CONSTANCY OF CHARACTERS. 



Iii any event, the student must have a considerable body of knowledge, gained 

 by actual experiment, before his judgment is worth much. In the beginning he is 

 apt to depend too much on the constancy of organisms and is certain to be misled 

 by names. To illustrate : To him all agar is agar and all gelatin is gelatin. Not 

 so, perhaps, to the organism with which he is experimenting. Slight differences in 

 the composition of a culture-medium sometimes make considerable difference in 

 the growth and general appearance of the bacteria, and this must be taken into 

 account. After the student has passed this stage of development he can interpret 

 his results much better. If, then, on some culture-medium he obtains results slightly 



Fig. 145* 



different from those already published by some author, he is not immediately driven 

 to suppose (i) that he has a new species, or (2) that the earlier writer was in 

 manifest error. Other hypotheses now lie open to him. He is dealing with a 

 living and variable organism, and perhaps the conditions in his experiment are not 

 precisely like those to which it was subjected by the previous experimenter. It 

 may also be an organism which has already varied into many races having slightly 

 different peculiarities. Only when full weight has been given to these possibilities 

 is he entitled to fall back on the others. On the other hand, however, he must not 



FIG. 145. Colonies of Bacillus aroideae, radiatc-fimbriate when grown on an agar plate at 25 C. 

 Photograph by Townsend. 



