PHASES IN THE LIFE OF THE INDIVIDUAL 725 



at all, is not continually decomposing in some parts, while being 

 regenerated in others. No living molecule is spared this decom- 

 position. The latter, however, does not seize upon all molecules at 

 the same time; while one is decomposing, another is being 

 constructed, and so on. One living particle affords the conditions 

 for the origin of another or several others, but itself dies. The 

 particles newly formed in turn give rise to others and, likewise, die. 

 In this manner living substance is continually dying, without life 

 itself becoming extinct." ^ From this standpoint, therefore, there 

 can be no question of any kind of living substance being truly 

 immortal. The whole conception of a possible immortality arises 

 from a confusion of ideas. 



Minot,^ on the other hand, has elaborated a theory of senescence 

 which may be regarded as an extension of that of Weismann. 

 Like the latter, he seems to assume that death is not a universal 

 accompaniment of life, and that natural death has been acquired 

 in the course of evolutionary development. He proceeds to define 

 senescence as an increase in the differentiation of the protoplasm. 

 During the early periods of life the young material is produced, and 

 the protoplasm is undifferentiated. During the later stages of 

 existence cell differentiation goes on, and th,e organism gradually 

 becomes old. When the cells acquire the faculty of passing beyond 

 the simple stage to the more complete organisation, they lose some- 

 thing of their vitality, of their power of growth, and of their 

 possibilities of perpetuation. Just as senescence depends upon 

 the increase and differentiation of the cytoplasm, so, conversely, 

 rejuvenation depends upon the increase of the nuclear material; 

 and consequently the alternation of the two phases of the life 

 cycle (the early brief one when the young material is formed, and 

 the later prolenged one when the process of differentiation is going 

 on) is due to an alternation in the proportions of nucleus and 

 protoplasm. In criticism of this theory, it may be urged that it is 

 in reality nothing more than a descriptive account of a general type 

 of cellular change, and that it provides no sort of explanation as to 

 why this type of change occurs, nor how it is that differentiation is 

 apparently correlated with a reduction of vitality leading eventually 

 to death. 



Eobertson and Eay ^ have recently adduced evidence for the view 

 that the potential longevity of any given individual is determined 

 by the relative velocities of anabolism in the cellular and sclerou 



1 Verworn, General Physiology, Lee's Translation from the second German 

 Edition, London, 1899. • • , , , 



2 Minot, loc. cit. For the views of Steinach, etc., on the interstitial gland, 



see p. 327. 



^ Robertson and Ray, loc. cit. 



