§ 55. Does Structure originate Function, or does Function 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 organic 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 in truth this question is not determinable by any evidence now accessible to us. The very simplest forms of life known (even the non-nucleated, if there are any) consist of granulated protoplasm; and granulation implies structure. Moreover since each kind of protozoon, even the lowest, has its specific mode of development and specific activity—even down to bacteria, some kinds of which, otherwise indistinguishable, are distinguishable by their different reactions on their media—we are obliged to conclude that there must be constitutional differences between the protoplasms they consist of, and this implies structural differences. It seems that structure and function must have advanced pari passu: some difference of function, primarily determined by some difference of relation to the environment, initiating a slight difference of structure, and this again leading to a more pronounced difference of function; and so on through continuous actions and reactions.
§ 56. Function falls into divisions of several kinds according 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 passively 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 considered as dynamical in their actions. From another point of view Function is divisible into the accumulation of energy (latent in food); the expenditure of energy (latent in the tissues and certain matters absorbed by them); and the transfer of energy (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 comparatively slight, and transfer required mainly to facilitate accumulation. In animals the function of accumulation comprehends those processes by which the materials containing latent energy 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 energies they contain, are conveyed throughout the organism; and the function of expenditure comprehends those processes by which the energy is liberated from these materials and transformed into properly co-ordinated motions. Each of these three most general divisions includes several more special divisions. The accumulation of energy 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 energy 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 energy come nervous actions and muscular actions: though not absolutely co-extensive with expenditure 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 excretion 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 epidermic 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 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 generalities.
§ 57. The first induction to be here set down is a familiar and obvious one; the induction, namely, that complexity of function is the correlative of complexity of structure. The leading aspects of this truth must be briefly noted.
Where there are no distinctions of structure there are no distinctions of function. A Rhizopod will serve as an illustration. From the outside of this creature, which has not even a limiting membrane, there are protruded numerous processes. Originating from any point of the surface, each of these may contract again and disappear, 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 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 many Algæ, everywhere devoted to the same process of assimilation, there arise, as in the higher plants, root and stem and leaves, there arise correspondingly unlike processes. Still more conspicuously among animals do there result varieties of function when the originally homogeneous mass is replaced by heterogeneous organs; since, both singly and by their combinations, modified parts generate modified changes. Up to the highest organic types this dependence continues manifest; 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 apparatus devoted to them. Thus among birds, which have more varied locomotive powers than mammals, the limbs are more widely differentiated; while the higher 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 rudiment, 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 successively 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 functional development is better displayed than in plants.
The first differentiation established separates the two fundamentally-opposed functions above named—the accumulation of energy and the expenditure of energy. 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 Cœlenterata, where definite tissues make their appearance, we observe that the only large functional distinction is between the endoderm, which absorbs nutriment, and the ectoderm which, by its own contractions and those of the tentacles it bears, produces motion: the contractility being however to some extent shared by the endoderm. That the functions of accumulation and expenditure 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 structure 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 different portions, œsophagus, stomach and intestine, perform different portions of this function. And instead of a contractility uniformly diffused through the ectoderm, there have arisen in the intermediate mesoderm some parts which have the office of contracting (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, 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 heart generate alternating currents in the nutriment occupying the peri-visceral cavity. How the function of transferring energy, thus vaguely indicated in these inferior 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-off from the others, at the same time that it becomes itself parted into subordinate functions.