Whilst all living bodies, whether animal or vegetable, are composed essentially of organic substances, there are nevertheless associated with the living organism larger or smaller amounts of matter which is practically dead. On the other hand there are numerous secondary organic products which at no time enter into the composition of living bodies, and which are therefore just as much "dead" substances as the genuine inorganic substances.
The general distinctions between dead and living matter are the following:
A Mode Of Increase. Living bodies possess the power of taking into their interior certain materials (food), foreign to. those composing their own substance, and of converting these into the materials of which they are themselves built up. This process is known as "assimilation," and it is in virtue of this that living bodies grow. The growth of the organism, therefore, and its increase in size, is not effected by the mere addition of matter from the outside, but by the taking of matter into the interior of the body, and its modification there.
On the other hand, when dead bodies increase in size (as crystals do in supersaturated solutions), this is effected simply by the addition of particles from the outside, or, as it is technically called, by the "accretion," instead of by the "intussusception," of matter. The newly 'added particles undergo no change from their previous constitution, and the essential element of "assimilation" is thus wanting, so that the process is in no sense one of "growth" properly so called.
B Cyclical Change. All dead matter tends to assume a condition of permanent stability and repose. Living matter, on the other hand, is pre-eminently distinguished by its tendency to pass through a series of cyclical changes, all the actions of living bodies being accompanied by a corresponding destruction of the matter by which these actions are effected. All these cyclical changes are effected by the slow but incessant reduction of the living matter of the organism to the non-living condition. Active life, therefore, can only be carried on by the constant destruction of portions of the living matter of the body; and to meet the loss thus caused, it is necessary that a corresponding amount of non-living matter should be constantly "assimilated," and raised from the statical condition of dead matter to the dynamical condition of living matter.
C Relations To The Outside World. Dead bodies are subject to the physical and chemical forces of the universe, and have no power of suspending these forces, or modifying their action, even for a limited period. On the other hand, living bodies, whilst subject to the same forces, are the seat of something in virtue of which they can override, suspend, or modify the actions of the physical and chemical forces by which dead bodies are exclusively governed. Dead matter is completely passive, unable to originate motion, and equally unable to arrest it when once originated. Living matter, so long as it is living, is the seat of energy, and can overcome the primary law of the inertia of matter. However humble it may be, and even if permanently rooted to one place, every living body possesses, in some part or other, or at some period of its existence, the power of independent and spontaneous movement - a power possessed by nothing that is dead. Similarly, the chemical forces, which work unresisted amongst the particles of dead matter, are in the living organism directed harmoniously to given ends, their action regulated under definite laws, and their natural working often strikingly modified, or even temporarily suspended, and this as effectually and as perfectly in the humblest as in the highest of created beings.
As a result of this, dead bodies exhibit nothing but reactions, and these purely of a physical and chemical nature, whilst they show no tendency to pass through periodical changes of state. On the other hand, living bodies exhibit distinct actions, and are pre-eminently characterised by their tendency to pass through a series of cyclical changes, which follow one another in a regular and determinate sequence.
D Reproduction. Every living body has the power of reproducing its like. Directly or indirectly, every living body has the power of giving off minute portions of its own substance, which, under proper conditions, will be developed into the likeness of the parent.