Spontaneous Generation. 143 



Here a double line of investigation is open. One 

 fixes upon a given organism, in a medium 

 wherein that organism alone is "culti- Argument. 

 vated." The other takes them indis- 1. By Cui- 



, . , ture. 



cnmmately as they appear in natural 

 conditions, and catches nature, as some one 'has 

 said, " off her guard." In the first line of investi- 

 gation, a fitting pabulum is prepared, that is, an ex- 

 tract of meat carefully filtered from other kinds of 

 life, or an extract of fruit; and into this is admitted 

 a germ of the solitary kind of microbe which comes 

 up for examination. In this pabulum it develops 

 and multiplies. Now you have only to watch it; 

 and that is done to the extreme degree of scientific 

 solicitude by what has been called the unbroken- 

 watching system; a couple of competent observers 

 relieving one another, so that the object under in- 

 spection is never lost to the eye for a moment, not 

 even during the space of many days. % 



158. As an example of special cultivation we may 

 mention among others Dr. Koch's treatment, two 

 or three years ago, of the germ which he considered 

 to be the cause of pulmonary comsumption, for it 

 was uniformly found in the epithelium of diseased 

 lungs. To test whether this was the specific cause 

 of that disease, he took a germ from a diseased lung, 

 and placed it in a medium where it would thrive and 

 multiply, but from which every other species of mi- 

 crobe had been carefully excluded. Then it was 

 necessary to go on cultivating this microscopic 



