32 ADAPTATION AND DISEASE 



colonies at first show no change from the usual type of growth, 

 but after several days little papillae appear on the surface of 

 the colonies. These have a more opaque white appearance, 

 and, if litmus has been added to the medium as an indicator, 

 the papillae take it up and assume a reddish colour, this indicat- 

 ing the production of acid in the papillae, whereas it is not being 

 produced elsewhere in the colonies. The Typhoid bacillus, for 

 instance, does not normally ferment isodulcite, but if the Typhoid 

 bacillus be planted upon isodulcite agar, in five days these 

 papillae appear and in these papillae the bacilli are " mutants " ; 

 they have gained the power of fermenting this sugar. Test the 

 bacilli from the non-papillated portions of the colonies, and they 

 have gained no such power. If bacilli be carefully removed from 

 one of the papillae and be plated on isodulcite agar, it is found 

 that a race of Typhoid bacilli has been gained, all the members 

 of which have this new power of immediately fermenting iso- 

 dulcite. The bacilli constituting the non-papillated areas are 

 unchanged ; they cause no dissociation of the isodulcite, although 

 if plated similarly on isodulcite agar, their colonies will repeat 

 the phenomenon : in five days scattered papillae will show them- 

 selves which present the new property. 



These, again, at first appear to be excellent examples of 

 what may be termed true or " chance " mutations. Obviously 

 not all the bacilli in the colonies take on the power of utilizing 

 isodulcite, dulcite, or other sugar, as a food-stuff, only certain 

 individuals. But on further study the further facts are to be 

 noted first, that the phenomenon is constant take any typhoid 

 bacillus, treat in this way, and in five days or so the dulcite- 

 fermenting papillae will show themselves ; and, secondly, these 

 dulcite -fermenting individuals present themselves constantly 

 in a particular relationship and superficial position. Bacilli 

 which are at or near the surface of the agar and obtain from it 

 abundant food have had no need to adapt themselves to the 

 new food-stuff. It is those furthest away, separated from the 

 agar by several layers of bacilli, that manifest this new property, 

 those which from their position receive only the diffused food- 

 stuffs which have passed by their more fortunately placed 

 comrades lower down. It looks very much as though, in the 

 struggle for existence, these bacilli that have found themselves 

 deprived of their accustomed food-stuffs have taken to absorbing 



