Consider how widely different this living particle is from the dead atoms with which the physicist and chemist have to do! The particle of gold falls to the bottom and rests--the particle of dead protein decomposes and disappears--it also rests: but the _living_ protein mass neither tends to exhaustion of its forces nor to any permanency of form, but is essentially distinguished as a disturber of equilibrium so far as force is concerned,--as undergoing continual metamorphosis and change, in point of form. Tendency to equilibrium of force and to permanency of form then, are the characters of that portion of the universe which does not live--the domain of the chemist and physicist. Tendency to disturb existing equilibrium,--to take on forms which succeed one another in definite cycles, is the character of the living world. What is the cause of this wonderful difference between the dead particle and the living particle of matter appearing in other respects identical? that difference to which we give the name of Life? I, for one, cannot tell you. It may be that, by and by, philosophers will discover some higher laws of which the facts of life are particular cases--very possibly they will find out some bond between physico-chemical phaenomena on the one hand, and vital phaenomena on the other. At present, however, we assuredly know of none; and I think we shall exercise a wise humility in confessing that, for us at least, this successive assumption of different states--(external conditions remaining the same)--this _spontaneity of action_--if I may use a term which implies more than I would be answerable for--which constitutes so vast and plain a practical distinction between living bodies and those which do not live, is an ultimate fact; indicating as such, the existence of a broad line of demarcation between the subject-matter of Biological and that of all other sciences. For I would have it understood that this simple Euglena is the type of _all_ living things, so far as the distinction between these and inert matter is concerned. That cycle of changes, which is constituted by perhaps not more than two or three steps in the Euglena, is as clearly manifested in the multitudinous stages through which the germ of an oak or of a man passes. Whatever forms the Living Being may take on, whether simple or complex, _production_, _growth_, _reproduction_, are the phaenomena which distinguish it from that which does not live.