46 INDIVIDUALITY IN ORGANISMS 



region. This change I have called decrease in receptiv- 

 ity of the subordinate part for the transmitted change. 

 The effect of physiological isolation of a part is 

 essentially the same as that of physical isolation. In 

 the lower organisms where its physiological and morpho- 

 logical characteristics as a part are less stable than in 

 the higher forms and it is able to respond to the altered 

 conditions accompanying physiological isolation, it 

 loses more or less completely its character as a part 

 because the conditions which determined and maintained 

 its specialization no longer act. Consequently it under- 

 goes dedifferentiation to a greater or less degree and so 

 approaches or returns to the undifferentiated or embry- 

 onic condition, and is then capable, if differences in meta- 

 bolic rate in the direction of the original gradient or 

 gradients still exist in it, or if conditions determine the 

 origin of new gradients in it, of development into a 

 new individual. I have shown that development and 

 differentiation are in general accompanied by a decrease 

 in metabolic rate which constitutes physiological senes- 

 cence and that the dedifferentiation of isolated parts 

 brings about rejuvenescence varying in degree with the 

 degree of dedifferentiation. 1 New individuals formed 

 from physiologically or physically isolated parts of pre- 

 existing individuals may therefore be physiologically 

 younger than the individuals from which they arose and 

 so be capable of repeating the developmental history 

 and process of senescence. Asexual reproduction in 

 general results from such physiological isolation of 

 parts and their dedifferentiation and ^differentiation 

 into individuals. In the higher animals physiological 



1 Child, Senescence and Rejuvenescence, 1915; particularly chaps, ii, 

 iv, v, vi, viii, x, xv. 



