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SENESCENCE AND REJUVENESCENCE 



only unnecessary but impossible. Germ plasm is any protoplasm 

 capable, under the proper conditions, of undergoing regression, 

 rejuvenescence, and reconstitution into a new individual, organism, 

 or part. In other words, germ plasm becomes merely an abstract 

 idea which connotes the sum-total of the inherent capacities or 

 "potencies" with which a reproductive element of any kind, natural 

 or artificial, agamic or gametic, giving rise to a whole or a part, 

 enters upon the developmental process. Germ plasm is then 

 merely another term for heredity. The process of inheritance is 

 concerned in every case of reproduction, whether it be agamic or 

 gametic, partial or total, and both experimental reproduction and 

 agamic reproduction in nature present opportunities for the study 

 of the process and mechanism of inheritance, which have thus far 

 been almost entirely neglected, but which are not found in con- 

 nection with the much more highly specialized process of gametic 

 reproduction. And, admitting that every reproductive element 

 of any kind is, before reproduction begins, an integral physiological 

 part of an organic individual, we may define heredity more briefly 

 as the capacity of a physiologically or physically isolated part for 

 reconstitution into a new individual or part. 



It does not by any means follow from this theory of reproduction 

 and inheritance that all the characteristics of the individual shall 

 reappear in the following generation. Many individual charac- 

 teristics which are the result of action of external factors or of 

 special functional activity of certain parts — such, for example, as 

 calloused areas in the skin, the functional hypertrophy or atrophy 

 from disuse of certain muscles, and many others — are evidently the 

 result of local quantitative changes in metabolism and as such 

 cannot be expected to alter at once the equihbrium of the whole 

 protoplasmic system in such a way that they will be reproduced 

 in following generations in the absence of the special conditions 

 which determined their first appearance. This is equally true for 

 agamic and for gametic reproduction. Nevertheless, since every 

 reaction represents to some extent a reaction of the whole organ- 

 ism and no change is purely local or entirely independent of 

 other changes, it is conceivable that if the special external or func- 

 tional conditions act in the same way through a sufficient number 



