Imagine a vessel full of water, at the ordinary temperature, in an atmosphere saturated with vapour. The _quantity_ and the _figure_ of that water will not change, so far as we know, for ever. Suppose a lump of gold be thrown into the vessel--motion and disturbance of figure exactly proportional to the momentum of the gold will take place. But after a time the effects of this disturbance will subside--equilibrium will be restored, and the water will return to its passive state. Expose the water to cold--it will solidify--and in so doing its particles will arrange themselves in definite crystalline shapes. But once formed, these crystals change no further. Again, substitute for the lump of gold some substance capable of entering into chemical relations with the water:--say, a mass of that substance which is called "protein"--the substance of flesh:--a very considerable disturbance of equilibrium will take place--all sorts of chemical compositions and decompositions will occur; but in the end, as before, the result will be the resumption of a condition of rest. Instead of such a mass of _dead_ protein, however, take a particle of _living_ protein--one of those minute microscopic living things which throng our pools, and are known as Infusoria--such a creature, for instance, as an Euglena, and place it in our vessel of water. It is a round mass provided with a long filament, and except in this peculiarity of shape, presents no appreciable physical or chemical difference whereby it might be distinguished from the particle of dead protein. But the difference in the phaenomena to which it will give rise is immense: in the first place it will develop a vast quantity of physical force--cleaving the water in all directions with considerable rapidity by means of the vibrations of the long filament or cilium. Nor is the amount of chemical energy which the little creature possesses less striking. It is a perfect laboratory in itself, and it will act and react upon the water and the matters contained therein; converting them into new compounds resembling its own substance, and, at the same time, giving up portions of its own substance which have become effete. Furthermore, the Euglena will increase in size; but this increase is by no means unlimited, as the increase of a crystal might be. After it has grown to a certain extent it divides, and each portion assumes the form of the original, and proceeds to repeat the process of growth and division. Nor is this all. For after a series of such divisions and subdivisions, these minute points assume a totally new form, lose their long tails--round themselves, and secrete a sort of envelope or box, in which they remain shut up for a time, eventually to resume, directly or indirectly, their primitive mode of existence. Now, so far as we know, there is no natural limit to the existence of the Euglena, or of any other living germ. A living species once launched into existence tends to live for ever.