2i6 SENESCENCE AND REJUVENESCENCE 



It is a well-known fact that a similar relation of dominance and 

 subordination exists in plants, the apical region or growing tip of 

 an axis being the dominant or controlling region of that axis. The 

 "law' of antero-posterior development in animals suggests that 

 the relations are at least primarily the same in embryonic develop- 

 ment as in experimental reproduction. The cases of apparent 

 mutual independence of different regions or parts of the embryo 

 represent beyond question a secondary condition, so far as the 

 independence shall prove to be real. 



As regards the longitudinal axis of the organism, then, the 

 region of highest rate of metabolism dominates other regions in the 

 earlier stages of development, and in general any region of higher 

 rate dominates regions of lower rate. The developmental gradients 

 along the axes of symmetry mentioned above (pp. 204-7) suggest 

 the existence of a dominance and subordination along these axes also. 



The remarkable parallelism between the relations of dominance 

 and subordination and the relations of metabolic rate along the 

 axis suggests that dominance and subordination may depend pri- 

 marily on rate of metabolism. As regards the plants, it is evident 

 that dominance depends on metabolic activity, for the effect on 

 other parts of decreasing or inhibiting the metabolism of the grow- 

 ing tip without killing it, for example, by inclosure in plaster or in 

 an atmosphere of hydrogen, is the same as that of killing it, or 

 removing it completely. In other words, the reproduction or 

 development of other growing tips which was previously inhibited 

 now proceeds. McCallum ('05) has demonstrated very clearly 

 that this relation of dominance and subordination in plants is not 

 dependent upon nutrition, water-content, or other more or less 

 incidental and widely varying conditions, but that it is a physio- 

 logical correlation of some sort apparently dependent upon funda- 

 mental factors in the plant constitution. As regards animals also, 

 there are many facts, some of which will be considered below, which 

 indicate clearly that dominance and subordination of parts in the 

 individual are primarily dependent upon rate of metabolism, al- 

 though with the development of a highly irritable conducting sys- 

 tem between dominant and subordinate parts, such as the nervous 

 system, it is conceivable that other factors may play a part. 



