DIFFERENTIALS AND EVOLUTION 601 



organisms, because of their great plasticity and ability to produce organs and 

 to restitute whole organisms, are potentially immortal, in the restricted sense 

 which applies to beings living on a planet and in a universe over which they 

 have no control. The higher organisms, because of their rigid organization 

 and lack of plasticity, because of their greater individualization, have lost 

 the power to restitute the whole organism and to be potentially immortal ; at 

 best, only small constituent parts still possess such power, and this can be 

 realized only under artificial experimental conditions. Higher organisms are 

 more readily discoordinated and disorganized. The delicate mechanisms of 

 adjustment to one another which their organs and tissues have developed, 

 no longer enable them to repair more extensive injuries experienced under 

 the influence of inner and outer environmental factors, to undergo compensa- 

 tory regulations and to propagate asexually. They have acquired senescence 

 and associated diseases in the attainment of individuality and one of the 

 prices they paid for individuality was the potentiality to immortal life. 



But while there is a parallelism between the ascending evolution of organis- 

 mal differentials, the specialization of organs and tissues, the increasing 

 rigidity of the organism, and the apparent inevitableness of senescence and 

 death, it is, in the first place, the increasing complexity in the structure, con- 

 stitution, and metabolic and functional interaction of tissues and organs rather 

 than the increasing specialization of the organismal differentials which is 

 responsible for these pathological consequences of ascending evolution. As a 

 result of the greater differentiation of the organs and their increasingly intri- 

 cate interaction the organs became more delicate and, in the course of time, 

 they were no longer quite adequate to the performance of their functions, 

 and this change becomes more and more cumulative with the advancing years 

 of the individual. The relative proportion of reversible cyclic and irreversible 

 non-cyclic processes is more and more altered to the advantage of the non- 

 cyclic with increasing age of the individual. 



Many processes in nature are cyclic, but other processes, as all those sub- 

 ject to the second law of thermodynamics, are nonreversible, proceeding only 

 in one direction. The disintegration of radio-active substances is non- 

 reversible, although under altered conditions also a creation of the latter 

 may occur. In organisms the essential functions must be cyclic; this is the 

 case with circulatory, respiratory, alimentary functions, with sleep and 

 hibernation, with the proliferation of certain tissues. The sexual processes are 

 also at least partly cyclic, but they sustain the life of the species rather than 

 the life of the individual. However, these cyclic processes are grafted on 

 an irreversible process of a non-cyclic character, on one continuous process, 

 starting with birth and leading to growth, maturity, old age and death. This 

 process, irreversible as far as the individual is concerned, is the basis of 

 cyclicity in the species. But, also, the species may be subject to non-cyclic 

 changes and will be destroyed in the end, when external conditions cause at 

 an early age a decline in the organisms and make propagation impossible. 



We may then regard disease and death as manifestations of insufficient 

 adaptation between the different constituents of an organism and between or- 



