414 BIOLOGY: GENERAL AND MEDICAL 



duction is reached, for the gametes derived from the 

 parents mingle their substance in the zygocyte or fer- 

 tilized ovum and form the starting point of the new 

 generation and the germ-plasm universally pervades 

 the species and is handed down from generation to 

 generation. 



The soma-plasm that grows about the germ-plasm and 

 subserves the purpose of transmitting it, grows old and 

 dies, but before that time the germ-plasm is usually 

 transmitted to a new generation by which it is trans- 

 mitted to other individuals, and so on forever. 



'The germ-plasm is like some great legacy: the trustees 

 grow old and die, but the fund goes on forever." The 

 individual is but an incident in the life of the germ-plasm. 



The soma develops through activities contained in 

 the germ, all its conduct is predetermined in the germ, 

 the method by which the germ-plasm is to be trans- 

 mitted to a new soma is predetermined in the germ, 

 and the time of the decadence of the custodial soma is 

 predetermined by the germ. 



The more complexly differentiated the soma becomes, 

 the more difficult it is to sustain and the privilege of 

 conjugation by which rejuvenation of the cells seems 

 to be effected being reserved for the germ-plasm alone, 

 the decadence of the soma becomes inevitable. 



The longevity of the soma-plasm is extremely variable 

 and the laws by which it is determined are unknown. 

 Among different living things, great differences in lon- 

 gevity obtain. Some insects live but days or even hours; 

 the great sequoia trees live for thousands of years. 



But even among closely related living things there are 

 great differences in longevity. The sequoia trees live 

 for thousands of years, but some of the firs become 

 decadent in twenty years. Ravens, crows, and parrots 

 live from fifty to a hundred years, but most other birds 

 only one or two decades. 



The whole subject of longevity is vague and few prin- 

 ciples regarding it can be formulated. Those creatures 



