Chemists have shown that a necessary condition to vital activity in animals is oxidation of certain matters contained in the body either as components or as waste products. The oxygen requisite for this oxidation is contained in the surrounding medium – air or water, as the case may be. If the organism be minute, mere contact of its external surface with the oxygenated medium achieves the requisite oxidation; but if the organism is bulky, and so exposes a surface which is small in proportion to its mass, any considerable oxidation cannot be thus achieved. One of two things is therefore implied. Either this bulky organism, receiving no oxygen but that absorbed through its integument, must possess but little vital activity; or else, if it possesses much vital activity, there must be some extensive ramified surface, internal or external, through which adequate aeration may take place – a respiratory apparatus. That is to say, lungs, or gills, or branchiæ, or their equivalents, are predicable a priori as possessed by all active creatures of any size.
Similarly with respect to nutriment. There are entozoa which, living in the insides of other animals, and being constantly bathed by nutritive fluids, absorb a sufficiency through their outer surfaces; and so have no need of stomachs, and do not possess them. But all other animals, inhabiting media that are not in themselves nutritive, but only contain masses of food here and there, must have appliances by which these masses of food may be utilized. Evidently mere external contact of a solid organism with a solid portion of nutriment, could not result in the absorption of it in any moderate time, if at all. To effect absorption, there must be both a solvent or macerating action, and an extended surface fit for containing and imbibing the dissolved products: there must be a digestive cavity. Thus, given the ordinary conditions of animal life, and the possession of stomachs by all creatures living under these conditions may be deductively known.
Carrying out the train of reasoning still further, we may infer the existence of a vascular system or something equivalent to it, in all creatures of any size and activity. In a comparatively small inert animal, such as the hydra, which consists of little more than a sac having a double wall – an outer layer of cells forming the skin, and an inner layer forming the digestive and absorbent surface – there is no need for a special apparatus to diffuse through the body the aliment taken up; for the body is little more than a wrapper to the food it encloses. But where the bulk is considerable, or where the activity is such as to involve much waste and repair, or where both these characteristics exist, there is a necessity for a system of blood-vessels. It is not enough that there be adequately extensive surfaces for absorption and aeration; for in the absence of any means of conveyance, the absorbed elements can be of little or no use to the organism at large. Evidently there must be channels of communication. When, as in the Medusæ, we find these channels of communication consisting simply of branched canals opening out of the stomach and spreading through the disk, we may know, a priori, that such creatures are comparatively inactive; seeing that the nutritive liquid thus partially distributed throughout their bodies is crude and dilute, and that there is no efficient appliance for keeping it in motion. Conversely, when we meet with a creature of considerable size which displays much vivacity, we may know, a priori, that it must have an apparatus for the unceasing supply of concentrated nutriment, and of oxygen, to every organ – a pulsating vascular system.
It is manifest, then, that setting out from certain known fundamental conditions to vital activity, we may deduce from them sundry of the chief characteristics of organized bodies. Doubtless these known fundamental conditions have been inductively established. But what we wish to show is that, given these inductively-established primary facts in physiology, we may with safety draw certain general deductions from them. And, indeed, the legitimacy of such deductions, though not formally acknowledged, is practically recognized in the convictions of every physiologist, as may be readily proved. Thus, were a physiologist to find a creature exhibiting complex and variously co-ordinated movements, and yet having no nervous system; he would be less astonished at the breach of his empirical generalization that all such creatures have nervous systems, than at the disproof of his unconscious deduction that all creatures exhibiting complex and variously co-ordinated movements must have an "internuncial" apparatus by which the co-ordination may be effected. Or were he to find a creature having blood rapidly circulated and rapidly aerated, but yet showing a low temperature, the proof so afforded that active change of matter is not, as he had inferred from chemical data, the cause of animal heat, would stagger him more than would the exception to a constantly-observed relation. Clearly, then, the a priori method already plays a part in physiological reasoning. If not ostensibly employed as a means of reaching new truths, it is at least privately appealed to for confirmation of truths reached a posteriori.
But the illustrations above given go far to show, that it may to a considerable extent be safely used as an independent instrument of research. The necessities for a nutritive system, a respiratory system, and a vascular system, in all animals of size and vivacity, seem to us legitimately inferable from the conditions to continued vital activity. Given the physical and chemical data, and these structural peculiarities may be deduced with as much certainty as may the hollowness of an iron ball from its power of floating in water.
It is not, of course, asserted that the more special physiological truths can be deductively reached. The argument by no means implies this. Legitimate deduction presupposes adequate data; and in respect to the special phenomena of organic growth, structure, and function, adequate data are unattainable, and will probably ever remain so. It is only in the case of the more general physiological truths, such as those above instanced, where we have something like adequate data, that deductive reasoning becomes possible.
And here is reached the stage to which the foregoing considerations are introductory. We propose now to show that there are certain still more general attributes of organized bodies, which are deducible from certain still more general attributes of things.
In an essay on "Progress: its Law and Cause," elsewhere published,8 we have endeavoured to show that the transformation of the homogeneous into the heterogeneous, in which all progress, organic or other, essentially consists, is consequent on the production of many effects by one cause – many changes by one force. Having pointed out that this is a law of all things, we proceeded to show deductively that the multiform evolutions of the homogeneous into the heterogeneous – astronomic, geologic, ethnologic, social, &c., – were explicable as consequences. And though in the case of organic evolution, lack of data disabled us from specifically tracing out the progressive complication as due to the multiplication of effects; yet, we found sundry indirect evidences that it was so. Now in so far as this conclusion, that organic evolution results from the decomposition of each expended force into several forces, was inferred from the general law previously pointed out, it was an example of deductive physiology. The particular was concluded from the universal.
We here propose in the first place to show, that there is another general truth closely connected with the above; and in common with it underlying explanations of all progress, and therefore the progress of organisms – a truth which may indeed be considered as taking precedence of it in respect of time, if not in respect of generality. This truth is, that the condition of homogeneity is a condition of unstable equilibrium.
The phrase unstable equilibrium is one used in mechanics to express a balance of forces of such kind, that the interference of any further force, however minute, will destroy the arrangement previously existing, and bring about a different arrangement. Thus, a stick poised on its lower end is in unstable equilibrium: however exactly it may be placed in a perpendicular position, as soon as it is left to itself it begins, at first imperceptibly and then visibly, to lean on one side, and with increasing rapidity falls into another position. Conversely, a stick suspended from its upper end is in stable equilibrium: however much disturbed, it will return to the same position. Our meaning is, then, that the state of homogeneity, like the state of the stick poised on its lower end, is one that cannot be maintained; and that hence results the first step in its gravitation towards the heterogeneous. Let us take a few illustrations.
Of mechanical ones the most familiar is that of the scales.