148 SUMMARY [CH. x 



the strain of bacteria under observation is a pure one that 

 is to say, derived from a single organism by the methods sug- 

 gested by Barber and others. 



2. The more readily a new character is "impressed " on an 

 organism the longer it is retained conversely, the more slowly 

 and reluctantly an organism takes on a new character the more 

 easily is that character lost. The behaviour of B. typhosus is 

 a good illustration. This organism can be trained to ferment 

 dulcite in a few days and will then retain the power for many 

 weeks in the absence of that sugar. It cannot be trained to 

 ferment lactose in less than two years and then loses the power 

 in a few days if the lactose is withdrawn. 



It must be understood that we are here speaking of " im- 

 pressed " variations. In the case of " spontaneous " variations 

 the reverse holds true. Bacteria which vary spontaneously 

 with great readiness often revert with equal facility, while 

 those that are tenacious of their normal characters often 

 prove tenacious of any new character they may spontaneously 

 develop. 



3. The longer an organism which has undergone a varia- 

 tion continues to be exposed to the influence which caused it, 

 the longer will the variation persist after that influence has 

 been withdrawn. 



For example, Rosenow (1912-13) isolated a streptococcus, 

 from a number of cases of general infection, possessing unusual 

 morphological and cultural characters which, however, showed 

 reversion on cultivation outside the body. He found that 

 strains isolated from the peritoneal exudate and blood at a 

 later stage in the disease showed these modifications in char- 

 acter to a greater degree than those isolated earlier in the 

 attack. 



The behaviour of B. typhosus again illustrates this point. 

 If a strain is grown on dulcite medium it acquires, in a few 

 days, the power of fermenting that sugar and this power is 

 retained for some weeks after the strain has been removed 

 from the dulcite medium. Reversion then occurs and the 

 power is lost. If however the strain is grown on a dulcite 

 medium continuously for three months the power to ferment 



