OLD AGE AND DEATH 359 



duction is exhausting and often fatal. But this does not 

 apply to the Protozoa. 



Among unicellular organisms the process of multiplica- 

 tion is simple and inexpensive, and correspondingly rapid. 

 One becomes two by dividing, and we cannot speak of 

 death when there is nothing left to bury. A little one soon 

 becomes a thousand. From one Infusorian there may be 

 a million in four days. 



It seems, however, that there is, in some cases at least, 

 a limit to the number of successive divisions that can occur 

 in an isolated family of Infusorians all descended from one, 

 and in the absence of that pairing of unrelated forms 

 which normally occurs in natural conditions. After a 

 certain number of generations which can be extended by 

 using stimulants, such as strychnin senile decay sets in, 

 the new members are born old, they are dwarfish and de- 

 generate ; soon they cease to be able to feed or divide. 

 Even at this low level of organisation it is only through the 

 fire of love that the phoenix of the species can renew its 

 youth. Without conjugation of unrelated forms the family 

 comes to an end. 



Now it may be that this limitation to the number of 

 successive cell-divisions has a deep significance in regard 

 to the growth of higher animals, which is, of course, accom- 

 plished by successive cell-divisions. Here, too, there are 

 limits to multiplication and renewal. In brains, notably, 

 the cells stop dividing or multiplying at a very early date 

 it may be before birth ! Thus we see the force of Professor 

 Weismann's sentence : " The real cause of death is to 

 be looked for, not in the using up of the body-cells, but 

 rather in the limits to the reproductive power of cells." 



What has been said throws light upon the problem 

 of senescence in the higher forms of life. It is to be re- 

 garded not simply as an intrinsic necessity ; it is incident 



