LIFE. No definition of life has ever proved quite satisfactory. Some include too much, others omit certain phenomena, a third class assumes conditions purely hypothetical, while many are unintelligible. Bichat says that life is "the sum total of the forces that resist death"; Treviranus, that it is "the constant uniformity of phenomena with diversity of ex ternal influences"; Duges, that it is "the special activity of organized bodies," and Beclard, that it is "organization in action." De Blainville's definition is: "Life is the twofold internal movement of composition and decomposition, at once general and continuous?' But according to Herbert Spencer this conception is in some re spects too narrow and in others too wide. Of his own definition, that it is "the co-ordination of actions,)) he says ---((like the others this definition includes too much, for it may he said of the solar system with its regularly recurring movements and its self-balancing perturbations, that it also exhibits co-ordination of actions." His amended conception of life is: "the definite combination of heterogeneous changes, both simultaneous and successive, in correspondence with external coexistences, and sequences." G. H. Lewes suggests the definition: "Life is a series of definite and successive changes, both of structure and composition, which take place within an individual without destroying its identity." However, this amounts to defining ignolunt per ignotius. The most recent at tempts have been in the direction of proving that life is merely a form of energy or motion, and show the influence which the physical sci ences have had in recent years over knowledge and speculation, which formerly were mainly based on biology and theology. The simplest answer to the question probably is: Life is metabolism.
Leaving the subject of attempted definition, it will be profitable to observe some of the char acteristics of life as compared with its absence; that is, substantially, a comparison of the or ganic with the inorganic part of the universe.
The boundary between living and not-living mat ter is much less distinct than rough inspection suggests, but some of the most striking charac teristics which distinguish living organisms from other objects of our experience which are not living may be pointed out. The distinctive prop erties of living matter are, first, the fact that it is organized; second, that it has the power of perpetuating itself within definite limits by chemically taking and adapting (assimilating) suitable material from the surrounding media, and in the process generating heat (energy) in the absence of which it disappears; and, third, that it has the power of self-reproduction. Life cannot exist without that chemical interchange between the organism and its inorganic environ ment, and between the constituent parts of the organism which is summed up in -the word metabolism (q.v.). Objections have been made to some of the definitions quoted above, and to others, that they assumed the existence of or ganization. But so far no evidence is present of any living thing without organization. The simplest one-celled animals and plants (see AMMA ; PROTISTA) consist of organized proto plasm. This fundamental living substance, called protoplasm (q.v.), is of complicated structure and chemical composition. Its struc ture differs in different organisms; but it every where consists of a combination of viscous "plasma" and water. The plasma contains the chemical substances upon whose changes life depends. Protoplasm is, however, not homo geneous. At least two kinds are found in every mass, the cytoplasm, constituting the major part of the whole; and usually the nucleus, the nutritive and reproductive centre. The combination of cytoplasm and nucleus forms a cell. Combinations of cells constitute the bodies of all organisms, large or small, past or present. These combinations are accumula tions resulting from the property, characteristic, regularly and fundamentally, only of living organisms, of the doubling of molecules, by which growth is effected.