SOME GENERAL CONCLUSIONS 465 



as they do the course of individual development, and we must admit 

 the possibility of sudden changes of considerable magnitude, so- 

 called mutations, although even these may be determined by pre- 

 viously existing internal conditions, as, for example, metamorphosis 

 in individual development which is primarily the result of internal 

 factors. And, finally, as our ability to control the process of indi- 

 vidual development has increased so greatly with the advance in 

 knowledge of experimental methods, we may perhaps expect that 

 in the course of time our ability to control the evolutionary pro- 

 cess may increase, although the difficulties involved in controlling 

 and modifying to any very great degree internal conditions which 

 are the result of millions of years of alternating progressive and 

 regressive change will perhaps make progress in this direction slow. 

 Senescence and rejuvenescence result from a combination of 

 factors which is found nowhere except in organisms, but there is no 

 reason to believe that any one of the factors which make up the 

 complex is peculiar to living things. Changes in the permeability 

 of membranes and other changes in aggregate condition of the 

 colloids, changes in proportion of active and inactive substance in 

 chemical systems, changes in water-content all these and many 

 others occur in non-living as well as in living systems. But we 

 may make our basis of comparison broader than this and use for 

 definitions somewhat more general terms than heretofore. In such 

 terms senescence is a retardation resulting from continued dynamic 

 activity under certain conditions in a system, and rejuvenescence 

 an acceleration resulting from elimination or transformation 

 of the retarding factors under altered conditions. These defini- 

 tions still hold good for the organism, but they also apply to many 

 other changes in nature. Senescence and rejuvenescence in this 

 sense are going on all about us, in some cases with short, in others 

 with very long, periods. The age changes in the organism are 

 merely one aspect of Werden und Vergehen, the becoming and 

 passing away, which make up the history of the universe. 



