514 



SCIENCE 



[N. S. Vol. XLHI. No. 1111 



believe that it exists before our very eyes 

 in one of the most characteristic features 

 of environmental relation, namely in spatial 

 quantitative differences in the action of 

 external factors on protoplasm. A brief 

 consideration of a simple case will serve to 

 make the point clear. 



Let us begin with a mass of protoplasm, 

 or a cell mass which is undifferentiated, 

 i. e., in which no morphological differences 

 and no localized quantitative or qualitative 

 differences in the metabolic reaction be- 

 yond those characteristic of protoplasm or 

 cells in their simplest terms are present. 

 Such a protoplasmic or cell aggregate may 

 include individualities of various kinds, as 

 I have already pointed out, but it is not 

 integrated into a physiological individual- 

 ity, as a whole, nor does it possess any in- 

 herent capacity for such integration. 



If now such an aggregate is subjected 

 to the differential action of environment 

 by permitting an exciting factor, a stimu- 

 lus, to act upon some point of its surface 

 the first result is an increase in dynamic 

 activity in the region immediately affected. 

 It is a familiar fact of physiology that the 

 dynamic effect of such a local excitation 

 does not remain limited to the region di- 

 rectly affected by the external exciting 

 factor. The local excitation is followed 

 by the spreading or transmission through 

 the protoplasm or over its limiting sur- 

 faces from the point immediately affected, 

 of some sort of dynamic change, which 

 itself acts as an exciting factor. For 

 present purposes the fact of transmission, 

 rather than the nature of the transmitted 

 change concerns us. 



Secondly, we know that in protoplasm 

 in general such transmitted excitations de- 

 crease in energy, intensity, or in our pres- 

 ent ignorance we may say in physiological 

 effectiveness, with increasing distance from 

 the point of origin, so that at a greater or 



less distance they become inappreciable or 

 ineffective. This range or limit of effec- 

 tiveness depends, of course, on various fac- 

 tors, the degree or intensity of the orig- 

 inal excitation, the capacity of the proto- 

 plasm for transmitting excitations, etc. 

 The case is analogous in certain respects to 

 the spreading of a wave in water, air or 

 any other physical medium from the point 

 of disturbance. 



Since a decrement in effectiveness occurs 

 in transmission, the degree of excitation 

 associated with it will be greatest at the 

 point of origin and will decrease with in- 

 creasing distance from this point. That is 

 to say, a gradient in excitation appears in 

 which the point of original excitation by 

 the external factor constitutes the region 

 of highest rate, intensity, or effectiveness. 

 Such a dynamic gradient represents, I be- 

 lieve, the simplest form and the starting 

 point of physiological integration in liv- 

 ing protoplasm. 



If the action of the external factor is of 

 short duration this dynamic gradient usu- 

 ally exists for only a short time and leaves 

 little or no appreciable persistent change in 

 the protoplasmic substratum. If, however, 

 the action of the external factor is suffi- 

 ciently long continued or sufficiently often 

 repeated, the protoplasmic changes sooner 

 or later become more or less evident and 

 more or less persistent. These changes are 

 fundamentally changes in irritability, in 

 the capacity of the protoplasm to react, and 

 since these changes are in general propor- 

 tional to the rate or intensity of dynamic 

 activity in the protoplasm the dynamic 

 gradient may produce in the protoplasm an 

 irritability gradient. The differences in 

 irritability at different levels are more or 

 less persistent, and when once established 

 tend in general to become intensified up to 

 a certain point. In short, a dynamic or 

 metabolic gradient arising as the result of 



