194 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



are very similar. Homoiogenous and heterogeneous individuality differentials 

 represent one of the most important injurious factors opposing the perpetual 

 life of tissues if separated from their normal connections. However, even 

 within the animal's own body, where the individuality differentials are autog- 

 enous, the return to the original tissue equilibrium after a disequilibration 

 has been established may be incomplete, owing to the fact that the various 

 tissues live under environmental conditions to which they are not fully 

 adapted and which, under some circumstances may become injurious. These 

 factors, step by step, cause the old age changes and, finally the death 

 of the tissues and of the individual in which, for a time, they have func- 

 tioned. The cells have to live under such injurious conditions because they 

 exert functions which concern the organism as a whole, and they are 

 acted upon by cells and substances which likewise function in the interest 

 of the whole organism; it is this condition which causes their ultimate 

 destruction. Hence potential immortality does not apply to the higher organ- 

 ism as a whole, but only to certain types of cells or organs which constitute 

 parts of this organism. The individual as such, as far as is known at present, 

 cannot avoid death. 



