31. These remarks may justify a selection of definitions, classified under the two heads. The selection is fitly opened by the Aristotelian definition which prevailed for centuries.
Aristotle distinguishes Life, which he says means “the faculties of self-nourishment, self-development, and self-decay,” from the Vital Principle. Every natural body manifesting life may be regarded as an essential existence (οὐσία); but then it is an existence only as a synthesis (ὡς συθέτη); and since an organism is such a synthesis, being possessed of Life, it cannot be the Vital Principle (ψυχή). Therefore it follows that the Vital Principle must be an essence, as being the Form of a natural body holding life in potentiality. The Vital Principle is the primary reality of an organism. “It is therefore as idle to ask whether the Vital Principle and Organism are one, as whether the wax and the impress on it are one.... Thus if an eye were an animal, Vision would be its Vital Principle: for Vision is, abstractedly considered, the essence of the eye; but the eye is the body of Vision, and if Vision be wanting, then, save in name, it is no longer an eye.”
Apart from certain metaphysical implications, inevitable at that period, there is profound insight in this passage. His adversary Telesio quite misconceives the meaning here assigned to the Vital Principle.[8]
32. Let us pass over all the intermediate forms of the hypothesis, and descend to Kant, who defines Life “an internal principle of action” (this does not distinguish it from fermentation); an organism he says is “that in which every part is at once means and end.” “Each part of the living body has its cause of existence in the whole organism; whereas in non-living bodies each part has its cause in itself.” Johannes Müller adopts a similar view: “The harmonious action of the essential parts of the individual subsist only by the influence of a force, the operation of which is extended to all parts of the body, and does not depend on any single parts; this force must exist before the parts, which are in fact formed by it during the development of the embryo.... The vital force inherent in them generates from the organic matter the essential organs which constitute the whole being. This rational creative force is exerted in every animal strictly in accordance with what the nature of each requires.”
33. This is decidedly inferior to Aristotle, who did not confound the vegetative with the rational principle. It rests on the old metaphysical error of a vis medicatrix, an error which cannot sustain itself against the striking facts which constantly point to a vis destructrix, a destructive tendency quite as inexorable as the curative tendency. And the experimental biologist soon becomes impressed with the fact that the tissues have indeed a selective action, by which from out the nutrient material only these substances are assimilated which will enter into combination with them; but this selective action is fatal, no less than reparative: substances which poison the tissue are taken up as readily as those which nourish it. The idea of prescience, therefore, cannot be sustained; it is indeed seldom met with now in the writings of any but the Montpellier school, who continue the traditions of Stahl’s teaching. It has been so long exploded elsewhere that one is surprised to find an English physiologist clinging to a modification of it—I mean Dr. Lionel Beale, who repeatedly insists on Life as “a peculiar Force, temporarily associated with matter,” a “power capable of controlling and directing both matter and force,” an “undiscovered form of force having no connection with primary energy or motion.” “The higher phenomena of the nervous system are probably due primarily to the movements of the germinal matter due to vital power, which vital power of this the highest form of germinal matter is in fact the living I.”
34. Apart from the primary objection to all these definitions, namely, that they seek to express organic phenomena in terms of an extra-organic principle, to formulate the facts observed in terms of a cause inferred, there is the fatal objection that they speak confidently on what is avowedly unknown. If the force be, as Dr. Beale says, “undiscovered,” on what grounds can he assert that it has no connection with the forces which are known? All that the observed facts warrant is the assertion that organic phenomena are special (which no one denies), and must therefore depend on special combinations of matter and force. But on this ground we might assume a crystallizing Force, and a coagulating Force, having no connection with the molecular forces manifested elsewhere: these also are special phenomena, not to be confounded with each other.
35. Schelling defines Life as “a principle of individuation” and a “cycle of successive changes determined and fixed by this internal principle.” Which is so vague that it may be applied in very different senses. Bichat’s celebrated definition (which is only a paraphrase of a sentence in Stahl), “the sum of the functions which resist Death,” although an endeavor to express the facts from the Intra-organic point of view, is not only vague, but misrepresents one of the cardinal conditions, by treating the External Medium as antagonistic to Life, whereas Life is only possible in the relation to a Medium.
36. Were it not so vague, the definition proposed by Dugès and Béclard would be unexceptionable: the former says it is “the special activity of organized beings”; the latter, “the sum of the phenomena proper to organized bodies.” When supplemented by a description of organized bodies, these formulæ are compendious and exact. The same remark applies to the definition of Lamarck: “that state of things which permits organic movements; and these movements, which constitute active life, result from a stimulus which excites them.”
37. De Blainville, and after him Comte and Charles Robin, define it thus: “Life is the twofold internal movement of composition and decomposition at once general and continuous.” This, excellent as regards what is called vegetal life, is very properly objected to by Mr. Herbert Spencer in that it excludes those nervous and muscular functions which are the most conspicuous and distinctive of vital phenomena. The same objection must be urged against Professor Owen’s definition: “Life is a centre of intussusceptive assimilative force capable of reproduction by spontaneous fission.”
38. In 1853, after reviewing the various attempts to express in a sentence what a volume could only approximately expound, I proposed the following: “Life is a series of definite and successive changes, both of structure and composition, which take place within an individual without destroying its identity.” This has been criticised by Mr. Herbert Spencer and by Dr. Lionel Beale, and if I had not withdrawn it before their criticisms appeared, I should certainly have modified and enlarged it afterwards. I mention it, however, because it is an approach to a more satisfactory formula in so far as it specifies two cardinal characteristics distinguishing organisms from all anorganisms, namely, the incessant evolution through definite stages, and the preservation of specific integrity throughout the changes; not only the organism as a whole is preserved amidst incessant molecular change, but each tissue lives only so long as the reciprocal molecular composition and decomposition persist. On both of these points I shall have to speak hereafter. The definition, however, is not only defective in its restriction to the molecular changes of Nutrition, taking no account of the Properties and Functions of the organism; but defective also in giving no expression to equally important relations of the organism to the medium.