THE FOUR LAWS OF AGE 229 



at a slower rate. The mammals take a much longer 

 period to pass through their infancy and reach their 

 maturity, but after they have reached their maturity 

 they do not sustain themselves so long. Their later 

 cytomorphosis occurs at a higher speed than the bird's. 

 This is a field of study which we can only recognise 

 the existence of at present, and which needs to be 

 explored before, to any general, or even to a special 

 scientific, audience, any promising hypotheses can be 

 presented. Definite conclusions are of course still 

 more remote. 



Next as regards death. The body begins its de- 

 velopment from a single cell, the number of cells 

 rapidly increases, and they go on and on increasing 

 through many years. Their whole succession we may 

 appropriately call a cycle. Each of our bodies repre- 

 sents a cell cycle. When we die, the cycle of cells 

 gives out, and, as I have explained to you in a pre- 

 vious lecture, the death which occurs at the end of 

 the natural period of life is the death which comes 

 from the breaking down of some essential thing 

 some essential group of members of this cell cycle ; 

 and then the cycle itself collapses. But the death is 

 the result of changes which have been going on 

 through the successive generations of cells making 

 up this cycle. There are unicellular organisms; these 

 also die ; many of them, so far as we can now deter- 

 mine, never have any natural death, but there are 

 probably others in which natural death may occur. It 

 is evident that the death of a unicellular organism is 



