Life, in Biology, is a general term for the external and internal activities of an organism in relation to its environment. These relations may be referred to the organism as a unity, or they may be expressed more fundamentally, though incompletely, in terms of the physical and chemical changes in the living matter. Between the habits of an organism and the changes in the protoplasm there are, for higher plants and animals, three intermediate grades of interpretations—in terms of the functions of organs, the properties of tissues, and the phases of cell-life. But, beyond the higher and lower limits of observable organism on the one hand and of analysable protoplasmic changes on the other, the biologist can speak with no special authority, whatever his opinions may be as to the common denominator—matter and energy, or about the transcendental interpretation of an intelligent organism (see BIOLOGY, CELL, PROTOPLASM).
Characteristics of Organisms.—The boundary between living and not-living matter is much less distinct than rough inspection suggests, but it is quite possible to point out certain characteristics which distinguish living organisms from other objects of our experience which are not-living. Some of the most striking of these characteristics may be summed up in the three words—Continuity, Rhythm, and Freedom. (a) So far as our experience goes, all organisms originate from other organisms, and in normal conditions become themselves parents. This fact of continuity is real and literal enough to lend a certain attribute of immortality to life, as may be gathered from the articles
HEREDITY, EMBRYOLOGY, EVOLUTION. (b) Organisms take in matter and energy as they live and grow, while on the other hand they also expend energy and are subject to material waste; they feed and work, rest and act, grow and reproduce, in fact pass through a rhythmic cycle of changes such that waste and repair are for variable periods kept in approximate equilibrium. From what we know of the living-matter or 'physical basis of life,' it seems that two vital processes of upbuilding and down-breaking, of composition and decomposition, of synthesis and analysis, of anabolism and katabolism, sum up the changes in the protoplasm (see ANABOLISM, FUNCTION, PROTOPLASM). (c) As to freedom, while organisms are much more dependent upon their environment than are inorganic bodies, it is equally true that they attain more apparent freedom. The sustained equilibrium of an organism is unified and dynamical; it admits of direct action upon surroundings, of active thrust as well as more passive parry, of activity which is sometimes called 'automatic' or 'spontaneous,' because it does not occur in direct or traceable response to stimulus from without (see ENVIRONMENT).
Definitions.—Life, like other fundamental facts, eludes definition. Bichat described it as 'the sum of the functions which resist death,' a definition superficially contradictory to Claude Bernard's epigram, 'La vie, c'est la mort.' According to De Blainville, 'life is the twofold internal movement of composition and decomposition, at once general and continuous,' while Spencer's often-quoted definition describes it as 'the definite combination of heterogeneous changes, both simultaneous and successive, in correspondence with external co-existences and sequences.' Lewes defines life as 'a series of definite and successive changes, both of structure and composition, which take place within an individual without destroying its identity,' while Joseph Cook as a transcendentalist calls life 'the invisible, individual, co-ordinating cause directing the forces involved in the production and activity of any organism possessing individuality.' Finally, Lafitte, as an expositor of Comte, regards life as 'a general, internal, and continuous phenomenon of composition and decomposition, occurring in a definite organism, placed in a fit medium.' For practical purposes, life is the internal and external activity of an organism in relation to its environment.
The conditions of life vary enormously, for organisms are able to adapt themselves to most diverse environment, including under that term conditions of space and pressure, moisture and oxygen, food, heat, and light, &c. The animal life of the deep-sea illustrates interesting adaptations to great though doubtless unfelt pressure, to darkness, and other peculiar conditions; the minimum life of dried-up spores, Protozoa, ova, small animals, and seeds shows the possibility of persistence for prolonged periods without water; the fauna and flora of arctic snows and seas on the one hand, and of hot springs on the other, illustrate extreme adaptations to diverse temperature conditions; and there are abundant illustrations, from fasting men upwards, of the length of time during which life may continue without food. A few facts may be cited: small nematodes will survive desiccation for fourteen years, and, though the tales of germinating mummy wheat are highly unsatisfactory, it is certain that cereal grains may germinate after ten years' desiccation, and seeds of Leguminosæ after several decennia. Seeds rich in ferments and oils have much less power of surviving than those in which starch predominates. As to temperature, dry yeast will live after exposure to 70° C., and a portion survives even at 100° C.; Pasteur heated dry fungoid spores without fatal results to 120° C., but the same when moist were killed at the boiling-point. Some bacteria are said to resist boiling, but the reverse is usually true. Kühne killed marine Amœbæ at 35° C., while fresh-water forms stood 10° more. Even seeds have been known to withstand 100° C., but it is familiar that a longer exposure to much lower temperature is usually fatal. Higher plants have been known to survive burial under a glacier for four years; and fishes, frogs, &c. have often revived after being frozen hard in ice. Dry yeast, according to Cagniard de la Tour (quoted by Huxley), survived - 60° C., but when moist was killed at - 5° C.; yet Cohn cooled bacteria to - 18° C. without death, and seeds have survived such an extreme as - 120° C. To illustrate the diverse sensitiveness of animals, Semper notes that a temperature about the freezing-point of fresh water kills Infusoria but not pond-snails, that the minimum of vital activity in the former was seen at 4° C., in the latter at 12° C., yet the optimum for both is the same—viz. about 25° C. No better instance of experimental work can be referred to than Dr Dallinger's researches, in which he was able slowly to educate Monads which normally lived at a temperature of 18° C. to thrive at over 70° C. Of the internal conditions of chill-coma, and of the vita minima under extremes of heat, desiccation, &c., we know almost nothing.
See DESICCATION, ENVIRONMENT, HIBERNATION; also Huxley's Anat. of Invert. Animals (Lond. 1877), Semper's Animal Life (Inter. Sc. Series, Lond. 1881), and Wiesner's Biologie der Pflanzen (Vienna, 1889).
Origin of Life.—It is not a dogma, not yet a 'law of Biogenesis,' but a fact of experience that all living organisms arise from other living organisms—omne vivum e vivo. See ABIOGENESIS, BATHYBIUS, HEREDITY, SPONTANEOUS GENERATION.
But those who advance beyond an agnostic position as to this problem, and speculate beyond the limits of our experience, give the following four answers to the question of the historical origin of living organisms: (1) Life originated under conditions beyond the sphere of scientific inquiry. Thus, Alfred Russel Wallace postulates a 'spiritual influx' at the origin of life, while theologians are usually more explicit (see CREATION). (2) Organisms or germs of organisms were brought to the earth by meteorites from elsewhere. This hypothesis, supported by Sir William Thomson, shifts the responsibility of the problem off the shoulders of our planet, and leaves the problem of origin—elsewhere. (3) 'The question as to the origin of life,' Professor W. Preyer says, 'is not less transcendental than that as to the origin of matter and energy. In regard to the latter, it is axiomatic that they had no origin, but are eternal, otherwise matter and energy have arisen out of nothing.' So in regard to life, he argues that it had no thinkable beginning, and that it is as legitimate to suppose that the inorganic originated from organisms as to suppose the converse. In regard to this suggestion it may be noted that while it is quite true that much of the inorganic on the earth has arisen from the work and waste, remains and decomposition of organisms, the forms of life supposed to have persisted in the ancient 'tracts of fluent heat' must have been extraordinarily different from any which we now observe. (4) Living matter evolved of itself from matter which was not living, as the outcome of unexplained processes of chemical upbuilding or synthesis. Professor Ray Lankester suggests further that the first protoplasm fed upon 'the antecedent steps in its own evolution,' 'upon the albuminoids and such other compounds that had been brought into existence by those processes, which culminated in the development of the first protoplasm.' This hypothesis is most in harmony with the general theory of evolution, of which however it forms no integral part. It has against it the constant fact of experience and result of experiment that all life springs from life, besides serious difficulties in connection with that chemical upbuilding or synthesis, which it is so easy to postulate and so difficult to understand. See also LONGEVITY, IMMORTALITY, TRANSMIGRATION, PRE-EXISTENCE; and for Life Assurance, see INSURANCE.