PROCESS OF REPRODUCTION IN ORGANISMS. 5 



conditions and in the presence of nutritive material. If this 

 conclusion is correct then senescence is a necessary feature of 

 life. But senescence in the lower forms does not lead inevitably 

 to death, for any conditions which determine the use as a source 

 of energy or the elimination of a part of these structural sub- 

 stances and so make possible a higher rate of metabolism bring 

 about rejuvenescence. It is perhaps more strictly correct to 

 say that w T hen the formation and deposition of these inactive 

 substances which retard metabolism is more rapid than their 

 break-down and removal then the organism is growing old, 

 but whenever the processes of breaking down and elimination 

 of the.se accumulations are in excess of the processes which 

 form them the organism is growing young, provided nutrition 

 is available for the increased rate of metabolism which is thus 

 made possible. Sexual reproduction is not the only means by 

 which the organism returns so to speak to or toward the starting 

 point. Every experimental reproduction resulting from the 

 physical isolation of a piece involves necessarily a greater or 

 less degree of rejuvenescence. 



We have then in the reconstitution of a part into a new whole 

 after experimental isolation all the essential features of true 

 reproduction and of inheritance. The new individual formed 

 is physiologically and morphologically younger than that from 

 which it originated and it possesses the essential character- 

 istics of the species. 



III. ASEXUAL REPRODUCTION IN NATURE. 



In a recent paper (Child, 'na) I have attempted to show 

 that at least many forms of asexual reproduction in nature 

 are essentially similar to the regulatory reconstitution of a 

 piece experimentally isolated. As a matter of fact I believe 

 that all forms of asexual reproduction are of this type. The 

 chief difference between asexual reproduction in nature and the 

 reproduction induced by experimental isolation of pieces is 

 that the isolation in the latter case is physical and complete, 

 while in the former it is usually at first physiological and very 

 often only partial. The simplest types of asexual reproduction 

 which result from autotomy, self-laceration, etc., are identical 



