VOLUNTARY ACTION 187
this experience leaves a trace in it, i.e., in some way modi fies it ; and as a result of this modification the response to a subsequent stimulus, of the same or of a different kind, will not be quite the same as before. These traces left in the organism and the resulting modification of subsequent re sponses may be so slight as to escape the most discriminat ing observation. Indeed, in the lower ranges of life they are hardly observable, and the truth of the statement as applied to the lowest ranges may be fairly called in ques tion. It is probable, however, that wherever there is life some slight organic modification results from experience but on the inferior levels it is of negligible importance so far as the history of the individual organism is concerned.
The decreasing importance of these modifications in the lower grades of life is only one aspect of the general truth that responsiveness to environment increases as the scale of life is ascended. In fact, the relative position of an or ganism in the scale of life is determined by its responsive ness to environment. In the vegetable kingdom the rose bush responds to climatic or seasonal changes, but the limits within which it may respond are very narrow. It is rooted to one spot, unless transplanted by human skill. In that fixed locality it may dress itself in green and blush with red blossoms under the caressing touch of Summer. But how much more restricted is its adaptability than that of the wild goose, which feels the approach of Winter from afar and wings its way after the retreating Summer; or of the animals which freely rove abroad in search of food and protect themselves from the cold blasts by heavy coats of hair or even acquire the skill to build themselves shelters against the storms? But animal adaptability sinks into insignificance as compared with the capacity of man to bring himself into satisfactory relations with a complex and changing environment.
The modes of responsiveness which characterize these three grades of life the vegetable, the animal and the human are sensitivity, sensitivity plus motility, and sen-
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