CH. x] SUMMARY 149 



dulcite is found to persist afterwards, on ordinary media, 

 " permanently." 



This observation, based on the results of laboratory experi- 

 ments, provides a clue, as Adami observes, to the nature of the 

 process by which new races of bacteria are developed. In the 

 laboratory organisms can be exposed to certain modifying 

 influences for many months or even years and the new char- 

 acters developed by such means are found to persist for long 

 periods before reversion takes place. In nature agencies which 

 possess the power of modifying the characters of bacteria may 

 exert their influence for an indefinite period and the process 

 of reversion in this case may be indefinitely postponed. In 

 other words the new characters developed may appear to be 

 permanent. A variant, however, may retain its new characters 

 indefinitely and show no tendency whatever to revert under 

 ordinary conditions of growth and yet it may still be capable 

 of reverting immediately under suitable conditions. Examples 

 of this are common in the laboratory and may be found in 

 nature. Laurent describes a decolourised strain of B. ruber 

 which was grown for 12 months at a temperature of 25-35 C., 

 being subcultured 32 times in this period, without once show- 

 ing any trace of pigment. On lowering the temperature to 18C. 

 pigmentation at once reappeared. Again, the diphtheria ba- 

 cillus is far removed from its mycelial ancestry but under 

 suitable conditions will still display a partial reversion to a 

 mycelial structure. We do not on this account deny the title 

 of " species " to the diphtheria bacillus, for we recognise that 

 the idea of absolute permanence in character is not essential 

 to our conception of a species in the case of bacteria. It is not 

 permanence in character but the degree of resistance to al- 

 teration in character displayed by an organism that determines 

 our opinion of its specific nature. In spite of the many minor 

 variations they display there is exhibited by most species of 

 bacteria a resistance to modification a " vis inertia " which 

 constitutes true racial stability. 



We have seen, then, that the difference between variation 

 and transmutation is one of degree alone. It is a question of 

 the extent of the modification and the degree of permanence 



