2 24 SENESCENCE AND REJUVENESCENCE 



organism, and that it seems to depend primarily upon impulses or 

 changes of some sort transmitted from the dominant region, rather 

 than upon the transportation of chemical substances. Chemical 

 substances arising in the course of metabolism are undoubtedly 

 important factors in determining the constitution and character of 

 particular organs and parts, but it is difficult to understand how 

 they can account for the definite and orderly spatial characteristics 

 of living things. Hormones, internal secretions, and other chemi- 

 cal substances unquestionably play a very essential role in physio- 

 logical correlation, particularly in the higher animals where 

 different organs are highly differentiated, but for the production of 

 such different specific substances different organs are necessary. 

 At present we are concerned with the question of the primary 

 origin of these organs, with the appearance and localization of 

 differences which make possible the production of different specific 

 substances in different parts of the individual, and it is evident 

 that these primary specializations and differentiations, their locali- 

 zation and orderly and definite spatial arrangement, cannot be 

 accounted for by the action or interaction of such substances. 



According to the conception developed above, the dominance of 

 a region depends primarily upon its rate of metabolism as compared 

 with that of other regions within the range of its influence. Where 

 the region of high rate is the primary factor in maintaining the 

 gradient, as it undoubtedly is in the lower organisms and in the 

 early stages of development of many higher forms, it is of course 

 the chief factor in determining the metabolic rate in other regions 

 and so maintains its original dominance. But in more highly 

 differentiated forms, or in later developmental stages, where rela- 

 tively permanent structural differentiations have arisen along the 

 course of the gradient, so that it has become structurally fixed, 

 the region of highest rate still remains dominant because it gives 

 rise to more powerful impulses than do other regions and conse- 

 quently influences them more than they do it. Lastly, in the higher 

 animals, where, in all except early embryonic stages, transmission 

 through nerves is the chief factor in physiological integration (see 

 Sherrington, '06), the original gradient in metabohc rate may 

 persist chiefly, or perhaps in some cases only, in the efferent con- 



