Life has been variously defined by different writers. Bichat defines it as "the sum total of the functions which resist death;" Treviranus, as "the constant uniformity of phenomena with diversity of external influences;" Duges, as "the special activity of organised bodies;" and Beclard, as "organisation in action." All these definitions, however, are more or less objectionable, since they either really mean nothing, or the assumption underlies them that life is inseparably connected with organisation. In point of fact, no rigid definition of life appears to be at present possible, and it is best to regard it as being simply a tendency exhibited by certain forms of matter, under certain conditions, to pass through a series of changes in a more or less definite and determinate sequence. The essential phenomenon of vitality is, therefore, in the words of Herbert Spencer, "the continuous adjustment of internal relations to external relations," and life, in its effect, is the totality of the functions of a living being. Life, however, may also be considered as a cause, since amongst the phenomena presented by all living beings there are some which cannot be referred to the action of known physical or chemical laws, and which, therefore, temporarily at any rate, we must term "vital."
Whilst the nature of life thus does not admit of rigid definition, we find that the phenomena of vitality can only be manifested under certain conditions, some of these being intrinsic and indispensable, whilst others are extrinsic, and not in themselves, or collectively, essential.
The only intrinsic condition of life appears to be the existence of a special "physical basis," as it has been termed. We do not find, namely, that the phenomena of vitality can be manifested by any and every form of matter. On the contrary, and as might have been expected upon a priori grounds, all living bodies appear to be composed of a special substance, which is the material basis of life, and which seems to be substantially identical in all alike. No living body is throughout composed of this living basis, but all contain a greater or smaller amount of other materials, which are in one sense dead. The real phenomena of vitality are conditioned, therefore, by certain special portions of the organism, which are alone formed of this living matter; and this matter in chemical composition and physical characters appears to be identical in all living beings whether animal or vegetable. To this physical basis the names of "protoplasm" or "bioplasm" are applied. The lowest organisms consist of little else but simple unmodified protoplasm; but even in the most complex organisms it can be shown that their essential parts, in which alone vitality is inherent, are similarly composed of protoplasmic matter.
As regards its nature, protoplasm, though capable of forming the most complex structures, does not necessarily exhibit anything which can be looked upon as organisation, or differentiation into distinct parts; and its chemical composition is the only constant which can be approximately stated. It consists, namely, in all its forms, of the four elements, carbon, hydrogen, oxygen, and nitrogen, united into a proximate compound to which Mulder applied the name of "proteine," and which is very nearly identical with albumen or white-of-egg. It further appears probable that all forms of protoplasm can be made to contract by means of electricity, and "are liable to undergo that peculiar coagulation at a temperature of 40°-50° centigrade, which has been called 'heat-stiffening'" (Huxley). As viewed under the microscope, protoplasm presents itself as a clear viscous, semi-fluid substance, which is commonly rendered granular by the presence of disseminated particles of fatty matter, and which is deeply stained by immersion in a solution of carmine.
In addition to the physical and chemical properties of protoplasm, many writers are in the habit of speaking of the "vital properties " of this substance. These so-called "vital properties " are necessarily the same as those manifested by living beings in general, and consist in the power exhibited by living protoplasm of assimilating foreign matter, of reproducing itself by the detachment of portions of its substance, and of having certain relations with the world outside itself. As regards the last of these points, protoplasm, in its living state, when uncon-fined by any rigid wall or outer envelope, possesses the power of throwing out longer or shorter prolongations or processes of its own substance ("pseudopodia"), by means of which it can obtain food, or, if free, move about. Even in vegetable cells, where a rigid cell-wall is as a rule present, the protoplasm in the interior is often capable of rotation as a whole, or of exhibiting an active circulation of granules similar to that observed in many masses of animal protoplasm. Moreover, the researches of Mr Francis Darwin have shown that the cells of the glandular hairs of the Common Teasel (Dipsacus sylvestris) emit mobile filaments of protoplasm quite similar to the "pseu-dopodia" of many of the lower animals; while wall-less masses of protoplasm, capable of emitting pseudopodia, are met with in the life-history of some of the lower plants.
In speaking, however, of the power of nutrition and reproduction, or of the power of emitting pseudopodia, or of exhibiting irritability as being "vital properties" of protoplasm, a fallacious mode of reasoning is employed. These powers belong to living protoplasm, and it remains to be shown that they are even potentially present in dead protoplasm - as protoplasm. At any rate, they stand in a different category to the physical and chemical "properties" of protoplasm, since we must suppose these to be invariably and constantly present in protoplasm, whether alive or dead; unless we are to deny that protoplasm is a definite compound at all.
Apart from this, however, we may admit that protoplasmic matter * is "the formal basis of all life" (Huxley); and that the phenomena of vitality cannot be manifested save through the vehicle of protoplasm. Nevertheless, there remain certain conditions equally indispensable to the external manifestation of vital phenomena; though life itself, or the power of exhibiting vital phenomena, may be preserved for a longer or shorter period, even though these conditions be absent. These extrinsic conditions of vitality are, firstly, a certain temperature varying from near the freezing-point to 120o or 130o; secondly, the presence of water, which enters largely into the composition of all living tissues; thirdly, the presence of oxygen in a free state, though some of the lower forms of vegetable life are capable of existing in an atmosphere devoid of oxygen.