WHAT IS PHYSICAL LIFE 



that more than a tithe of the laws of life 

 have been as yet thoroughly discovered and 

 known. 



Because it was not from lack of sus- 

 ceptibility on the part of unicellular or- 

 ganisms to every alleged influence of en- 

 vironment that their unchangeableness 

 was due, for their living forms now show 

 as much sensitiveness to such influences as 

 do other living forms, and hence presum- 

 ably they always have done so. Among 

 bacteria, for example, a severe struggle 

 is always going on between different 

 kinds for possession of the field. One of 

 the greatest difficulties of the bacteriol- 

 ogist, when he tries to isolate any one form 

 on his culture medium, is to prevent it from 

 being invaded by other bacteria which soon 

 exterminate his chosen kind. It is the 

 word, contamination, which vexes this sci- 

 entific experimenter's soul, more than that 

 of any gardener when he sees weeds multi- 

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