CONSTANCV OF C 11 ARACTKKS. 



183 



In an\' event, the stndent nuist have a considerable })ody of knowhil'^L, oai/icd 

 by actual cxpcriDiciit, before liis judgment is worth much. In the l)eoinning lie is 

 apt to depend too much on the constanc)' 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 slightl)- 



Fig. 145.* 



different from tliose ahead)' published by some author, he is not immediately dri\-eu 

 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 oi'ganism, and perhaps the conditions in his experiment are not 

 precise!}' like tliose to which it was subjected by the previous experimenter. It 

 ma)' also be an organism which has already varied into many races having slightl)- 

 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, howe\'er, he must not 



Fig. 145. — Colonies of Bacillus aroideae, radiate-firabriate when grown on an agar plate at 25° C. 

 Photograph by Townsend. 



