278 FKAGMENTS OF SCIENCE. 



The great source of error here has been already 

 alluded to in this discourse. The observers worked in 

 an atmosphere charged with the germs of different or- 

 ganisms; the mere accident of first possession render- 

 ing now one organism, now another, triumphant. In 

 different stages, moreover, of its fermentative or putre- 

 factive changes, the same infusion may so alter as to be 

 successively taken possession of by different organisms. 

 Such cases have been adduced to show that the earlier 

 organisms must have been transformed into the later 

 ones, whereas they are simply cases in which different 

 germs, because of changes in the infusion, render them- 

 selves valid at different times. 



By teaching us how to cultivate each ferment in its 

 purity in other words, by teaching us how to rear the 

 individual organism apart from all others, Pasteur 

 has enabled us to avoid all these errors. And where 

 this isolation of a particular organism has been duly 

 effected it grows and multiplies indefinitely, but no 

 change of it into another organism is ever observed. 

 In Pasteur's researches the Bacterium remained a Bac- 

 terium, the Vibrio a Vibrio, the Penicillium a Penicil- 

 lium, and the Torula a Torula. Sow any of these in a 

 state of purity in an appropriate liquid; you get it, and 

 it alone, in the subsequent crop. In like manner, sow 

 small-pox in the human body, your crop is small-pox. 

 Sow there scarlatina, and your crop is scarlatina. Sow 

 typhoid virus, your crop is typhoid cholera, your crop 

 is cholera. The disease bears as constant a relation to 

 its contagium as the microscopic organisms just enume- 

 rated do to their germs, or indeed as a thistle does to its 

 seed. No wonder then, with analogies so obvious and 



some cases been conducted, will do well to consult the Rev. W. 

 H. Dallinger's excellent ' Notes on Heterogenesis ' in the October 

 number of the Popular Science Review, 



