Page:Popular Science Monthly Volume 11.djvu/440

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from the undigested stuff mechanically lodged for a while in the body) are of three classes: those preparatory to and culminating in the conversion of the food into protoplasm; those concerned in the discharge of energy; and those tending to economize the immediate products of the second class of changes by rendering them more or less useful for the first.

5. It is respiratory.—Taken as a whole, the metabolic changes are preëminently processes of oxidation. One article of food—i. e., one substance taken into the body, viz., oxygen—stands apart from all the rest; and one product of metabolism peculiarly associated with oxidation—viz., carbonic acid—stands also somewhat apart from all the rest. Hence, the assumption of oxygen and the excretion of carbonic acid, together with such of the metabolic processes as are more especially oxidative, are frequently spoken of together as constituting the respiratory processes.

6. It is reproductive.—The individual amœba represents a unit. This unit, after a longer or shorter life, having increased in size by the addition of new protoplasm in excess of that which it is continually using up, may by fission (or by other means) resolve itself into two (or more) parts, each of which is capable of living as a fresh unit or individual.

Such are the fundamental vital qualities of the protoplasm of an amœba; all the facts of the life of an amœba are manifestations of these protoplasmic qualities in varied sequence and subordination. The higher animals, we learn from morphological studies, are in reality groups of amœbæ peculiarly associated together. All the physiological phenomena of the higher animals are similarly the results of these fundamental qualities of protoplasm peculiarly associated together. The dominant principle of this association is the physiological division of labor corresponding to the morphological differentiation of structure. Were a larger or "higher" animal to consist simply of a colony of undifferentiated amœbæ, one animal differing from another merely in the number of units making up the mass of its body, without any differences between the individual units, progress of function would be an impossibility. The accumulation of units would be a hindrance to welfare rather than a help. Hence, in the evolution of living beings through past times, it has come about that in the higher animals (and plants) certain groups of the constituent amœbiform units or cells have, in company with a change in structure, been set apart for the manifestation of certain only of the fundamental properties of protoplasm, to the exclusion or at least to the complete subordination of the other properties.

These groups of cells, thus distinguished from each other, at once by the differentiation of structure and by the more or less marked exclusiveness of structure, receive the name of "tissues." Thus, the units of one class are characterized by the exaltation of the contrac-