If now, from this same point of view, 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. How of form the like holds, is equally manifest. The sphere, which is • This paragraph originally formed part of a review-article on " Transcenden- tal Physiology," published in 1857.
DEVELOPMENT. 149 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 objects 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 aug- 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 partial 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- i-ant 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 each 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. Further, 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. 151 development is a change from incoherent, indefinite homo- geneit}", to coherent, definite heterogeneity, becomes a self- evident one, when observation has shown us the state in which organisms begin, and the state in wliich they end.
Just in the same way 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 within 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 w^hole, 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. Evidentlv this selective assimilation illustrates the general truth, demonstrable a 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 showm 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 into 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. No 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 THE 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. How 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 uncora- 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 plays so important a part in organic development, will not be difiicult 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? is a question about which, there has 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?
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 difiPers 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 Rhizopods, which pre- 154 THE IJJDUCTIONS 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 which 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 exhalatioUy 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, elastic 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 THE 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 Rhizopods 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 serving as hand and mouth; or it may 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 Algce, 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 hetero- FUNCTION. 157 geneous organs; since both singly and by their 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 having 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 Ccelenterata, where definite tissues make their first appearance, we observe that the only marked functional distinction is between the endo- 158 THE INDUCTIONS OF BIOLOGY.
derm, whicli absorbs nutriment, and the ectoderm, which, by- its own contractions and those of the tentacles it bears, pro- duces motion. That the functions of accumulation and ex- 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, sesophagus, 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. 159 heart generate alternating currents in the crude and dihite nutriment occupying the peri- visceral cavity. How 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 general function becomes more clearly marked-ofi" from the others, at the same time that it becomes itself parted into subordinate functions.
In a developing embryo, the functions, or more strictly the structures which are to perform them, arise in the same general order. A like primary 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- cal 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, has 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 difierences 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. Fully to understand this change from homogeneity to heterogeneity of function, which accompanies the change FXTNCTION. 101 from homo^^encity to lieterogeneity 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- scribe'd; but by a simultaneous process they are rendered more mutually dependent. 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, 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 11 162 THE INDUCTIONS OF BIOLOGY.
the 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 ofiice 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 nervomuscular 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 well-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. 163