226 SENESCENCE AND REJUVENESCENCE 



other parts. With a sufficiently high rate of metabolism and when 

 not subordinated to other parts, any part of the simpler organisms 

 is capable of developing into an apical region or head. 



The objection may be raised that even if such a metabolic 

 gradient is established, there is nothing to maintain it with the 

 necessary degree of constancy to produce definite results. As a 

 matter of fact, regional differences in metabolism do maintain 

 themselves to a remarkable degree and may even be accentuated. 

 Certain muscles frequently or strongly -stimulated become capable 

 of greater activity, and little-used parts gradually lose their 

 capacity for activity. There is good reason to believe that within 

 certain limits an increase in rate of metabolism in a protoplasmic 

 substratum changes the condition of the substratum so that a still 

 higher rate is possible, and vice versa. The analogy between the 

 organism and the stream referred to in chap, i is perhaps of service 

 here. An increase in rate of flow of the stream alters the channel 

 so that a still higher rate is possible, and a decrease in rate of flow 

 produces conditions which bring about further decrease. Moreover, 

 the region of high rate of metabolism in the organism once estab- 

 lished is more susceptible because of its high rate to the action of 

 external conditions: in animals, particularly in motile forms, this 

 region becomes the seat of the special sense-organs and is therefore 

 the most important part of the body as regards relations between 

 the organism and the external world. These conditions result from 

 the original high metabolic rate of the region, but they also con- 

 tribute toward maintenance of a relatively high rate of metabolism. 



And, finally, the question whether purely quantitative differences 

 along an axis are sufficient to account for the morphological differ- 

 ences which arise along that axis is one which can be answered only 

 after the most extended and painstaking investigation. At present 

 we know that morphological characters can be altered very widely 

 by conditions whose effect upon the organism is primarily quanti- 

 tative. The different types of anterior end in pieces of Planaria 

 (see pp. 111-12) are cases in point. The very general belief that 

 qualitatively different substances or entities of some kind are 

 necessary as a basis for morphological development does not rest 

 upon direct or experimental evidence, but is an inference from the 



