WHAT IS PHYSICAL LIFE 



the facts of both reproduction and hered- 

 ity. No one of the known forces can re- 

 produce itself, still less can it go through 

 the cycles of successive generations, any 

 more than matter itself can. A hereditary 

 unit of heat is as absurd as hereditary 

 helium. But life without coming by 

 hereditary descent and then going on to 

 reproduction does not exist. Both these 

 elements are as inseparable from life as 

 attraction is inseparable from gravitation. 

 But where else is there even an analogue, 

 not only to both, but to either one of them? 

 Vital reproduction is difficult enough, but 

 heredity is ten-fold more so, and we have 

 already seen to what grievous mental 

 strain some biologists have been subjected 

 in trying to explain how every cell and 

 part in an adult body exists, potentially 

 if not actively, in the single primordial 

 cell " when as yet there were none of 

 them," and exists there according to that 

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