Page:Popular Science Monthly Volume 21.djvu/376

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contraction; hence protoplasm may be said to be "contractile" and this is another of its so-called vital properties.

Protoplasm also feeds upon nutritive material brought into contact with its surface. This it does by flowing around the substance, whatever it may be, which serves for its food, thus inclosing it in a temporary stomach improvised anew for each occasion, and becoming gradually obliterated as the new material slowly dissolves and is absorbed, mingling and chemically combining with the already existing protoplasm, and thenceforth forming a part of its substance. In other words, certain kinds of dead matter called food are assimilated, converted into living protoplasm by those processes of absorption and chemical union which constitute nutrition in all living things. Hence, protoplasm is "assimilative" and this is another of its vital properties. Side by side with this process of taking in new material and converting it into its own substance, there is also another process going on—that of rejection of old, broken-down, effete matter which has not only become useless to the living protoplasm, but would be injurious if retained.

The life of protoplasm is thus seen to consist in a double series of chemical changes, by one of which its substance is constantly renewed and built up; by the other, it as constantly breaks down, the products of decomposition being gradually rejected from the living, ever-fluctuating mass, which thus becomes the theatre, the arena, of life.

What is the outcome of this constant play of chemical and physical forces—this incessant interchange of matter between the mass of protoplasm and its environment? In other words, what is the meaning of the life thus manifested? Its significance is this: The production and manifestation of new and higher kinds of force than any belonging to inanimate, inorganic matter.

In the life of protoplasm we behold the dawning of voluntary motion—of those spontaneous movements especially characteristic of animals (though shown to a slight extent by plants as well), and exhibited in the highest degree by man in the thousand muscular adaptations displayed in his complex mechanism.

But the doctrine of the correlation of forces formulates the fact that the amount of force in the universe of matter is constant and unvarying; that, as matter is indestructible, so the forces which it manifests are persistent—never increasing, never diminishing. Whence, then, comes this new and higher kind of force called spontaneous motion? It is a law of physics that, as elemental molecules aggregate to form those which are more complex and massive, the force previously manifested by the simpler molecules becomes potential or latent, as it was formerly expressed; and that in the breaking down of these more complex molecules, in their return to their former simple state, this hidden force springs into activity again, not necessarily reappearing, however, as the same kind of force; there is not only a storing