THE HISTORY OF BACTERIOLOGY. 447 



work reached a positive conclusion when De 

 Bary proved that parasitic moulds occurring 

 upon different plants, and till then regarded 

 as different species, belong in the cycle of 

 development of a single species, and showed 

 that such a change of host or hetercecism 

 took place among the rusts or Uredineae. 

 Bassi and Balsamo discovered in 1835 (pub- 

 lished in 1837) that the disease of silkworms 

 known as muscardine is caused by a mould, 

 and Audouin found that this mould forms 

 spores, by the help of which the germs of the 

 disease live through the winter and are able 

 to spread the disease anew the next year. 

 Schoenlein discovered the mould that causes 

 scald-head in 1839. At the same time the itch- 

 mites were discovered anew. 



On the basis of these discoveries regard- 

 ing the processes of fermentation and disease, 

 Eisenmann, and still more acutely Henle, 

 again urged (1840) the doctrine of a con- 

 tagium animatum, and with such success 

 that in spite of Virchow's opposition the doc- 

 trine henceforward never disappeared from 

 the order of the day. Another thing contrib- 

 uted also to the establishment of this doctrine, 

 namely, that the need of assuming external 

 causes of disease was making itself felt among 

 epidemiologists. Pettenkofer especially, from 



