1920] CHILD 6r BELLAMY— BRYOPHY LLC M 251 



one part over another is probably primarily a matter of the dif- 

 ferences in electric potential and resulting currents. In general, 

 the "high end," the dominant region of a physiological gradient, is 

 externally electro-negative to other levels of the gradient, and in 

 this respect it is similar to a region of excitation, which is also 

 externally electro-negative to less excited or unexcited regions. 

 From this viewpoint the physiological gradient may be regarded. 

 at least tentatively, as the physiological expression or effect of the 

 potential gradient and the resulting currents which arise in rela- 

 tion to a region of high metabolic rate. In fact, the physiological 

 gradient in its simple form shows all the characteristics of an 

 excitation-transmission gradient in protoplasm. In all except the 

 simplest animals a nervous system with definite morphological 

 conducting paths develops as an expression and resultant of the 

 physiological gradients, and after definite nervous relations are 

 established between parts, the dominance of a particular region, 

 for example, the head, is no longer necessarily dependent upon the 

 persistence of the metabolic 'conditions which originally determine 

 its dominance. While the rate of metabolism concerned in the 

 initiation of a nervous impulse is undoubtedly high, its total 

 amount may be exceedingly small, yet the impulse may determine 

 an enormous amount of metabolism in the organ affected by it. 



In plants, how r ever, no nervous system develops, and the rela- 

 tions of dominance and subordination apparently depend through- 

 out life upon essentially quantitative physiological differences of 

 the same sort as those in which the relation originates. The 

 nervous structure of higher animals is capable of conducting 

 impulses for long, perhaps for indefinite distances; but in the less 

 highly specialized protoplasms of the simpler animals and the 

 plants the dynamic effects of excitation undergo a decrement with 

 increasing distance from their point of origin. Such a decrement 

 determines the existence of the physiological gradient, and it is 

 evident that under such conditions physiological dominance of any 

 part must be limited in range, and that therefore the possibility 

 of what the senior author has called physiological isolation (1-6), 

 that is, of escape or isolation from such dominance without physical 

 separation of parts, exists. Theoretically physiological isolation 



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