CHAPTER XVII 



SOME GENERAL CONCLUSIONS AND THEIR SIGNIFICANCE FOR 

 BIOLOGICAL PROBLEMS 



It remains only to review briefly in a connected way some of the 

 more important conclusions of the preceding chapters and to make 

 a few further suggestions as to their bearing upon certain biological 

 problems. In the first place, a full consideration of the facts leads 

 unmistakably to the conclusion that the age cycle is simply one 

 aspect of the developmental cycle, or we might even say that the 

 developmental cycle is an aspect of the age cycle. Senescence 

 and rejuvenescence do not include special processes, they are 

 merely certain aspects of the relations between the metabolic reac- 

 tions and the protoplasmic substratum. The progressive changes 

 with which physiological senescence is associated are changes in 

 the direction of greater physiological stability of the protoplasm 

 and decreased dynamic activity. The regressive changes which 

 bring about rejuvenescence are not necessarily reversals in the 

 chemical sense of the progressive changes, but rather a substitution 

 of a new substratum for an old. As a structure built by man, when 

 it is no longer suited to existing conditions, may be torn down and 

 some parts of it used, together with new material, for building a new 

 structure which meets the demands of the new conditions, so in 

 organisms structural features built up under certain physiological 

 conditions may under others be broken down, and some of their 

 constituents may take part in the formation of a new structure. 



Both progression and regression are undoubtedly going on at 

 all times in the active organism, but under the usual conditions of 

 vegetative life the progressive changes overbalance greatly the 

 regressive because building material in the form of nutrition is 

 being added. But while growth and progressive development, 

 with its speciahzation and differentiation of parts, is the more con- 

 spicuous feature of the Hfe cycle, reduction and regression are none 

 the less essential parts of it. The life cycle consists of one or more 

 periods of senescence and one or more periods of rejuvenescence. 



459 



