82 



SCIENCE 



[N. S. Vol. XXXIX. No. 994 



In each case the peaks on the curve indi- 

 cate distinct centers of variation around 

 which the intermediate fluctuations are 

 grouped and these constitute biologic facts 

 of real importance, even though the types 

 overlap and appear to blend in the valleys 

 between the modes. 



Johannsen in his recent book (1913) has 

 discussed such bimodal curves with admir- 

 able clearness and points out that obvious 

 phenotypes (externally recognizable kinds) 

 may or may not represent true genotypes 

 (characterized by germinal differences), — 



LIQUEFACTION OF 

 GELATIN BY 

 AUROCOCa 



00. 03 08 13 18 23 2B 33 36 

 LIQUEFACTION IN CM. 



Pig. 2. 



breeding experiments being the only final 

 criterion. In our bacteriological work we 

 are making breeding experiments all the 

 time and even though our study of mass 

 reactions may be crude, it is free from the 

 grosser errors due to environmental varia- 

 tions, and our bimodal curves indicate real 

 protoplasmic diversity. For example, in a 

 study of the gelatine liquefying power of 

 the orange cocci, it appeared that the depth 

 of liquefaction after 30 days varied from 

 to over 3.5 cm., each intermediate .5 c.c. 



value being represented. Yet the frequency 

 with which various grades of liquefaction 

 occurred showed that only two distinct 

 types were common (Fig. 2), one failing 

 entirely to liquefy, including 30 per cent, 

 of the strains studied, and the other lique- 

 fying rapidly, to the extent of 2.0-3.5 cm., 

 including 45 per cent, of the strains. Ordi- 

 narily such a difference in proteolytic 

 power as that between a liquefaction of 1.0 

 cm. and one of 3.0 woidd be considered im- 

 portant as marking a distinction between 

 a very slowly and a rapidly liquefying 

 type. Yet in view of the frequency curve 

 it is both practically convenient and bio- 

 logically sound to say that we are dealing 

 with two and only two distinct types, so far 

 as this character is concerned, one not 

 liquefying at all and the other liquefying 

 vigorously to an extent of 2.0-3.5 cm. in 30 

 days, while slowly liquefying strains may 

 be considered as aberrant varieties. 



Another example of this conception of 

 frequency types may be taken from recent 

 studies of the fermentative power of the 

 colon bacilli and the streptococci. Both 

 these groups have been split up according 

 to their acid-producing power in a wide va- 

 riety of carbohydrate media and any one 

 sugar has been considered just as impor- 

 tant as any other, giving almost as many 

 types as there are permutations and combi- 

 nations of the test substances used. Howe 

 (1912) has shown for the colon bacilli, and 

 the same thing is true for the streptococci 

 (Winslow, 1912), that the various carbohy- 

 drate media are not fermented at random, 

 but stand to each other in a definite "order 

 of availability" forming what Howe calls 

 a "metabolic gradient," such that if any 

 member of the series is fermented the 

 chances are that those ahead of it will be 

 fermented also. Thus in the colon group 

 dextrose is most often attacked, then lac- 

 tose, then saccharose and then raffinose. 

 Certain steps in the gradient are qualita^ 



