TRANSITIONS IN MORPHOLOGICAL CHARACTERS 117 



In each of the above instances, when the Euglena was first 

 given to a single individual rotifer, sister individuals were invaria- 

 bly kept as controls and fed on the customary Paramecium diet. 

 In no case did mutation occur among the controls. Each series 

 continued to reproduce the saccate form only. 



The above cases of immediate mutation following upon changed 

 nutrition, even after long periods of pure saccate generations, and 

 also in generations varyingly removed from the resting egg, 

 offer excellent evidence that mutation depends chiefly, if not 

 wholly, upon food conditions, rather than upon internal cyclic 

 changes or any of the other factors which we had previouslj'' 

 investigated. Especially noteworthy is the fact that the addition 

 of but a single drop of the new food culture was capable of pro- 

 ducing the full transition between these markedly dissimilar types 

 of the species. 



Possible influence of matter dissolved in the water of the 

 Euglena culture 



To determine whether the above mutations were due wholly to 

 a change in food or whether they might be influenced as well by 

 substances dissolved in the water of the Euglena culture, some 

 of this medium was passed through a common filter paper, which 

 separated out the Euglena and particles of foreign matter. The 

 resulting filtrate was examined and found to contain no Protozoa. 

 Drops of this were added to a series of individual cultures; but 

 in no case was a mutation produced. Additional quantities were 

 again added with the same negative result. Hence it appears safe 

 to say that the factors which produce the mutations are really in 

 the ingested food and are not chemical substances dissolved in 

 the cultiu'e medium. 



Interchange of culture medium 



In order, however, to test still more conclusively the possible 

 influence of culture medium and its dissolved contents, the fluid 

 from ten mass cultures containing only saccate individuals and 

 from ten cultures containing the humped type was filtered in each 

 case and interchanged, culture for culture. In one instance only 



