IXDIVIDUATIOX AND REPRODUCTION 217 



THE NATURE AND LIMITS OF DOMINANCE 



As regards the nature of the influence of the dominant region 

 upon other parts, the physico-chemical theory of the organism 

 affords two alternatives. Physiological correlation in the organism, 

 the influence of one part upon another, so far as it is not directly 

 mechanical, is accomplished in two ways: by the production and 

 transportation of substances, commonly known as chemical corre- 

 lation, and by the transmission through the protoplasm in general, 

 or along specialized conducting paths, of excitations which have 

 often been regarded as electrical in nature, but which now appear 

 to be associated with chemical changes (Tashiro, '13a). If chemical 

 correlation is the basis of the influence of the dominant region on 

 other parts, then we must suppose that metaboHsm in the dominant 

 region gives rise to certain chemical substances which are trans- 

 ported in some way through the body, but are gradually used up or 

 transformed so that their effects cease at a certain distance from 

 the region of origin. We may assume, further, that different sub- 

 stances are transported at different rates or are completely used up 

 at different distances from the point of origin. On the other hand, 

 the dominance and subordination of parts may conceivably be 

 accomplished by transmitted impulses. On the basis of this 

 alternative the metabolic activity of the dominant region must 

 produce certain changes or excitations which are transmitted 

 through the protoplasm, but which decrease in energy or effective- 

 ness as they are transmitted, so that finally a limit is reached beyond 

 which they are ineffective. 



Many facts favor the second alternative. In the first place, 

 chemical substances may be transported to any distance in the 

 fluids of an organism, and it is difiicult to see how any definite and 

 characteristic limit of effectiveness of such substances could exist, 

 unless we could assume that they were difi"using through a homoge- 

 neous medium or being transported at a definite rate and under- 

 going destruction also at a definite rate during transportation. 

 But it is certain that neither of these possibiUties is realized in all 

 organisms in which a limit of effectiveness of dominance ajipears, 

 and it is a fact that the existence of a decrement and a limit of 

 effectiveness in transmission has been obser\-ed in nian>- cases 



