72 THE EVOLUTION QF_PEATH 



may therefore expect that the changes which cause the 

 diminution wll be more noticeable in embryos than in older 

 animals. 



I have not succeeded in determining with absolute cer- 

 tainty the cause of the inhibition of growth. We find, 

 however, a close correlation between the alterations which 

 occur in the cells of the embryo and the inhibition, which 

 renders it probable that the alterations of the cells are at 

 least one essential cause of the diminution of the growth. 

 The alterations which here come into play are those of differ- 

 entiation, and in fact differentiation proceeds in young 

 embryos with extraordinary rapidity and in older embryos 

 more slowly. At the time of tiirth the differentiation is for 

 the most part far advanced, and thereafter continues extraor- 

 dinarily slowly. Up to the present at least it has been im- 

 possible to express our observations of the rapidity of differ- 

 entiation in statistical form because we do not yet know how 

 to measure differentiation quantitatively. We can merely 

 estimate the degree of differentiation. In spite of the 

 incomplete reliability of this method, I believe that the es- 

 timate which has been made answers to the truth. That a 

 causal relation exists between the diminution of differentia- 

 tion and the rate of growth is confirmed by the fact that 

 direct observation teaches us that undifferentiated cells 

 may divide rapidly and that differentiated cells divide more 

 slowly, and finally that the most completely differentiated 

 cells do not divide at all. The indicated considerations have 

 led me to the conclusion that differentiation is to be con- 

 sidered the essential cause of senescence. 



I have already asked you to give heed to the fact that 

 differentiation occurs principally as a transformation of proto- 

 plasm. At the same time we learn that in order to render the 



