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The veriest tyro in the matter of exercise knows that exercise develops a muscle; that repeated flexion and extension of the arm, for instance, will strengthen the muscles of that limb, not cause them to lose their contractibility.
Through the changes in matter produced by nourishment and throwing off, as actual functions of the protoplasm, and through its own plasticity, proceed all the other most simple factors of life, sensibility which consists in the interchange between the protoplasm and its food, contractibility which shows itself at a very low stage in the consumption of food, possibility of growth which is shown in the lowest stages of development by splitting, and internal motion without which neither the consumption nor assimilation of food is possible.
A very little alteration in feature might have made a plain face into a beautiful one; and some slight change in the position or the contractibility of certain of the muscles might have made the most awkward of manners and gaits into the most dignified and graceful. All that we all understand.
Rough and repulsive in appearance, and sluggish in habit, it has great power of contractibility. It may assume a dumpy oval shape, and again drag out its slow length until it resembles an attenuated German sausage, black in colour. Its "face" may be obtruded and withdrawn at pleasure, or rather will, for what creature could have pleasure in a face like a ravelled mop.
But simple as it was it had all the fundamental properties of living things irritability, contractibility, assimilation, and reproduction. It was a compound which seemingly deserved the name of "physical basis of life", which was soon given to it by Huxley. With this conception of protoplasm as the physical basis of life the problems connected with the study of life became more simplified.
In the wasted bodies of those who have suffered starvation, the muscles are shrunk and unnaturally soft, and have lost their contractibility; all those parts of the body which were capable of entering into the state of motion have served to protect the remainder of the frame from the destructive influence of the atmosphere.
The four properties of irritability, contractibility, assimilation, and reproduction, belong to these vital units the cells, and it is these properties which we are trying to trace to their source as a foundation of vital activity. We may first ask whether we have any facts which indicate that any special parts of the cell are associated with any of these fundamental activities.
Such, in brief, is the cell to whose activities it is possible to trace the fundamental properties of all living things. Cells are endowed with the properties of irritability, contractibility, assimilation and reproduction, and it is thus plainly to the study of cells that we must look for an interpretation of life phenomena.
What is attraction without molecules attracting each other? What is contractibility without muscular fibre, or secretion without a secreting gland? One combination of molecules exhibits the phenomena of life, another combination exhibits the phenomena of mind. All this was taught by the old heathen philosopher more than two thousand years ago.
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