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Updated: May 18, 2025
While mammals and birds maintain a heat which continues almost unaffected by external variations, and is often greater than that of the air by seventy, eighty, ninety, and even a hundred degrees. Once more, in greater self-mobility a progressive differentiation is traceable. Dead matter is inert: some form of independent motion is our most general test of life.
Thus, on contemplating the various grades of organisms in their ascending order, we find them more and more distinguished from their inanimate media in structure, in form, in chemical composition, in specific gravity, in temperature, in self-mobility. It is true that this generalization does not hold with regularity.
But it is only when we come to air-breathing creatures that we find the vital characteristic of self-mobility manifested in the highest degree. Flying insects, mammals, birds, travel with velocities far exceeding those attained by any of the lower classes of animals; and so are more strongly contrasted with their inert environments.
Of the Coelenterata, a great part are either permanently rooted or habitually stationary, and so have scarcely any self-mobility but that implied in the relative movements of parts; while the rest, of which the common jelly-fish serves as a sample, have mostly but little ability to move themselves through the water.
The contained protoplasm of a vegetal cell, which has self-mobility and when liberated sometimes performs amoeba-like motions for a time, may be regarded as an imprisoned amoeba; and when we pass from it to a free amoeba, which is one of the simplest types of first animals, or Protozoa, we naturally meet with kindred phenomena.
A word suggests itself which, apart from expressing adequately the peculiar self-mobility which we have just brought to our experience, goes well alongside the word 'inert' by forming a kind of rhyme with it. This is the term 'alert'. With its help we shall henceforth distinguish between matter in the inert and alert conditions.
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