144 Cells; or, Evolution. 



thing, letting it grow, while modifying its surround- 

 ings in various ways, to see what would become of 

 it, and whether perhaps it would turn into some- 

 thing else. Dr. Koch cultivated his stock of 

 phthisis microbes so far that they lost nearly all 

 their virus, and were brought to the very verge of 

 sterility, and still the stock remained unchanged in 

 species. Identifying it thus as a distinct species, 

 he wanted to find out further, whether the identical 

 disease of consumption could be communicated by 

 it to a healthy lung. Now the cat is understood to 

 be particularly exempt from the attacks of this dis- 

 ease. So he communicated the microbe to a cat. 

 The animal became consumptive. And thus he 

 proved the disease to be contagious. 



159. In the case before us, the object is merely 

 to see whence it is that the given organism under 

 view takes its rise; what is its life-history, or life- 

 cycle; and whether it has the character of a spe- 

 cies remaining unchanged, as like always produces 

 like. To cite the results of the Rev. Dr. Dallin- 

 ger's observations on the bacterium termo, which he 

 identifies as the exciting cause of all putrefaction, 

 just as the yeast-plant is the exciting cause of what 

 is commonly called fermentation, " these organ- 

 isms, lowly and little as they are, arise in fertilized 

 parental products. There is no more caprice in 

 their origin than in that of a crustacean or a bird." 

 Referring to the negative proof given before against 

 spontaneous generation, he says: " By experiment 



