DIFFERENTIATION AND REJUVENATION 161 



From all that has been said it seems to me legiti- 

 mate to conclude that there is an intimate correlation 

 between the rate of differentiation and the rate of 

 growth. I am inclined to go the one step farther, 

 and bring them into the relation of cause and effect; 

 and I present to you as the main general conclusion of 

 this first part of our series of lectures, the conception 

 that the growth and differentiation of the protoplasm 

 are the cause of the loss of the power of growth}- 



Now if cells become old as their protoplasm in- 

 creases and becomes differentiated, we should expect 

 to find that there would be a provision for the pro- 

 duction of young cells. It is rather mortifying to 

 reflect that the simple conception which I have now 

 to express to you, although it lay close at hand, failed 

 to combine itself in my mind for many years with the 

 conception of the process of senescence as I have just 

 described it to you. It is somewhat, it seems to me, like 

 two acquaintances of mine who lived long side by side, 

 seeing one another frequently until they were fairly 

 past the period of youth, when their attachment 

 became very close and by a sacrament they were 

 permanently joined together. So in the minds of 

 men often two ideas lie side by side which ought to 

 be married to one another, and there is no one ready 

 so dull is the owner of the mind, to pronounce the 

 sacramental words which shall join them, and the rite 



1 The hypothesis that the increase of protoplasm is the cause of old age was 

 originally put forward in i8go(C. S. Minot, "On Certain Phenomena of Grow- 

 ing Old," Proc. American Assoc. Adv. Science, xxix., Address of the Vice- 

 President, Section F). 



