102 III. NON-ELEMENTAL1STIC STRUCTURES
always give relational, structural data, that, under such and such conditions, such and such results follow. The non-el attitude and language, as opposed to the old elementalism, is a part of a broader and more fundamental semantic problem; namely, similarity of structure between language and the external world. Such similarity leads to similarity of 'logical' relations, predictability, and so forth, and, in general, to the understanding of the structure of the world and new s.r.
There are many examples of such organism-as-a-whole terms, but for the present we will mention only the terms 'tropism' in the generalized sense of Loeb, and the 'dynamic or physiological gradients' of Professor Child. The term 'tropism' means the response of the organism-as-a-whole to special external stimuli. For instance, the term 'helio-tropism' is applied in cases when the organism responds to the influence of light; 'chemotropism', when it reacts to chemical stimuli; 'galvano-tropism', when the organism responds to galvanic (electrical) stimulation,. The term 'dynamic or physiological gradient' is the foundation of the A biological system of Professor Child. Because of its importance, I shall explain the meaning of this term in some detail.1
All protoplasm exhibits empirically a structural characteristic which may be called 'irritability', which appears as a reaction of living protoplasm to external dynamic influences. That 'irritability', as a structural characteristic, becomes obvious when we consider that structurally disintegrated protoplasm is colloidally inactive and becomes 'dead'. Many of the most important characteristics of living protoplasm are strictly bound up with structural integrity.
This 'irritability' occurs in a structural plenum and is transmitted to other regions of the protoplasm with differing yet fjinite velocities, and not in 'no time', as Alice would say. Let us imagine a non-differentiated, except for the limiting surface (A), and living bit of protoplasm. This limiting surface represents that part of the protoplasm which is in direct contact with the environment. If the external dynamic factor (S) excites this living bit of protoplasm at a point (B), this stimulus will be the strongest at (B), and it will spread to the further removed portions of (A) in a dimin ishing gradient. If the decrement is not too sharp, the stimulus will reach the furthest regions of (A) ; namely,'(C), (D), (E), (F),.