C. DOBELL 335 



By a further set of nice experiments wliich cannot here be con- 

 sidered in detail, Burri has tried to shew that the power to split 

 saccharose does not appear all at once^ Between the non-saccha- 

 rose-splitting imperfect um and the saccharose-splitting perfectum 

 intervene many generations of individuals shewing every transitional 

 stage. The power of fermenting saccharose is gradually acquired in 

 small successive steps, until it manifests its full development in the 

 actively fermenting form B. perfectum. The power once acquired is 

 never lost — it always persists in the offspring. Moreover, individuals 

 which are in a transitional stage do not lose such partial activity as 

 they have acquired. Individuals which have only " half-acquired " the 

 power of splitting saccharose- may be transplanted to a saccharose-free 

 medium. If their offspring subsequently come in contact with the 

 sugar they then acquire complete power of fermenting it in half the 

 time necessary for an ordinary imperfectum race to do so. 



Burri supposes that the power to ferment saccharose is latent in 

 every imperfectum individual, probably in the form of a zymogen or 

 pro-ferment of some sort. The enzyme is produced gradually by the 

 constant action of the sugar on successive generations. He thinks it 

 probable that the newly acquired power of attacking saccharose does 

 not rejaresent a " regeneration " of a power originally present in the 

 race but temporarily lost^ 



Ail workers whose records we have so far considered have been 

 unanimous on one point — namely, that when a race has once acquired 

 the power of fermenting a certain sugar, it remains constant in this 

 respect (i.e. " breeds true "). A few observations have been made, 

 however, which shew that a reversion to the original state may occur 

 in some races — that is, the acquired power may be subsequently lost 

 under certain conditions. An instance of this sort is to be found in 

 the work of Bernhardt and Markoff (1912). They isolated^ a coli- 

 typhosus organism (" No. 4-59 ") which behaved exactly like B. coli 

 mutabile. It grew as a blue colony on Drigalski-Conradi agar^, and con- 

 stantly gave rise to red nodules in the mother-colony. The organisms 



' The change is said to be "relatively quick, but not sudden." 



2 That they have partially acquired the power is inferred from this and other experiments. 

 There is no outward and visible sign (gas production, etc.) of this "half-acquisition." 



2 Burri will not call the change a "mutation,'' because it is a gradually acquired 

 adaptation. According to him the change is really not the acquirement of a new 

 character, but the realization of a faculty already existing potentially. This appears 

 to me, however, to be applicable to all variations — of whatever sort — in all organisms. 



■* From a patient suffering from an intestinal complaint. 



^ See p. 328, footnote. 



