56 ADAPTATION AND DISEASE 



mere momentum : the functional activity once started, at least 

 in the order of events which come under my ken, continues too 

 long to be comparable with physical momentum. I think I 

 see the setting in motion of a serial or cyclic process of inter- 

 cellular reactions and counteractions, the one reaction starting 

 the other a process not contemplated by the physicist when 

 he speaks of either inertia or momentum. I prefer therefore 

 to employ for the present a non-committal term, and to speak 

 of the " law of habit." 



Once again, we see this law in action from the bottom to the 

 top of the tree of life. Take, for instance, a culture of a pig- 

 ment-producing microbe, such as the B. prodigiosus, which 

 grows normally at room temperature, producing on moist bread 

 and other farinaceous foods colonies and plaques that are blood- 

 red : subject the culture for a few minutes to a temperature 

 just below that which will destroy the bacilli, or to a temperature 

 which kept up for another five minutes would kill all the members 

 of the colony. Now from this tube make cultures upon potato 

 flour or starch paste in fact, upon the most favourable medium 

 and these first cultures are absolutely colourless. According 

 to the temperature chosen and the time of exposure so, it may 

 be that in three or four days the colonies assume a faint pinkish 

 tinge, or it may be that repeated cultures and weeks of growth 

 on the optimum medium and at optimum temperature have 

 to happen before the pigment production is re-established, 

 thousands of colourless generations having elapsed. More is 

 needed than merely optimum environment. It is as though 

 the biophoric molecules of the bacilli have been converted to a 

 simpler constitution by the heat, or certain specific side-chains 

 destroyed, or it may be certain enzymes in the bacterial body, 

 so that, while the living molecules are still capable of multiplying, 

 the habit of pigment production is lost, and although all the 

 components are present in the foodstuffs presented, only very 

 gradually are they built together to produce the pigment. And 

 the same is equally true, as I have already indicated, with regard 

 to positive acquirements : it takes days and sometimes weeks 

 before bacteria take on the active dissociation of a foreign sugar, 

 or glucoside, or alcohol, or fat, but once this power is acquired 

 it is apt equally to be retained for generations in the absence of 

 that particular fermentescible substance, just, to recall, as the 



