TISSUE-CULTURE IN VITRO 121 



at the beginning and at the end of life and much more rapid 

 during youth. 



Consequently, there are two kinds of time. One corre- 

 sponding to the classical notion, the sideral, physical time, 

 without beginning and without end, flowing in a continuous, 

 uniform, rigid fashion. The other, the physiological time, the 

 duration of our organism, which begins and ends with us, 

 and which does not affect identically in our youth and our old 

 age the phenomena of which we are the seat. It is a time which 

 remembers. It is no longer the impersonal, rigorous time 

 measured by the rotation of the earth, the immutable and 

 arithmetical time in which the universe evolves. Its flow 

 seems to be submitted to periodical fluctuations. It rebounds 

 with each germ: a living time. 



But, on the whole, what is time? Is it not absurd to say that 

 it flows? Can we measure physiological time with the same 

 units as the other, the time of inert things? Are they really 

 diflferentiable? We will now try to study these questions 

 successively. 



