l88 PSYCHOLOGY AND PREACHING
sitivity plus motility plus rationality. " Sensitivity " is, I know, a very questionable word with which to indicate the mode of responsiveness which characterizes vegetable life, because it has acquired its meaning in application to animal organisms; but there is no existing word which is more appropriate. Its etymology is against this use of it; but in the absence of a suitable term I venture to stretch the proprieties of language so far as to use it in this appli cation. By motility is meant the ability of an organism to move itself from place to place by the contraction of the muscles of some of its organs. Precisely what is meant by rationality will be explained a little later. It will be noted that each higher grade retains the mode or modes of adap tability of that which is below it, while on the higher level these modes are far more highly developed. The plant has what I have called " sensitivity," for want of a better term; but the animal has sensitivity more highly and variously developed than the plant and has motility in addition. Man has both sensitivity and motility far more highly and variously developed than the animal, notwithstanding the fact that in some specific senses and in some specific forms of locomotion he may be inferior to some animals; and has in addition the wondrous capacity of rationality. Of course there are no absolute lines of demarcation between these modes; each lower one merges into the next higher. There are plants, for instance, which seem to possess in some small measure the mode of adaptation which we call motility; e.g,. the sensitive plants have contractility, which is the funda mental element in motility. It is even more difficult to determine at what point exactly rationality is added to motility; and yet, broadly speaking, we know that it is a distinctively human characteristic, though there may be suggestions of its presence in the higher animals.
If we look at this advance from the lower to the higher modes of adaptability from another point of view, there is at once evident a corresponding increase in the complexity of the physical organization. The organization of the plant
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