; so in any mass of unstably-arranged atoms that passes into a stable arrangement, each atom conforms to the forces exercised on it by all the other atoms. This is a corollary from the general law of equilibration. "We saw (First Principles, § 130) that every change is towards equilibrium ; and that change can never cease until equilibrium is reached. Organisms, above all other aggregates, conspicuously display this progressive equilibration ; because their units are of such kinds, and so conditioned, as to admit of easy re-arrangement. Those GENESIS, HEREDITY, AND VARIATION. 275 extremely active changes which go on during the early stages of evolution, imply an immense excess of the mole¬ cular forces over those antagonist forces which the aggregate exercises on the molecules. While this excess continues, it is expended in growth, development, and function — expendi¬ ture for any of these purposes, being proof that part of the force embodied in molecular tensions, remains unbalanced. Eventually, however, this excess diminishes. Either, as in organisms which do not expend much force, decrease of assi¬ milation leads to its decline ; or, as in organisms which ex¬ pend much force, it is counterbalanced by the rapidly-increas¬ ing re-actions of the aggregate (§ 46). The cessation of growth, when followed, as in some organisms, by death, im¬ plies the arrival at an equilibrium between the molecular forces, and those forces which the aggregate opposes to them. When, as in other organisms, growth ends in the establish¬ ment of a moving equilibrium, there is implied such a de¬ creased preponderance of the molecular forces, as leaves no surplus beyond that which is used up in functions. The de¬ clining functional activity, characteristic of advancing life, expresses a further decline in this surplus. And when all vital movements come to an end, the implication is, that the actions of the units on ’the aggregate and the re¬ actions of the aggregate on the units, are completely bal¬ anced. Hence, while a state of rapid growth indi¬ cates such a play of forces among the units of an aggregate, as will produce active re-distribution ; tho diminution and arrest of growth, shows that the units have fallen into such relative positions that re-distribution is no longer so facile. When, therefore, we see that gamogenesis recurs only when growth is decreasing, or has come to an end, we must say that it recurs only when the organic units are approxima¬ ting to equilibrium — only when their mutual restraints pre¬ vent them from readily changing their arrangements in obe¬ dience to incident forces. That units of like forms can be built up into a more stable 276 THE INDUCTIONS OF BIOLOGY. aggregate than units of slightly unlike forms, is tolerably manifest, a priori. And we have facts which prove that mixing allied but somewhat different units, does lead to comparative in¬ stability. Most metallic alloys exemplify this truth. Com¬ mon solder, which is a mixture of lead and tin, melts at a much lower temperature than either lead or tin. The compound of lead, tin, and bismuth, called “ fusible metal,” becomes fluid at the temperature of boiling water ; while the temperatures at which lead, tin, and bismuth become fluid, are, respectively, G123, 442°, and 497°, F. Still more remarkable is the illustra¬ tion furnished by potassium and sodium. These metals are very near akin in all respects — in their specific gravities, their atomic weights, their chemical affinities, and the properties of their compounds. That is to say, all the evidences unite to show that their units, though not identical, have a close resem¬ blance. What now happens when they are mixed ? Potassium alone melts at 136°, sodium alone melts at 190°, but the alloy of potassium and sodium, is liquid at the ordinary temperature of the air. Observe the meaning of these facts, expressed in general terms. The maintenance of a solid form by any group of units, implies among them an arrangement so stable, that it cannot be overthrown by the incident forces. Whereas the assumption of a liquid form, implies that the incident forces suffice to destroy the arrangement of the units. In the one case, the thermal undulations fail to dislocate the parts ; while in the other case, the parts are so dislocated by the thermal undulations, that they fall into total disorder — a disorder admitting of easy re-arrangement into any other order. For the liquid state is a state in which the units become so far free from mutual restraints, that incident forces can change their relative positions very readily. Thus we have reason to conclude, that an aggregate of units which, though in the main similar to each other, have minor differences, must be more unstable than an aggregate of homogeneous units : the one will yield to disturbing forces which the other successfully resists. GENESIS, HEREDITY, AND VARIATION. 277 Now though the colloidal atoms of which organisms are mainly built, are themselves highly composite ; and though the physiological units compounded out oi these colloidal atoms, must have structures far more involved ; yet it must happen with such units, as with simple units, that those which have exactly like forms, will admit of arrangement into a more stable aggregate than those which have slightly- unlike forms. Among units of this order, as among units of a simpler order, imperfect similarity must entail imperfect polar balance, and consequent diminished ability to withstand disturbing forces. Hence, given two organisms which, by diminished nutrition or increased expenditure, are being ar¬ rested in their growths — given in each an appioacliing equilibrium between the forces of the units and the forces of the aggregate — given, that is, such a comparatively-balanced state among the units, that re-arrangement of tnem by inci¬ dent forces is no longer so easy ; and it will follow that by uniting a group of units from the one organism with a group of sli ghtly- different units from the other, the tendency to¬ wards equilibrium will be diminished, and the mixed units will be rendered more modifiable in their arrangements by the forces acting on them : they will be so far freed as to be¬ come again capable of that re-distribution which constitutes evolution. This view of the matter is m harmony with the results of observation on the initial stages of develop¬ ment. Some pages back, it was asserted that sperm- cell and germ- cell severally arrive, before their union, at a condition of equilibrium. Though approximately true, this is not liter¬ ally true. I learn from Dr W. II. Ransom, who has investi¬ gated the question with great care, that the unfertilized ovum continues, for a time, to undergo changes similar to those which the fertilized ovum undergoes; but that these changes, becoming languid and incomplete, are finally arrested by decomposition. Here we find what might be expected. In the first place, an organism which develops germ-cells, is not in a state of mole¬ cular equilibrium, but in a state of approach to such equili- 278 THE INDUCTIONS OF BIOLOGY. brium. Hence, a group of physiological units cast off from it, will not be wholly without a tendency to undergo the struc¬ tural re-arrangements which we call development ; but will have this tendency unduly restrained by partially-balanced polarities. In the second place, undue restraint of the phy¬ siological units, while it renders them as wholes less-easily altered in their relative positions by incident forces, thereby also renders them more liable to be individually decomposed bv incident forces : the same thermal undulations which, if the physiological units are comparatively free, will aid their re-arrangement by giving them still greater freedom, will, if they are comparatively fixed, begin to change the arrange¬ ments of their components — will decompose them. In the third place, their decomposition will be prevented as well as their re-distribution facilitated, by such disturbance of their polarities as we have seen must result from mixing with them the slightly-unlike units of another organism. And now let us test this hypothesis, by seeing what power it gives us of interpreting established inductions. § 93. The majority of plants being hermaphrodites, it has, until quite recently, been supposed that the ovules of each flower are fertilized by pollen from the anthers of the same flower. Mr Darwin, however, has shown that the arrange¬ ments are generally such as to prevent this : either the ovules and the pollen are not ripe simultaneously, or obstacles pre¬ vent access of the one to the other. At the same time, he has shown that there exist arrangements, often of a remarkable kind, which facilitate the transfer of pollen by insects from the stamens of one flower to the pistil of another. Simi¬ larly, it has been found that among the lower animals, herma- phrodism does not usually involve the production of fertile germs, by the union of sperm-cells and germ-cells developed in the same individual ; but that the reproductive centres of one individual are united with those of another, to produce fertile germs. Either, as in the Pyrosoma, the Perophora, and GENESIS, HEREDITY, AND VARIATION. 270 in many higher molluscs, tlie ova and spermatozoa are ma¬ tured at different times ; or, as in annelids, tliey are prevented by their relative positions from coming in contact. Remembering the fact that among the higher classes of organisms, fertilization is always effected by combining the sperm-cell of one individual with the germ-cell of another ; and joining with it the fact that among hermaphrodite organ¬ isms, the germ-cells developed in any individual, are usually not fertilized by sperm-cells developed in the same individual ; we see reason for thinking that the essential thing in fertiliz¬ ation, is the union of specially-fitted portions of different or¬ ganisms. If fertilization depended on the peculiar properties of sperm-cell and germ-cell, as such ; then, in hermaphrodite organisms, it would be a matter of indifference whether the united sperm-cells and germ-cells were those of the same in¬ dividual, or those of different individuals. But the circum¬ stance that there exist in such organisms, elaborate ap¬ pliances for mutual fertilization, shows that unlikeness of derivation in the united reproductive centres, is the deside¬ ratum. Now this is just what the foregoing hypothesis implies. If, as was concluded, fertilization has for its object the disturbance of that approximate equilibrium existing among the physiological units separated from an adult organ¬ ism ; and if, as we saw reason to think, this object is effected by mixture with the slightly- different physiological units of another organism ; then, we at the same time see reason to think, that this object will not be effected by mixture with physiological units belonging to the same organism. Thus, the hypothesis leads us to expect such provisions as we find exist. § 94. But here a difficulty presents itself. These proposi¬ tions seem to involve the conclusion, that self-fertilization is impossible. It apparently follows from them, that a group of physiological units from one part of an organism, ought to have no power of altering the state of approaching balance in 2S0 THE INDUCTIONS OF BIOLOGY. a group from another part of it. Yet self-fertilization does occur. Though the ovules of one plant, are generally fer¬ tilized by pollen from another plant of the same kind ; yet they may be, some of them, fertilized by the pollen of the same plant. And though, anjong hermaphrodite animals, self-fer¬ tilization is usually negatived by structural or functional ar¬ rangements ; yet in certain Entozoa, there appear to be special provisions by which the sperm-cells and germ-cells of the same individual may be united, when not previously united with those of another individual. Certainly, at first sight, these facts do not consist with the above supposition. Neverthe¬ less, there is a satisfactory solution of them. In the last chapter, when considering the variations that may result in offspring from the combination of unlike parental constitutions, it was pointed out that in an unfolding organism, composed of slightly-different physiological units derived from slightly-different parents, there cannot be main¬ tained an even distribution of the two orders of units. We saw that the instability of the homogeneous, negatives the uniform blending of them ; and that, by the process of differ¬ entiation and integration, they must be more or less separated ; so that in one part of the body the influence of one parent will predominate, and in another part of the body the influence of the other parent : an inference which harmonizes with daily observation. And we also saw, that the sperm-cells or germ- cells produced by such an organism, must, in virtue of these same laws, be more or less unlike one another. It was shown that through segregation, some of the sperm-cells or germ- cells will get an excess of the physiological units derived from one side, and some of them an excess of those derived from the other side : a cause which accounts for the unlikenesses among offspring simultaneously produced. Now from this segregation of the different orders of physiological units, in¬ herited from different parents and lines of ancestry, there arises the possibility of self-fertilization in hermaphrodite organisms. If the physiological units contained in the sperm- GENESIS, HEREDITY, AND VARIATION. 281 colls and germ- cells of tlie same flower, are not quite homo¬ geneous — if in some of the ovules the physiological units derived from the one parent greatly predominate, and in some of the ovules those derived from the other parent ; and if the like is true of the pollen-cells ; then, some of the ovules may be nearly as much contrasted with some of the pollen-cells, in the characters of their contained units, as were the ovules and pollen-cells of the parents from which the plant proceeded. Petween part of the sperm-cells and part of the germ- cells, the community of nature will he such that fertilization will not result from their union ; but between some of them, the differences of constitution will be such that their union will produce the requisite molecular instability. The facts, so far as they are known, seem in harmony with this deduction. Self-fertilization in flowers, when it takes place, is not so efficient as mutual fertilization. Though some of the ovules produce seeds, yet more of them than usual are abortive. From which, indeed, results the establishment of varieties that have structures favourable to mutual fertilization; since, being more prolific, these have, other things equal, greater chances in the “struggle for existence.” Further evidence is at hand in support of this interpreta¬ tion. There is reason to believe that self-fertilization, which at the best is comparatively inefficient, loses all efficiency in course of time. After giving an account of the provisions for an occasional, or a frequent, or a constant crossing between flowers ; and after quoting Prof. Huxley to the effect that among hermaphrodite animals, there is no case in which “ the occasional influence of a distinct individual can be shown to be physically impossible ; ” Mr Darwin writes — “ from these several considerations and from the many special facts which I have collected, but which I am not here able to give, I am strongly inclined to suspect that, both in the vegetable and animal kingdoms, an occasional intercross with a distinct in¬ dividual is a law of nature. * * * in none, as I suspect, can self-fertilization go on for perpetuity.” This conclusion, 13 / 282 THE INDUCTIONS OF BIOLOGY. based wholly on observed facts, is just the conclusion to which the foregoing argument points. That necessary action and the re-action between the parts of an organism and the organism as a whole — that power of the aggregate to re-mould the units, which is the correlative of the power of the units to build up into such an aggregate ; implies that any differences existing between the units inherited by an organism, must gradually diminish. Being subject in common to the total forces of the organism, they will in common be modified to¬ wards congruity with these forces ; and therefore towards likeness with each other. If, then, in a self-fertilizing organism and its self-fertilizing descendants, such contrasts as origin¬ ally existed among the physiological units, are progressive¬ ly obliterated — if, consequently, there can no longer be a segregation of different physiological units in different sperm- cells and germ-cells ; self-fertilization will become impossible : step by step the fertility will diminish, and the series will finally die out. And now observe, in confirmation of this view, that self- fertilization is limited to organisms in which an approximate equilibrium among the organic forces, is not long maintained. While growth is actively going on, and the physiological units are subject to a continually-changing distribution of forces, no decided assimilation of the units can be expected : like forces acting on the unlike units, will tend to segregate them, so long as continuance of evolution permits further segrega¬ tion ; and only when further segregation cannot go on, will the like forces tend to assimilate the units. Hence, where there is no prolonged maintenance of an approximate organic balance, self-fertilization may be possible for some gener¬ ations ; but it will be impossible in organisms distinguished by a sustained moving equilibrium. § 95. The interpretation which it affords of sundry pheno¬ mena familiar to breeders of animals, adds probability to the hypothesis. Mr Darwin has collected a large “ body of facts. genesis, heredity, and variation. 283 showing, in accordance with the almost universal belief ot breeders, that with animals and plants a cross between different varieties, or between individuals of the same variety but ot another strain, gives vigour and fertility to the offspring ; and on the other hand, that close interbreeding diminishes vigour and fertility,” — a conclusion harmonizing with the current belief respecting family-intermarriages in the human race. Have we not here a solution of these facts ? Relations must, on the average of cases, be individuals whose physiological units are more nearly alike than usual. Animals of different varieties must be those whose physiological units aic moie unlike than usual. In the one case, the unlikeness of the units may frequently be insufficient to produce fertilization ; or, if sufficient to produce fertilization, not sufficient to produce that active molecular change required for vigorous develop¬ ment. In the other case, both fertilization and vigoious development will be made probable. Nor are we without a cause for the irregular manifestation ot these general tendencies. The mixed physiological units com¬ posing any organism, being, as we have seen, more or less se¬ gregated in the reproductive centres it throws off ; there may arise various results, according to the degrees of difference among the units, and the degrees in which the units are segre¬ gated. Of two cousins who have married, the common grand¬ parents may have had either similar or dissimilar constitu¬ tions; and if their constitutions were dissimilar, the probability that their married grandchildren will have offspring will be greater than if their constitutions were similar. Or the brothers and sisters from whom these cousins descended, in¬ stead of severally inheriting the constitutions of their parents in tolerably equal degrees, may have severally inherited them in very different degrees : in which last case, intermaniages among. the grandchildren will be less likely to provo infertile. Or the brothers and sisters from whom these cousins de¬ scended, may severally have married persons very like, or very unlike, themselves ; and from this cause there may 234 THE INDUCTIONS OF BIOLOGY. have resulted, either an undue likeness, or a due un like¬ ness, between the married cousins. These several causes, conspiring and conflicting in endless ways and degrees, will work multiform effects. Moreover, differences of segrega¬ tion will make the reproductive centres produced by the same nearly-related organisms, vary considerably in their amounts of unlikeness ; and therefore, supposing their amounts of unlikeness great enough to cause fertilization, this fertiliza¬ tion will be effective in various degrees. Hence it may happen that among offspring of nearly- related parents, there may be some in which the want of vigour is not marked, and others in which there is decided want of vigour. So that we are alike shown why in-and-in breeding tends to diminish both fertility and vigour ; and why the effect cannot be a uniform effect, but only an average effect. § 96. While, if the foregoing arguments are valid, gamo- genesis has for its main end, the initiation of a new develop¬ ment by the overthrow of that approximate equilibrium arrived at among the molecules of the parent-organisms; a further end appears to be subserved by it. Those inferior organisms which habitually multiply by agamogenesis, have conditions of life that are simple and uniform ; while those organisms that have highly-complex and variable conditions of life, habitually multiply by gamogenesis. How if a species has complex and variable conditions of life, its members must be severally exposed to sets of conditions that are slightly different : the aggregates of incident forces cannot be alike for all the scattered individuals. Hence, as functional deviation must ever be inducing structural deviation, each individual throughout the area occupied, tends to become fitted for the particular habits which its particular conditions necessitate ; and in so far, tmfitted for the average habits proper to the species. But these undue specializations are continually checked by gamogenesis. As Mr Darwin remarks — 1 “ intercrossing plays a very important part in nature in GENESIS, HEREDITY, AND VARIATION. 285 keeping the individuals of the same species, or of the variety, true and uniform in character the idiosyncratic divergences obliterate each other. Gamogenesis, then, is a means of turning to positive advantage, the individual differentiations which, in its absence, would result in positive disadvantage. "Were it not that individuals are ever being made unlike each other by their unlike conditions, there would not arise among * them those contrasts of molecular constitution, which we have seen to be needful for producing the fertilized germs of new individuals. And were not these individual differentiations ever being mutually cancelled, they would end in a fatal narrowness of adaptation. This truth will be most clearly seen if we reduce it to its purely abstract form, thus : — Suppose a quite homogeneous species, placed in quite homogeneous conditions ; and suppose the constitutions of all its members in complete concord with their absolutely-uniform and constant conditions; what must happen ? The species, individually and collectively, is in a state of perfect moving equilibrium. All disturbing forces have been eliminated. There remains no force which can, in any way, change the state of this moving equilibrium ; either in the species as a whole or in its members. But we have seen ( First Principles , § 133) that a moving equilibrium is but a transition towards complete equilibration, or death. The absence of differential or un-equilibrated forces among the members of a species, is the absence of all forces that can cause changes in the conditions of its members — is the ab¬ sence of all forces which can initiate new organisms. To say, as above, that complete molecular homogeneity existing among the members of a species, must render impossible that mutual molecular disturbance which constitutes fertilization, is but another way of saying, that the actions and re-actions of each organism, being in perfect balance with the actions and re-actions of the environment upon it, there remains in each organism, no force by which it differs from any other — no force which any other does not meet with an exactly 286 THE INDUCTIONS OF BIOLOGY. equal force — no force which, can set up a new evolution among the units of any other. And so we reach the remarkable conclusion, that the life of a species, like the life of an individual, is maintained by the unequal and ever-varying actions of incident forces on its different parts. An individual homogeneous throughout, and having its substance everywhere continuously subject to like actions, could undergo none of those changes which life con¬ sists of; and similarly, an absolutely- uniform species, having all its members exposed to identical influences, would be deprived of that initiator of change which maintains its existence as a species. Just as, in each organism, incident forces constantly produce divergences from the mean state in various directions, which are constantly balanced by opposite divergences indi¬ rectly produced by other incident forces ; and just as the combination of rhythmical functions thus maintained, consti¬ tutes the life of the organism ; so, in a species, there is, through gamogenesis, a perpetual neutralization of those contrary de¬ viations from the mean state, which are caused in its different parts by different sets of incident forces ; and it is similarly by the rhythmical production and compensation of these con¬ trary deviations, that the species continues to live. The moving equilibrium in a species, like the moving equilibrium in an individual, would rapidly end in complete equilibration, or death, were not its continually- dissipated forces continually re-supplied from without. Besides owing to the external world, those energies which, from moment to moment, keep up the lives of its individual members ; every species owes to certain more indirect actions of the external world, those energies which enable it to perpetuate itself in successive generations. §97. What evidence still remains, may be conveniently woven up along with a recapitulation of the argument pursued through the last three chapters. Let us contemplate the facts in their synthetic order. * / GENESIS, HEREDITY, AND VARIATION. 287 Tnat compounding and re-compounding through which we pass from the simplest inorganic substances to the most com¬ plex organic substances, has several concomitants. Each successive stage of composition, presents us with atoms that are severally larger or more integrated, that are severally more heterogeneous, that are severally more unstable, and that are more numerous in their kinds ( First Principles , § 111). And when we come to the substances of which living bodies are formed, we find ourselves among multiplied, divergent groups and sub-groups of compounds, the units of which are large, heterogeneous, and unstable, in high degrees. There is no reason to assume that this process ends with the formation of those complex colloids which characterize organic matter. A more probable assumption is, that out of the complex colloidal atoms, there are evolved, by a still further integration, atoms that are still more heterogeneous, and of kinds that are still more multitudinous. What must be their properties P Al¬ ready the colloidal atoms are extremely unstable — capable of being variously modified in their characters by very slight incident forces ; and already the complexity of their polarities prevents them from readily falling into those positions of polar equilibrium which result in crystallization. Now the organic atoms composed of these colloidal atoms, must be simi¬ larly characterized in far higher degrees. Far more numerous must be the minute changes that can be wrought in them by minute external forces ; far more free must they remain for a long time to obey forces tending to re-distribute them ; and far greater must be the number of their kinds. Setting out with these physiological units, the existence of which various organic phenomena compel us to recognize, and the production of which the general law of Evolution thus leads us to anticipate ; we get an insight into the phenomena of Genesis, Heredity, and Variation. If each organism is built of certain of these highly- plastic units peculiar to its species — units which slowly work towards an equilibrium of their complex polarities, in producing an aggregate of the specific 288 THE INDUCTIONS OF BIOLOGY. structure, and wliicli are at the same time slowly modifiable by the re-actions of this aggregate — we see. why the mul¬ tiplication of organisms proceeds in the several ways, and with the various results, which naturalists have observed. Heredity, as shown not only in the repetition of the specific structure, but in the repetition of ancestral deviations from it, becomes a matter of course ; and it falls into unison with the fact that, in various simple organisms, lost parts can be re¬ placed, and that, in still simpler organisms, a fragment can develop into a whole. While an aggregate of physiological units continues to grow,, by the assimilation of matter which it moulds into other units of like type ; and while it continues to undergo changes of structure ; no equilibrium can be arrived at between the whole and its parts. Under these conditions, then, an un- differentiated portion of the aggregate — a group of phy¬ siological units not bound up into a specialized tissue — will be able to arrange itself into the structure peculiar to the species ; and will so arrange itself, if freed from controlling forces, and placed in fit conditions of nutrition and temper¬ ature. Hence the continuance of agamogenesis in little- differ¬ entiated organisms, so long as assimilation continues to be greatly in excess of expenditure. But let growth be checked and development approach its completion — let the units of the aggregate be severally exposed to an almost constant distribution of forces ; and they must begin to equilibrate themselves. Arranged as they will gradually be, into comparatively stable attitudes in relation to each other, their mobility will diminish ; and groups of them, partially or wholly detached, will no longer readily re¬ arrange themselves into the specific form. Agamogenesis will be no longer possible ; or, if possible, will be no longer easy. When we remember that the force which keeps the Earth in its orbit, is the gravitation of each particle in the Earth towards every one of the group of particles existing 91,000,000 of miles off; we cannot reasonably doubt, that each unit in GENESIS, HEREDITY, AND VARIATION. 289 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. 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 nearly- related 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, HE EE DIXY, 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 be 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 Yariation into har¬ mony with 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 which 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 which affords us this interpretation. CHAPTER XL CLASSIFICATION. § 98. That orderly arrangement of objects called Classi¬ fication, lias 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, different kinds of pure Literature, under the heads o -lCLion, 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, Go- CLASSIFICATION. 293 o!°gy, Chemistry, Physiology, &c. ; he 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 Light, 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 their 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 different 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 may be placed in an ascending succession. But groups severally iormed of things distinguished by some common attribute which implies many other attributes, do not admit of serial arrangement. You cannot rationally say, either that His¬ torical 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 tilings equal, the relations among phenomena are recognized in the order of their conspicuousness ; and that, uner things equal, they are recognized in the order of their THE INDUCTIONS OF BIOLOGY. 294 simplicity. Tlie first classifications are sure, therefore, to bo 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 concentions and the realities. CLASSIFICATION. 295 § 99. Biological classifications illustrate well these phases, through which 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 jilaced 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-thin o-s, 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 m the water, creatures that crawl. And these groups were thought of in the order of their importance. . ^le ^rst> arrangements made by naturalists were based either on single characters, or on very simple combinations of characters. Describing plant-classifications, Bindley says : — “ Bivinus invented, in 1690, a system depend¬ ing ujaon the formation of the corolla ; Kamel, in 1693, upon the fruit alone ; Magnol, in 1720, on the calyx and corolla , and finally, Linnaeus, 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 gieat measure serial, is based on the degrees in which these THE INDUCTIONS OF BIOLOGY. 296 attributes are possessed. In 1703, some thirty years before the time of Linmeus, our countryman Hay had sketched the outlines of a more advanced system. He said that — Plants are either Flowerless, or Flowering ; and these are Dicotyledones, or Monocotyledones. Among the minor groups which he placed under these general heads, “ were Fungi, Mosses, Ferns, Composites, Cichoracese TJmbellifers, 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, Bay’s ideas remained dormant until the time of J ussieu ; 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 Natural System has now taken the following form ; which I copy (adding the alliances to the classes) from Prof. Findley’s Vegetable Kingdom * * From this table I have omitted the class Ehizogens , which other botanists do not agree Avith Lindley in regarding as a separate class. The plants respect¬ ing which there has arisen this difference of opinion, are certain floivering plants, which groAV parasitically on the roots of trees. The reasons assigned by Endlicher and Lindley, for erecting them into a separate group of Phsenogams, 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 ; ana mat they are without chlorophyll. Mr Griffith and Dr Hooker, however, have given preponderating reasons AAdiy 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 Iloober 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. 297 Asexual, or Flowerless Plants. Stems and leaves undistinguishable Steins and leaves distinguishable O rAlgales I. Thallogens < Eungales ■ tLichenales rMuscales II. Achogens s Lycopotlalcs tFilicales Sexual, or Flowering Plants. Wood of stem youngest in centre ; cotyledon single. Leaves parallel-veined, permanent ; wood confused III. Endogens •< Leaves net-veined, deciduous ; wood, when perennial, arranged in a circle with a central pith Wood of stem youngest at circum¬ ference, always concentric ; coty¬ ledons two or more. Seeds quite naked IV. Dictyogens. 'Glum ales A rales Pal males Hydrales Narcissales Amomales Orchidales Xyridalcs J uncales Liliales bAlismales V. Gymnogens. f Diclinous Seeds enclosed in seed-vessels VI. Exogens - Hypogynous Perigynous .Epigynous {Amentales Urtieales Euphorbiales &c. &c. {Violales Cistales Malvaies &c. &c. {Eicoidales Daphnales Rosales &c. &c. {Campanales Mvrtales Cactales &c. &c. Here, linear arrangement lias disappeared : there is a breaking up into groups and sub-groups and sub-sub- groups, which 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 Rhizogens , 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 Entozoa , exists for the production by Rhizogens , of seeds that are great iu number and almost spore-like in size. 298 THE INDUCTIONS OF BIOLOGY. same principle of co-orclination 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 are 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¬ naeus. He grouped them thus :* — Cl. 1. Mammalia. Ord. Primates, Pruta, Perm, Glires, Pecora, Bellum, Cete. Cl. 2. Aves. Ord. Accipitres, Picse, Anseres, Grallse, Gallinse, Passeres. Cl. 3. Amphibia. Ord. lleptiles, Serpentes, Nantes. Cl. 4. Pisces. Ord. Apodes, Jugulares, Thoracici, Abdominales. Cl. 5. Insecta. Ord. Coleoptera, Ilemiptera, Lepidoptera, Neuroptera, Diptera, Aptera. Cl. 6. Vebmes. Ord. Intestina, 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. Not 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/5 This con¬ ception survived till the time of Cuvier. “Naturalists/5 * This classification, and the three which follow it, i quote (auridgmg Some of them) from Prof. Agassiz’s “Essay on Classification.” CLASSIFICATION. says Agassiz, “were bent upon establishing one continual uniform series to embrace all animals, between the links of which it was supposed there were no unequal intervals. The watchword of their school was : Natura non facit saltum. They called their system la chaine dcs etres .” 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 are constructed; and in pursuance of this assertion, he drew out the following scheme. First Branch. Animalia Yf.rtebrata Cl. 1. Mammalia. Cl. 2. Birds. Cl. 3. Beptilia. Cl. 4. Fishes. Second Branch. Animalia Mollusca. Cl. 1. Cephalapoda. Cl. 2. Pteropoda. Cl. 3. Gasteropoda. Cl. 4. Acephala. Cl. 5. Brachiopoda. Cl. 6^ Cirriiopoda. Third Branch. Animalia Artictjlata. Cl. 1. Annelides. Cl. 2. Crustacea. Cl. 3. Arachnides. Cl. 4. Insects. Fourth Branch. Animalia Badiata. Cl. 1. Eciiinoderms. Cl. 2. Intestinal Worms. Cl. 3. Acalepile. tvL. 4. Polypi. Cl. 5. Infusoria. 300 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. INVEKTEBKATA. Apathetic Animals. Cl. 1. Infusoria. Cl. 2. Polypi. Cl. 3. P ADI ARIA. Cl. 4. Tuntcata. Cl. 5. Yermes. II. Sensitive Animals. Cl. 6. Insects. Cl. 7. Arachnids. Cl. 8. Crustacea. Cl. 9. Annelids. Cl. 10. Cirripeds. Cl. 11. CoNCHIFERA. Cl. 12. Mollusks. Do not feel, and move only by their excited irritability. No brain, ■ not elongated medullary mass ; no senses ; forms varied ; rarely articu¬ lations. Peel, but obtain from their sensa¬ tions only perceptions of objects, a sort of simple ideas, which 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. VEKTEBKATA. III. Intelligent Animals. Cl. 13. Fishes. Cl. 14. Peptides. Cl. 15. Birds. Cl. 16. Mammalia. Peel; 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 skele- , ton ; form symmetrical, the parts being in pairs. CLASSIFICATION. 301 Passing 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 Yon Paer, led him to arrange animals as 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. (Artictjlata.) Evolutio gemma. The development produces identical parts arising on both sides of an axis, and closing up along a line opposite the axis. IV. Doubly Symmetrical type. ( Vertebrata.) Evolutio higemina. The development produces identical parts arising on both sides of an axis, growing up¬ wards 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 Vertebrata 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 rela- 302 THE INDUCTIONS OF BIOLOGY. tions 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 : — Vertebrata ( Abranchiata ) Mammalia Aves Reptilia ( Branchiatd ) Amphibia Pisces Mollusca Cephalopoda Heteropoda Gasteropoda' dioecia ( Tulmonata Gasteropoda' 1 Pteropoda monoecia Lamellibranchiata Annulosa Articulata Insecta Arachnida Myriapoda Crustacea Annuloida Annellata Scoleid® Echinodermata Trematoda Potifera Toeniad® Turbellaria Nematoidea CtELENTERATA Ilydrozoa Actinozoa. Protozoa Infusoria Spongiad® Gregarinid® Noctilucidce Foraminifera Thallassicollidce What remnant there may seem to be of linear succession in some of these sub-groups, is merely an accident of typo¬ graphical convenience. Each of them is to be regarded simply as a cluster. Were Prof. Iluxley 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 Hunterian Lectures delivered at the College of Sur¬ geons in 1863. And if he were further to develop the arrangement, by dispersing the sub-groups and sub-sub¬ groups on the same principle, there would result an arrange- CLASSIFICATION. 303 ment perhaps not very much unlike that shown in the an¬ nexed diagram. • • • ••'Arts • ; He/itiUn V E RTEB'RATA \ • • • ^ • • « Am fiLibia hsccs Aj^achnida Insec ts between them, unite them together as one great class of Aryan lan¬ guages ; radically distinguished from the classes of lan¬ guages spoken by the other great divisions of the human race. § 123. How this kind of subordination of groups, which we see arises in the course of continuous descent, multiplica¬ tion, and divergence, is just the kind of subordination of groups which plants and animals exhibit : it is just this kind of subordination which has thrust itself on the attention of naturalists, in spite of pre- conceptions. The original idea was that of arrangement in linear order. We saw that even after a considerable acquaintance with the structures of organisms had been acquired, naturalists con¬ tinued their efforts to reconcile the facts with the notion of a uni-serial succession. The accumulation of evidence necessi¬ tated the breaking up of the imagined chain into groups and sub-groups. Gradually there arose the conviction that these groups do not admit of being placed in a line. And the conception finally arrived at, is, that of certain great sub¬ kingdoms, very widely divergent, each made up of classes much less widely divergent, severally containing orders still less divergent; and so on with genera and species. The diagram on page 303, shows the general relations of these divisions in their degrees of subordination. Hence this “grand fact in natural history of the subordina¬ tion of group under group, wdiich from its familiarity does not always sufficiently strike us,” is perfectly in harmony with the hypothesis of evolution. The extreme significance of this kind of relation among organic forms, is dwelt on by hlr Dai win ; who shows how an ordinary genealogical tree represents, on a small scale, a system of grouping analogous to that which exists among organisms in general, and which is THE ARGUMENTS FROM CLASSIFICATION. 359 explained on the supposition of a genealogical tree by wliich all organisms are affiliated. If, wherever we can trace direct descent, multiplication, and divergence, this formation of groups within groups takes place 5 there results a siiong presumption that the groups within groups which constitute the animal and vegetal kingdoms, have arisen by direct descent, multiplication, and divergence — that is, by evolu¬ tion. r 124. Stron" confirmation of this inference is furnished by the fact, that the more marked differences which divide groups, are, in both cases, distinguished fiom the less marked differences which divide sub-groups, by this, that they are not simply greater in degree , but they are more radical in hind. Objects, as the stars, may present them¬ selves in small clusters, which are again more or less aggie¬ gated into clusters of clusters, in such manner that the in¬ dividuals of each simple cluster, are much closer together than are the simple clusters composing a compound cluster : in which case, the kinship that unites groups of groups differs from the kinship that unites groups, not in nature , but only in amount. But this is not the case either with the groups and sub-groups which we know have resulted from evolution, or with those which we here infer have re¬ sulted from evolution. Among these, we find the highest or most general classes, are separated from one another by fundamental differences that have no common measure with the differences that separate small classes. Observe the pa¬ rallelism. We saw that each sub-kingdom of animals is marked off from the other sub-kingdoms, by a total unlikeness in its plan of organization : that is, the members of any sub-kingdom are bound together, not by some superficial attribute which they all have, but by some attribute determining the general nature of their organizations. While, contrariwise, the members of the smallest groups are united together, and se¬ parated from the members of other small groups, by modi- 360 THE EVOLUTION OF LIFE. fications which, do not affect the essential relations of party. That this is just the kind of arrangement which results from evolution, the case of languages will show. If we compare the dialects spoken in different parts of England, we find scarcely any differences hut those of pro¬ nunciation : the structures of the sentences are almost uniform. Between English and the allied modern languages, there are decided divergences of structure : there are some unlikenesses of idiom ; some unlikenesses in the ways of modifying the meanings of verbs ; and considerable unlike¬ nesses in the uses of genders. But these unlikenesses are not sufficient to hide a general community of organization. A greater contrast of structure exists between these modern lan¬ guages of Western Europe, and the classic languages. That differentiation into abstract and concrete elements, which is shown by the substitution of auxiliary words for inflections, has produced a higher specialization distinguishing these languages as a group from the older languages. Neverthe¬ less, both the ancient and modern languages of Europe, to¬ gether with some Eastern languages derived from the same original, have, under all their differences of organization, a fundamental community of organization ; inasmuch as all of them exhibit the formation of words by such a coalescence and integration of roots as destroys the independent meanings of the roots. These Aryan languages, and others which have the amalgamate character, are united by it into a class distinguished from the aptotic and agglutinate languages ; in which the roots are either not united at all, or so incompletely united that one of them still retains its independent meaning. And philologists find that these fundamental differences which severally determine the grammatical forms, or modes of com¬ bining ideas, are really characteristic of the primary divisions among languages. That is to say, among languages, where we know that evolution has been going on, the greatest groups are marked off from one another by the strongest structural contrasts ; and as the like holds among groups of organisms, there re- THE ARGUMENTS FROM CLASSIFICATION. 861 suits a further reason for inferring that these have been evolved. § 125. There is yet another parallelism of like meaning. We saw (§ 101) that the successively-subordinate classes, orders, genera, and species, into which zoologists and botan¬ ists segregate animals and plants, have not, in reality, those definite values conventionally given to them. There are well-marked species, and species so imperfectly defined that certain systematists regard them as varieties. Between genera, strong contrasts exist in many cases ; and in other cases, contrasts so much less decided as to leave it doubtful whether they constitute generic distinctions. So, too, is it with orders and classes : in some of which there have been introduced intermediate sub-divisions, having no equivalents in others. Even of the sub-kingdoms the same truth holds. The contrast between the Molluscoida and the Mollusca , is far less than that between the Mollusca and the Annulosa ; and there are naturalists who think that the Vertebrata are so much more widely separated from the other sub-kingdoms, than these are from one another, that the Vertebrata should have a classificatory value equal to that of all the other sub¬ kingdoms taken together. Eow just this same indefiniteness of value, or incomplete¬ ness of equivalence, is observable in those simple and com¬ pound and re-compound groups, which we see arising by evolution. In every case, the endeavour to arrange the divergent products of evolution, is met by a difficulty like that which would meet the endeavour to classify the branches of a tree, into branches of the first, second, third, fourth, &c., orders — the difficulty, namely, that branches of intermediate degrees of composition exist. The illustration furnished by languages will serve us once more. Some dia¬ lects of English are but little contrasted ; others are strongly contrasted. The alliances of the several Scandinavian tongues with one another are different in degree. Dutch is much 362 THE EVOLUTION OF LIFE less distinct from German than Swedish is ; while between the Danish and Swedish there is so close a kinship, that they might almost be regarded as widely-divergent dialects. Similarly on comparing the larger divisions, we see that the various languages of the Aryan stock, have deviated from the original to very unlike distances. The geneiai conclusion is manifest. While the kinds of human speech fall into groups, and sub-groups, and sub-sub-groups ; yet the groups are not equal to one another in value, nor have the sub-groups equal values, nor the sub-sub-groups. If, then, the classification of organisms results in several orders of assemblages, such that assemblages of the same order are hut indefinitely equivalent ; and if, where evolution is known to have taken place, there have arisen assemblages between which the equivalence is similarly in¬ definite ; there is additional reason for inferring that organisms are products of evolution. § 126. A fact of much significance remains. If groups of organic forms have arisen by divergence and re-diver- gence ; and if, while the groups have been develojiing from simple groups into compound groups, each group and sub-group has been giving origin to more complex forms of its own type ; then it is inferable that there once ex¬ isted greater structural likenesses between the members of allied groups, than exist now. Hence, if we take the simplest members of any group to be those which have undergone the least change ; we may expect to find a greater likeness between them and the simplest members of an allied group, than we find between the more complex members of the two groups. This, speaking generally, proves to be so. Between the sub-kingdoms, the gaps are extremely wide ; but such distant kinships as may be discerned, bear out an¬ ticipation. Speaking of that extremely-degraded vertebrate animal the Amphioxus, which has several molluscous traits the arguments from classification. 363 111 its organization, Dr Carpenter remarks, tliat it “ furnishes an apt illustration of another important fact, that it is by the lowest rather than by the highest forms of two natural groups, that they are brought into closest relation.” What are the faint traces of community between the Annulosa and the Mollusca ? They are the thread-cells which some of their inferior groups have in common with the Goelenterata. More decided approximations exist between the lower members of classes. In tracing down the Crustacea and the Arachnida from their more complex to their simpler forms, zoologists meet with difficulties : respecting some of these simpler forms, it becomes a question which class they belong to. The Lepidosiren , about which there have been disputes whether it is a fish or an amphibian, is inferior in the organization of its skeleton, to the great majority ot both fishes and amphibia. Widely as they differ from them, the lower mammals have some characters in common with birds, which the higher mammals do not possess. Mow since this kind of relationship of groups is not ac¬ counted for by any other hypothesis, while the hypothesis of evolution gives us a clue to it ; we must include it among the evidences of this hypothesis, which the facts of classification furnish. § 127. What shall we say of these several leading truths when taken together? That naturalists have been gradually compelled to arrange organisms in groups within groups ; and that this is the arrangement which we see arises by descent, alike in individual families and among races of men, is a striking circumstance. That while the smallest groups are the most nearly related, there exist beween the great sub-kingdoms, structural contrasts of the profoundest kind ; cannot but impress us as remarkable, when we see that where it is known to take place, evolution actually produces these feebly-distinguished small groups, and these strongly-dis¬ tinguished great groups. The impression made by these two 364 THE EVOLUTION OF LIFE. parallelisms, which add meaning to eacli oilier, is deepened by tlie third parallelism, which enforces the meaning of both — the parallelism, namely, that as, between the species, genera, orders, classes, &c., which naturalists have formed, there are transitional gradations ; so between the groups, sub-groups, and sub-sub-groups, which we know to have been evolved, groups of intermediate values exist. And these three correspondences between the known results of evolution, and the results here ascribed to evolution, have further weight given to them by the circumstance, that the kinship of groups through their lowest members, is just the kinship which the hypothesis of evolution implies. Even in the absence of these specific agreements, the broad fact of unity amid multiformity, which organisms so strik¬ ingly display, is strongly suggestive of evolution. Freeing ourselves from pre-conceptions, we shall see good reason to think with Mr Darwin, “ that propinquity of descent — the only known cause of the similarity of organic beings — is the bond, hidden as it is by various degrees of modification, which is partly revealed to us by our classifications.” When we consider that this only known cause of similarity, joined with the only known cause of divergence, which we have in the influence of conditions, gives us a key to these likenesses obscured by unlikenesses, to which no consistent interpreta¬ tion can otherwise be given, even if purely hypothetical causes be admitted; we shall see that were there none of those very remarkable harmonies above pointed out, the truths of classification would still yield strong support to our conclusion. CHAPTER Y. THE ARGUMENTS FROM EMBRYOLOGY. 3 123. There was briefly set forth in § 52, a remarkable induction established by Yon Baer ; who “ found that in its earliest stage, every organism has the greatest number of characters in common with all other organisms in their earliest stages ; that at a stage somewhat later, its structure is like the structures displayed at corresponding phases by a less extensive multitude of organisms ; . that at each subse¬ quent stage, traits are acquired which successively distin¬ guish the developing embryo from groups of embryos that it previously resembled — thus step by step diminishing the class of embryos which it still resembles ; and that thus the class of similar forms is finally narrowed to the species of which it is a member/’ Though this generalization is to be taken with qualifications, yet, as an average truth, it may be regarded as beyond question ; and as an average truth, it has a profound significance. For if we follow out in thought the implications of this truth — if we conceive the germs of all kinds of organisms simultaneously developing ; if after taking their first step together, we imagine at the second step, one half of the vast multitude diverging from the other half ; if, at the next step, we mentally watch each of these great assemblages beginning to take two or more routes of development ; if we represent to ourselves this bifurcation simultaneously going on, stage after stage, in all the THE EVOLUTION OF LIFE 366 brandies ; we shall see that there must result an aggregate analogous, in its arrangement of parts, to a tree. If this vast genealogical tree be contemplated as a whole, made up cf trunk, great branches, secondary branches, and so on, as far as the terminal twigs ; it will be perceived that all the various kinds of organisms represented by these terminal twigs, forming the periphery of the tree, will stand related to each other in small groups, which are united into groups of groups, and so on. The embryological tree, expressing the developmental relations of organisms, will be similar to the tree which symbolizes their classificatory relations. That subordination of classes, orders, genera, and species, to which naturalists hgve been gradually led, is just that subordination which results from the divergence and re-divergence of embryos, as they all unfold. On the hypothesis of evolution, this parallelism has a meaning — indicates that primordial kinship of all organisms, and that progressive differentiation of them, which the hypothesis alleges. Tut on any other hypothesis the parallelism is meaningless : or rather, it raises a difficulty ; since it implies either an effect without a cause, or a design without a purpose. § 129. It was said above, that this great embryological law is to be taken with certain qualifications. The resem¬ blances which hold together great groups of embryos in their early stages, and which hold together smaller and smaller groups in their later and later stages, are not special or exact, but general or approximate ; and in some cases, the conformity to this general law is very imperfect. These irregularities, however, instead of being at variance with the hypothesis of evolution, afford further support to it. Observe, first, that the only two other possible suppositions respecting developmental changes, are negatived, the one by this general law and the other by the minor nonconformities to it. If it be said that the conditions of the case necessi¬ tated the derivation of all organisms from simple germs, and TIIE ARGUMENTS FROM EMBRYOLOGY. 307 therefore necessitated a morphological unity in their primitive states ; there arises the obvious answer, that the morphologi¬ cal unity thus implied, is not the only morphological unity to be accounted for. 'Were this the only unity, the various kinds of organisms, setting out from a common primordial form, should all begin from the first to diverge individually, as so many radii from a centre ; which they do not. If, other¬ wise, it be said that organisms were framed upon certain types, and that those of the same type continue developing together in the same direction, until it is time for them to begin putting on their specialities of structure ; then, the answer is, that when they do finally diverge, they ought severally to develop in direct lines towards their final forms. Iso reason can be assigned why, having once parted company, some should progress towards their final forms by irregular or circuitous routes. On the hypothesis of design, such de¬ viations are inexplicable. The hypothesis of evolution, however, while it pre-supposes those general relations among embryos which are found to exist, also affords explanations of these minor nonconformities. If, as any rational theory of evolution pre- supposes, the pro¬ gressive differentiations of organic forms from one another during past times, have resulted, as they are resulting still, from the direct and indirect effects of external conditions — if organisms have become different, either by immediate adaptations to unlike habits of life, or by the mediate adapta¬ tions resulting from preservation of the individuals most fitted for such habits of life, or by both ; and if the embryonic changes are related to the changes that were undergone by ancestral races ; then these irregularities must be expected. For the successive changes in modes of life pursued by successive ancestral races, can have had no regularity of sequence. In some cases they must have been more numerous than in others ; in some cases they must have been greater in degree than in others ; in some cases they must have been to lower modes, in some cases to higher modes, and in some 368 TIIE EVOLUTION OF LIFE. cases to inodes neither higher nor lower. Of two connate races which diverged in the remote jrnst, the one may have had descendants that have remained tolerably constant in theii habits, while the other may have had descendants that have passed through widely-aberrant modes of life ; and yet some of these last may have eventually taken to modes of life like those of the divergent races derived from the same stock. And if the metamorphoses of embryos, indicate, in a general way, the changes of structure undergone by ancestors ; then, the later embryologic changes of such two allied races, will be somewhat different, though they may end in very similar forms. An illustration will make this clear. Mr Darwin says : — “ Petrels are the most aerial and oceanic of birds, but in the quiet sounds of Tierra del Fuego, the Puffinuria berarcli , in its general habits, in its astonishing power of diving, its manner of swimming, and of flying when un¬ willingly it takes flight, would be mistaken by any one for an aide or grebe ; nevertheless, it is essentially a petrel, but with many parts of its organization profoundly modified/’ Now if we suppose these grebe-like habits to be continued through a long epoch, the petrel-form to be still more ob¬ scured, and the approximation to the grebe-form still closer ; it is manifest that while the chicks of the grebe and the Puffinuria will, during their early stages of development, display that likeness involved by their common derivation from some early type of bird, the chick of the Puffinuria will eventually begin to show deviations, representative of the ancestral petrel-structure, and will afterwards begin to lose these distinctions, and assume the grebe- structure. Hence, remembering the perpetual intrusions of organisms on one another’s modes of life, often widely different ; and remembering that these intrusions have been going on from the beginning ; we shall be prepared to find that the general law of embryologic parallelism, is qualified by irregularities that are mostly small, in many cases considerable, and THE ARGUMENTS FROM EMBRYOLOGY. 36) occasionally great. The hypothesis of evolution accounts for these : it does more — it implies the necessity of them. § 130. The substitutions of organs and the suppressions of organs, are among those secondary embryological phe¬ nomena which harmonize with the belief in evolution but cannot be reconciled with any other belief. There are cases where, during its earlier stages of development, an embryo possesses organs that afterwards dwindle away, as there arise other organs to discharge the same functions. And there are cases where organs make tlicir appearance, grow to certain points, have no functions to dischaigo, and disappear by absorption. We have a remarkable instance of this substitution in the successive temporary appliances for aerating the blood, which the mammalian embryo exhibits. During the first phase of its development, the mammalian embryo circulates its blood through a system of vessels distributed over what is called the area vasculosa — a system of vessels homologous with one which, among fishes, serves for aerating the blood until the permanent respiratory organs come into play. After a time, there buds out from the mammalian embryo, a vascular membrane called the allantois, homologous with one which, in birds and reptiles, replaces the first as a breathing apparatus. But while in the higher oviparous vertebrates, the allantois serves the purpose of a lung during the rest of embryonic life, it does not do so in the mamma¬ lian embryo. In implacental mammals, it aborts, having no function to discharge; and in the higher mammals, it becomes “ placentiferous, and serves as the means of inter¬ communication between the parent and the offspring oe- comes an organ of nutrition more than of respiration. Now since the first system of external blood-vessels, not being in contact with a directly-oxygenated medium, cannot be very serviceable to the mammalian embryo as a lung , and smco 370 THE EVOLUTION OF LIFE. the second system of external blood-vessels is, to the im- placental embryo, of no greater avail than the first ; and since the communication between the embryo and the placenta among placental mammals, might as well or better have been made directly, instead of by metamorphosis of the allantois ; these substitutions appear unaccountable as results of design. But they are quite congruous with the supposition, that the mammalian type arose out of lower vertebrate types. For in such case, the mammalian embryo, passing through states representing, more or less distinctly, those which its remote ancestors had in common with the lower F'ertebrata, develops these subsidiary organs in like ways with the lower Vertebrata. Even more striking than the substitutions of organs are the suppressions of organs. Mr Darwin names some cases as “ extremely curious ; for instance, the presence of teeth in foetal whales, which when grown up have not a tooth in their heads ; * * * It has even been stated on good authority that rudiments of teeth can be detected in the beaks of certain embryonic birds.” Hot even temporary functions can be assigned for these organs that are first built up and then pulled down again. They are absolutely useless — their formation is absolutely superfluous. Irrecon¬ cilable with any teleological theory, they do not even har¬ monize with the theory of fixed types which are maintained by the development of all the typical parts, even where not wanted ; seeing that the disappearance of these incipient organs during foetal life, spoils the typical resemblance. But while to all other hypotheses these facts are stumbling- blocks, they yield strong support to the hypothesis of evolu¬ tion. Allied to these cases, are the cases of what has been called retrograde development. Many parasitic creatures and creatures which, after leading active lives for a time, eventu¬ ally become fixed, lose, in their adult states, the limbs and senses which they had when young. It may be allcgecb THE ARGUMENTS FROM EMBRYOLOGY. 371 however, that these creatures could not secure the habitats needful for them, without possessing during their larval stages, eyes and swimming appendages which eventually become useless ; that though, by losing these, their organiza¬ tion retrogresses in one direction, it progresses in another direction ; and that, therefore, they do not exhibit the need¬ less development of a higher type on the way to a lower type. Nevertheless there are instances of a descent in organization, following an apparently- superfluous ascent. Mr Darwin says that in some genera of cirripedes, “ the larvae become developed either into hermaphrodites having the ordinary structure, or into what I have called comple¬ ments males, and in the latter, the development has assuredly been retrograde ; for the male is a mere sack, which lives for a short time, and is destitute of mouth, stomach, or other organ of importance, excepting for reproduc¬ tion.” § 131. Comparative embryology shows us that besides substitutions of organs, there are wdiat may be called substi - tuted modes of development. The same kind of structure is not always produced in the same way ; and some allied groups of organisms have modes of evolution which appear to be radically contrasted. The two modes are broadly dis¬ tinguishable as the direct and the indirect . They may severally characterize the general course of evolution as a whole, and the course of evolution in particular organs. Thus in the immense majority of articulate animals, metamorphoses, more or less marked and more or less numerous, are passed through on the way to maturity. The familiar transformations of insects show us how circuitous is the route by which the embryo-form arrives at the adult form, among some divisions of the Articulata. But there are other divisions, as the lower Arachnida, in which the unfold¬ ing of the egg into the adult takes place in the simplest manner : the substance grows towards its appointed shape 372 THE EVOLUTION OF LIFE. by tlie shortest route. The Mollusca furnish contrasts which, though less marked, are essentially of the same nature. Among some Gasteropods, according to Yogt, the germ-mass, after undergoing its earliest changes in the same way as germ- masses in general, begins to transform itself bodily into the finished, structure : in one part, the component cells coalesce to form the heart, in another part to form the liver, and so on. But in other classes of molluscs, as the Cephalopods, the eiubryo is moulded out of the blastoderm, or superficial layer of the germ-mass ; and the various organs, mostly aris¬ ing out of this blastoderm by a process of budding, reach their ultimate shapes through successive modifications, while they grow at the expense of the nutriment absorbed from the rest of the germ-mass. And this indirect development is universal among the Pertebrata. Aow on contemplating in their ensemble , the facts thus briefly indicated, we may trace among these irregularities something like a general rule. The indirect development characterizes the most-higlily- organized forms. In the sub-kingdom Vcrtebrata, 'which, considered as a whole, stands far above the rest in complexity, the development is uniformly indirect. It is indirect in the great mass of the Articulata. It is indirect in the highest Mollusca. Conversely, it is direct in a large proportion of the lower types. The eggs of Protozoa, of Coelenterata , of inferior Annuloicla, originate the respective structures proper to them, by transformations that are almost immediate ; each of the cycle of forms passed through, is assumed, when the proper time comes, in the simplest way ; and where they multiply by budding, the substance of the bud passes by as short a process as may be, into the finished form. Where among the simpler types of animals, the evolution is indirect, its indirectness generally appears to be related to some transitional mode of life, which the larva passes through on its way to maturity ; and where we find direct evolution among the more complex types, it is THE ARGUMENTS FROM EMBRYOLOGY. 373 in their most degraded members : instance the Acari among the Articulata .* We have before found that the facts of social organization, furnish us with hints towards interpreting the phenomena exhibited in individual organisms. Let us see whether analogies hence derived, do not help us here. A factory, or other producing establishment, or a town made up of such establishments, is an agency for elaborating some commodity consumed by society at large \ and may be regarded as analogous to a gland or viscus in an individual organism. If, now, we inquire what is the primitive mode in which one of these producing establishments grows up, we find it to be this. A single worker, who himself scfLls the produce ot his labour, is the germ. His business increasing, he employs helpers — His sons or others ; and having done this, he be¬ comes a vendor not only of his own handiwork, but of that of others. A further increase of his business compels him to multiply his assistants, and his sale grows so rapid that he is obliged to confine himself to the process of selling ; that is, he ceases to be a producer, and becomes simply a channel through which the produce of others is conveyed to the public. Should his prosperity rise yet higher, he finds that he is unable to manage even the sale of his commodities, and has to employ others, probably of his own family, to aid him in sellins: ; that is, to him as a main channel are now added subordinate channels ; and so on continuously. Moreover, * It may be urged that the mode of development is obviously related to the size of the mass which is to be transformed into the embryo. Doubtless it is true that direct transformation is characteristic of small ova, and indirect trans¬ formation of large ova ; and some such connexion may be necessary. Yery pos¬ sibly that polarity of the physiological units, which determines the specific structure, will not act throughout a large mass in such way as to transform it bodily into the specific structure ; though it will thus act throughout a small mass. But that the bulk of the ovum is not the sole cause of this difference of method, is proved by the fact that in some cases where the development is comparatively direct, as in Actcon , the ovum is very much larger than in cases where it is comparatively indirect, as in minute insects. THE EVOLUTION OF LIFE. 374 when there grow up in one pla*ce, as a Manchester or a Birmingham, many establishments of like kind, this process is carried still further. There arise factors and agents, who are the channels through which are transmitted the pro¬ duce of many mills ; and we believe that primarily, these factors were manufacturers who undertook to dispose of the produce of smaller houses as well as their own, and ultimately became salesmen only. blow this, which is the original mode in which social agencies of all kinds are evolved, does not continue to be the mode. There is a tendency everywhere manifested to substitute a direct process for this indirect process. Manufacturing establishments are no longer commonly developed through the series of modifica¬ tions above described ; but mostly arise by the immediate transformation of a number of persons into master, clerks, foremen, workers, &c. Instead of business-partnerships being formed, as they originally were, by some slow unob¬ trusive union between traders and their sons or assistants ; we now have joint-stock-companies resulting by sudden metamorphoses of groups of citizens. The like is true with larger and more complex social agencies. A new town in the United States arises not at all after the old method of gradual accumulations round a nucleus, and successive small modifications of structure accompanying increase of size ; but it grows up over a large area, according to a pre- deter¬ mined plan ; and there are developed at the outset, those various civil, ecclesiastical, and industrial centres, which the incipient city will require. Even in the formation of colonies we may similarly see, that the whole tj^pe of social organization proper to the race from which the colony comes, begins at once to show itself. There is not a gradual passing through all those developmental phases passed through by the mother- society ; but there is a comparatively direct transformation of the assemblage of colonists, into a social organism allied in structure to the social organism of which it was an offset. TIIE ARGUMENTS FROM EMBRYOLOGY. 375 Let us now return to the development of individual organisms ; carrying back this idea with us. On the hypothesis of evolution, all organs must have been originally formed after the indirect method, by the accumulation of modifications upon modifications ; and if the development of the embryo repeats the development of ancestral races, organs must be thus formed in the embryo. To a consider¬ able extent they are thus formed. There is a striking parallelism between the mode in which, as above described, manufacturing agencies are originally evolved, and the mode in which secreting organs are evolved. Out of the group of bile- cells forming the germ of the liver, some centrally-placed ones, lying next to the intestine, are trans¬ formed into ducts through which the secretion of the peri¬ pheral bile-cells is poured into the intestine ; and as the peri¬ pheral bile-cells multiply, there similarly arise secondary ducts emptying themselves into the main ones ; tertiary ones into these ; and so on. 13 ut while in this and in other organs, the development remains in a great degree indirect ; there are organs, as the heart, in which it is comparatively direct. The heart of the vertebrate embryo does not arise from a bud ; but it is first traceable as an aggregated mass of cells, becoming distinct from the cells amid which it is imbedded : its transformation into a contractile chamber, is effected by the consolidation of its outer cells while its inner cells liquify. And the comparatively direct development thus displayed in some organs of the higher embryos, is, as we have seen, characteristic of the entire development in many lower embryos. On the hypothesis of evolution, the direct mode of de¬ velopment in animals, must have been substituted for the indirect mode ; as we see that it is substituted in societies. How comes it to have been substituted ? 33y studying the cause of the substitution in the social organism, we may perhaps get some insight into its cause in the individual or¬ ganism. The direct mode of forming social agencies 370 TIIE EVOLUTION OF LIFE. replaces tlie indirect mode, when these social agencies have either been so long established, or have become so prevalent, or both, as to modify the people’s habits and ideas. Groups of citizens unite into corporate bodies which quickly organ¬ ize, because the habit of forming such combinations has so far modified the thoughts and feelings of citizens, that it becomes natural to them thus to arrange themselves. So too, is it with the men who form a colony. The rapid as¬ sumption by them of a social structure, as similar as circum¬ stances permit to the structure of the mother- society, is manifestly due to the fact, that the organization of the mother-society has moulded the emotions and beliefs of its members into conformity with itself ; so that when some of its members are transferred to a colony, they arrange themselves directly into a structure of like type with that of the mother-society : they do not repeat all the stages through which the mother-society passed, because their natures have been too far modified to allow of their doing this. That action and reaction between a social organism and its units, which we here see accounts for changes in modes of social development, must be paralleled by the action and reaction between an individual organism and its units. Various classes of phenomena compelled us to conclude, that each kind of organism is composed cf physiological units, having certain peculiarites which force them to arrange themselves into the form of the species to which they are peculiar. And in the chapters on Genesis, Heredity, and Variation, we saw reason to believe, that while the polarities of the physiological units determine the structure of the organism as a whole ; the organism as a whole, if its structure is changed by incident forces, reacts on the physiological units, and modifies them towards con¬ formity with its new structure. Now this action and reac¬ tion between an organic aggregate and its units, tending ever to bring the two into absolute harmony, must be con¬ tinually making the developmental processes more direct ; THE ARGUMENTS FROM EMBRYOLOGY. 377 and will show its effects in all kinds of ways and degrees, according to the ancestral history of each species. Suppos¬ ing it were possible for a race of organisms to have con¬ tinued propagating itself through an indefinitely-long period without any change of conditions, necessitating change of structure ; there would be reached so complete a congruity between the organic aggregate and its physiologi¬ cal units, that the units would arrange themselves directly into a structure like that of the adult organism : the germ would put on the proper characters of the species, with little or no transposition of substance. But in the absence of any such constancy of conditions and structure, what may we expect ? We may expect that where the conditions and structure have been most constant, the mode of develop¬ ment will be the most direct ; and that it will be the most indirect, where there have been the greatest and most numerous changes in the habits and structures of ancestral races of organisms. And we may also expect that develop¬ mental changes corresponding to early ancestral forms, will undergo an obliteration that is great in proportion to the fixity of organization that has been since maintained. The facts appear in harmony with this conclusion. We see a comparatively- direct development in those inferior types of animals, which show us, by their inferiority, that they have not, since the commencement of organic life, passed through many sets of changes. And where we find direct de¬ velopment among higher types of animals, it characterizes the simpler rather than the more complex members of the types. Between different parts in the same embryo, there are un¬ likenesses in the method of formation, which seem to have kindred meanings. The heart, of which the development is in great measure direct, is an organ that appears compara¬ tively early among the ascending grades of organic forms ; and having appeared, retains throughout the character of a hollow muscle. Conversely, the organs which develop with 17 378 THE EVOLUTION Or LIFE. great indirectness, are the organs of external relation j which, in the progress of organic forms, undergo various metamorphoses. Some light, too, is thus thrown on certain irregularities in the order of development of organs. If we contemplate those continuous actions and reactions which tend ever to establish a balance between an organic aggregate and its units ; we shall see that the effect which the units composing any organ, produce on the organism as a whole, will depend, partly on the permanence of such organ, and partly on its proportional mass. The influence of any force, is a product of its amount multiplied into the time during which it has acted. Hence, a larger part of the aggregate acting for a shorter time, will impress itself on the phy¬ siological units, as much as a smaller part acting for a longer time ; and may thus begin to show its influence in the developmental changes, as soon as, or even earlier than, a part that has existed for a greater period. Thus it becomes comprehensible why, in certain Entozoa which have im¬ mensely-developed generative systems, the rudiments of the generative systems are the first to become visible. And thus are also explicable, anomalies such as those pointed out by Prof. Agassiz — the appearance, in some cases, of traits characterizing the species, at an earlier period of development than traits characterizing the genus. % § 132. So that while the embryologic law enunciated by Yon Baer, is in harmony with the hypothesis of evolution, and is, indeed, a law which this hypothesis implies ; the minor nonconformities to the law, are also interpretable by this hypothesis. Parallelism between the courses of develop¬ ment in species that had a common ancestry, is liable to bo variously modified in correspondence with the later ancestral forms passed through after divergence of such species. The substitution of a direct for an indirect process of formation, which we have reason to believe will show itself, both in the unfolding of the entire organism and in the unfolding of par- THE ARGUMENTS FROM EMBRYOLOGY. 379 ticular organs, must obscure the embryologic history. And the parts influencing the whole in degrees varying with their masses, there results a further influence which, from the out¬ set, must begin to modify the metamorphoses of each kind of embryo ; and cause it to show incipient divergences from embryos which had ancestral histories the same as its own. Thus we find three different causes conspiring in endless ways and degrees, to produce deviations from the general law — causes which are manifestly capable of producing, under special conditions, changes in apparent contradiction to this law. CHAPTER VI. THE ARGUMENTS FROM MORPHOLOGY. § Leaving out of consideration tlie parallelism of development which characterizes organisms belonging to each gioup, that community of plan which exists among them when they are mature, is extremely remarkable and extremely suggestive. As before shown (§ 103), neither the supposition that these combinations of attributes which unite classes are fortuitous, nor the supposition that no other combinations v ei e practicable, nor the supjiosition of adherence to pre° determined typical plans, suffices to explain the facts. An instance will best prepare the reader for seeing the true meaning of these fundamental likenesses. Under the immensely- varied forms of insects, greatly elon¬ gated like the dragon-fly, or contracted in shape like the lady-bird, winged like the butterfly, or wingless like the flea, we find this character in common — there are primarily twenty segments. These segments may be distinctly marked, or they may be so fused as to make it difficult to find the divisions between them. This is not all. It has been shown that the same number of segments is possessed by all the Crustacea. The highly- consolidated crab, and the squilla with its long, loosely-jointed divisions, are composed of the same number of somites. Though, in the higher crustaceans, some of these successive indurated rings, forming the exo¬ skeleton, are never more than partially marked off from each THE ARGUMENTS FROM MORPHOLOGY. 381 other; yet they are indentifiable as homologous with segments, which, in other crustaceans, are definitely divided. What, now, can he the meaning of this community of structure among these hundreds of thousands of species filling the air, burrowing in the earth, swimming in the water, creeping about among the sea-weed, and having such enormous differences of size, outline, and substance, as that no community would be suspected between them ? Why under the down-covered body of the moth and under the hard wing-cases of the beetle, should there be discovered the same number of divisions as in the calcareous framework of the lobster P It cannot be by chance that there exist just twenty segments in all these hundreds of thousands of species. There is no reason to think it was necessary , in the sense that no other number would have made a possible organism. And to say that it is the result of design — to say that the Cre¬ ator followed this pattern throughout, merely for the purpose of maintaining the pattern — is to assign a motive which, if avowed by a human being, we should call whimsical, bio rational interpretation of this and hosts of like morphological truths, can be given except by the hypothesis of evolution ; and from the hypothesis of evolution they are corollaries. If organic forms have arisen from common stocks by per¬ petual divergences and redivergences — if they have continued to inherit, more or less clearly, the characters of ancestral races; then there will naturally result these communities of fundamental structure among extensive assemblages of crea¬ tures, that have severally become modified in countless ways and degrees, in adaptation to their respective modes of life. To this let it be added, that while the belief in an intentional adhesion to a pre-determined pattern throughout a whole group, is totally negatived by the occur¬ rence of occasional deviations from the pattern ; such devi¬ ations are reconcilable with the belief in evolution. As pointed out in the last chapter, there is reason to think that remote ancestral traits, will be obscured more or less according 382 THE EVOLUTION OF LIFE. as tlie superposed modifications of structure, have or have not been great or long maintained. Hence, though the occur¬ rence of articulate animals, such as spiders and mites, having fewer than twenty segments, is fatal to the supposition that twenty segments was decided on for the three groups of superior Articulata; it is not incongruous with the supposition, that some primitive race of articulate animals, bequeathed to these three groups this common typical character — a character which has nevertheless, in many cases, become greatly ob¬ scured, and in some of the most aberrant orders of these classes, quite lost. § 134. Besides these wide- embracing and often deeply- hidden homologies, which hold together different animals^ there are the scarcely-less significant homologies between different organs of the same animal. These homologies, like the others, are obstacles to the supernatural interpreta¬ tions, and supports of the natural interpretation. One of the most familiar and instructive instances is furnished by the vertebral column. Snakes, which move sinuously through and over plants and stones, obviously need a segmentation of the bony axis from end to end ; and inasmuch as flexibility is required throughout the whole length of the body, there is advantage in the comparative uniformity of this segmentation : the creature’s movements would be impeded if, instead of a chain of vertebrae varying but little in their lengths, there existed in the middle of the series some long bony mass that would not bend. But in most of the higher Fertebrata , the mechanical actions and reac¬ tions demand that while some parts of the vertebral axis shall be flexible, other parts shall be inflexible. Inflexibility is especially requisite in that part of the vertebral column called the sacrum ; which, in mammals and birds, forms a fulcrum exposed to the greatest strains which the skeleton has to bear. How in both mammals and birds, this rigid portion of the vertebral column is not made of one long THE ARGUMENTS FROM MORPHOLOGY. 383 segment or vertebra, but of several segments fused to¬ gether. In man there are five of these confluent sacral £> vertebrae ; and in the ostrich tribe they number from seven¬ teen to twenty. Why is this ? Why, if the skeleton of each species was separately contrived, was this bony mass made by soldering together a number of vertebra) like those forming the rest of the column, instead of being made out of one simple piece ? And why, if typical uniformity was to be maintained, does the number of sacral vertebra) vary within the same order of birds P Why, too, should the develop¬ ment of the sacrum be by the round-about process of first forming its separate constituent vertebrae, and then de¬ stroying their separateness ? In the embryo of a mammal or bird, the substance of the vertebral column is, at the out¬ set, continuous. The segments that are to become vertebrae, arise gradually in the midst of this originally-homogeneous axis. Equally in those parts of the spine which are to remain flexible, and in those parts which are to grow rigid, these segments are formed ; and that part of the spine which is to compose the sacrum, having passed out of its ori¬ ginal unity into disunity, by separating itself into segments, passes again into unity by the coalescence of these segments. To what end is this construction and re- construction ? If, originally, the spine in vertebrate animals consisted from head to tail of separate moveable segments, as it does still in fishes and some reptiles— if, in the evolution of the higher Fertebrata, certain of these moveable segments were ren¬ dered less moveable with respect to each other, by the mechanical conditions to which they are exposed, and at length became relatively immoveable ; it is comprehensible why the sacrum formed out of them, should continue ever after to show more or less clearly its originally- segmented structure. But on any other hypothesis, this segmented structure is inexplicable. “We see the same law in comparing the wonderfully complex jaws and legs in crusta¬ ceans,” says Mr Darwin * referring to the well-known fact 384 THE EVOLUTION OF LIFE. that those numerous lateral appendages which, in the lower crustaceans most of them serve as legs, and have like shapes, are, in the higher crustaceans, some of them represented by enormously-developed claws, and others by variously-modi¬ fied foot-jaws. “It is familiar to almost every one,” he continues, “ that in a flower the relative position of the sepals, petals, stamens, and pistils, as well as their intimate structure, are intelligible on the view that they consist of metamorphosed leaves arranged in a spire. In monstrous plants we often get direct evidence of the possibility of one organ being transformed into another ; and we can actually see in embryonic crustaceans and in many other animals, and in flowers, that organs, which when mature become extremely different, are at an early stage of growth exactly alike.” * * * “ Why should one crustacean, which has an. ex¬ tremely complex mouth formed of many parts, consequently always have fewer legs ; or conversely, those with many legs have simpler mouths ? Why should the sepals, petals, stamens, and pistils in any individual flower, though fitted for such widely- different purposes, be all constructed on the same pattern ? ” To these and countless similar questions, the theory of evolution furnishes the only rational answer. In the course of that change from homogeneity to heterogeneity of struc¬ ture, displayed in evolution under every form, it will neces¬ sarily happen that from organisms made up of numerous like parts, there will arise organisms made up of parts more and more unlike : which unlike parts will nevertheless con¬ tinue to bear traces of their primitive likeness. § 135. One more striking morphological fact, near akin to some of the facts dwelt on in the last chapter, must be here set down — the frequent occurrence, in adult animals and plants, of rudimentary and useless organs, which are homologous with organs that are developed and useful in allied animals and plants. In the last chapter we saw that THE ARGUMENTS FROM MORPHOLOGY. d 85 during tlio development of embryos, there often arise organs which disappear on being replaced by other organs dis¬ charging the same functions in different ways ; and that in some cases, organs develop to certain points, and are then re-absorbed without performing any functions. But very generally, the partially-developed organs are retained throughout life. The osteology of the higher Vertebrate, supplies abundant examples. Vertebral processes which, in one tribe, are fully formed and ossified from independent centres, are, in other tribes, mere tubercles not having independent centres of ossification. While in the tail of this animal, the vertebrae are severally composed of centrum and appendages, in the tail of that animal, they are simple osseous masses without any appendages ; and in another animal, they have lost their individualities by coalescence with neighbouring vertebrae into a rudimentary tail. From the structures of the limbs, analogous facts are cited by comparative anatomists. The undeveloped state of certain metacarpal bones, characterizes whole groups of mammals. In one case we find the normal number of digits ; and, in another case, a smaller number with an atrophied digit to make out the complement. Here is a digit with its full number of phalanges ; and there a digit of which one phalange has been arrested in its growth. Still more remarkable are the instances of entire limbs bein