OLD AGE AND DEATH 357 



Our problem is to try to understand more clearly why 

 it is that the curve of life, after attaining what the thought- 

 ful biologist Treviranus called a vita maxima, should 

 begin to droop, sinking quickly or very slowly to a nadir 

 of vita minima, at the threshold of death. Why do 

 living creatures grow old, or, to be more accurate, why do 

 many living creatures grow old ? 



As this is a very difficult problem not admitting as 

 yet of complete solution we must work round about 

 it, instead of venturing on a direct attack. By patiently 

 approaching it from different sides, we may reach in the 

 end some greater clearness. Shakespeare had the problem 

 before him when he wrote : " And so, from hour to hour, 

 we ripe and ripe, And then, from hour to hour, we rot and 

 rot, And thereby hangs a tale." But the tale is not as yet 

 coherent. 



One of the interesting results of modern Biology is 

 expressed in the somewhat startling phrase, which we 

 owe to Professor Weismann the immortality of the Proto- 

 zoa. This implies that the simplest animals (and plants 

 too for that matter) the unicellular organisms are not 

 subject to natural death in the same degree as higher 

 forms are. It seems that many of them, at least, are 

 practically immortal. They may be devoured, they may 

 be crushed, they may be killed in a hundred ways ; some 

 are liable to fatal infection from microbes ; but of their 

 natural death there is no definite proof. 



In the open sea there are countless millions of uni- 

 cellular organisms both plants and animals Foramini- 

 fera, Radiolaria, Diatoms, Desmids, and so forth which 

 are continually being killed by vicissitudes of temperature 

 and the like. Dying, they sink slowly down, like a cease- 

 less, gentle snow-shower, it may be through miles of water, 

 to form no inconsiderable part of the food-supply of the 



