76 TEXT-BOOK OF BACTERIOLOGY. 



For instance, given a mixture of bacteria a putrid vegetable 

 infusion: separate the various species which it contains, isolate 

 them, and breed each separately. It will scarcely be possible to do 

 so. However carefully we go to work, however frequently we trans- 

 pose from glass to glass, however small may be the portions with 

 which we inoculate in order to obtain the greatest separation of 

 germs, we will scarcely ever succeed, even by a lucky chance. In- 

 deed, as a rule we will fail, and our investigations will be altogether 

 wanting in reliability. 



The case is almost more unfavorable still if it is necessary to 

 get one particular species, and this alone, out of the mixture. The 

 only way is to work on at a venture till good luck places the de- 

 sired bacterium at our disposal. If \ve now breed this artificially, 

 and if but one single foreign germ has found access to it, it may 

 be that the stranger finds the conditions particularly suitable to 

 him, he multiplies infinitely, pushes aside the lawful occupants of 

 the soil, overgrows them completely, and gives the culture quite a 

 new appearance. It can, therefore, readily be understood how this 

 striking phenomenon caused the serious belief in the transforma- 

 tion of one species into another. 



The fact is, we want some reliable means for dictating to the 

 germs the definite, controllable ways and limits which they are 

 not to leave or dispute without consent and aid. 



These facts are clear enough, and the great majority of investi- 

 gators have long since acknowledged their truth. The hindrances 

 and difficulties which are met at every turn in the use of liquid 

 media are, in fact, not less weighty than numerous, and although 

 the French school has not yet renounced this method of investiga- 

 tion, one must, nevertheless, pay the tribute of admiration to the 

 ability and dexterity which have enabled them to attain such great 

 results with such imperfect means. 



III. POTATO CULTURES. 



It is not surprising if even the most ingenious methods fail to 

 conquer the difficulties just described, since they lie in the nature 

 of the media themselves, yet all obstacles were removed, as it were, 

 by a single blow as soon as the employment of solid food bases 

 took the place of the liquid ones. 



The way in which the former came to be employed is sufficiently 

 curious to deserve mention. 



It was observed that when slices of boiled potato were left ex- 

 posed to the air for a time and then kept a day or two where they 



