C. DoBKLi. 345 



There is little to record concerning structural variations which are 

 permanent — mutations, that is, which when they have once appeared 

 breed true in subsequent generations. Only two cases of this sort will 

 be noted here. 



First, the work of Barber (1907) on Bacillus coli^ must be mentioned. 

 This observer began with pure cultures of bacteria, and grew all his 

 sub-cultures on the same media and under the same conditions. The 

 environment was therefore alike for all individuals — as far as possible. 

 Barber noticed that there were constantly present, among the typical 

 individuals in his cultures, a small number distinguished by their 

 greater length. By a special method which he devised*, he was able to 

 select these long individuals and propagate them further. No less 

 than 140 such individuals were so isolated in one series of experiments. 

 With a single exception they all gave rise to colonies consisting of 

 individuals with the normal dimensions'. The variations were, in other 

 words, modifications — not permanent changes. In the one exceptional 

 case, however, he succeeded in obtaining a new race of long individuals. 

 This race bred true. It was kept for 32 months — being frequently 

 transplanted — without undergoing any change. Selection of maximal 

 and minimal sized individuals of this race was also without effect: 

 neither the original nor a new race could be obtained. The mutated 

 race had partially lost its motility, and differed also in certain cultural 

 characters from the original race. In another series of experiments, 

 Barber isolated 50 long individuals from another strain of B. coli. In 

 this instance he succeeded in obtaining a similar long race which 

 proved to be constant as regards this character. It was found necessary 

 sometimes to make several successive selections from the colonies of 

 mutating organisms in order to attain a pure fixed race. Altogether 

 three new long races were finally established. 



Similar experiments were only partially successful in the case of 

 B. typhosus. No constant long race was obtained. 



The mutations, it will be seen, occurred spontaneously in all these 

 cases. New races were established by the selection of individuals 

 which had already varied. It seems that the long individuals which 

 occurred in the original cultures were of two classes — though outwardly 



^ Barber made a number of similar experiments with yeasts. 



' The method is fully described in the original. It consists essentially in a direct 

 selection, under the microscope, of a desired individual by means of a very fine glass 

 capillary tube. 



' That is, in those cases in which snccessfol cnltnres resulted. In many cases, the 

 isolated individuals grew badly or not at all. 



Joom. of Gen. n 24 



