1 74 Lectures on Bacteria. [^ xiv. 



12. In conclusion we must not omit to remark, that in a 

 large number of infectious diseases, and these too of the most 

 common occurrence, no one has as yet succeeded in discovering 

 a distinct Bacterium or any other microscopic parasite as the 

 cause of the particular disease, or the supposed discovery of 

 such organisms is quite untrustworthy. This is true of diarrhoea, 

 of typhus fever, of yellow fever, of whooping-cough, and of 

 acute exanthemata of the skin, such as scarlet fever, measles, 

 and small-pox in men and equivalent diseases in other animals. 

 We practise vaccination, as is well known, as a protection 

 against small-pox, and Pasteur applies his famous method for 

 attenuating the contagium of hydrophobia, for protective inocu- 

 lation and for curing the infected, but the organism which may 

 possibly be the real contagium has hitherto at least escaped 

 observation. It is not really necessary to repeat that Henle's 

 postulates remain unchanged as against these negative results 

 in the search for the contagium vivum. 



XIV. 



Diseases caused by Bacteria in the lower animals 

 and in plants. 



I. There is reason for assuming that Bacteria play a more 

 important part as disease-producing parasites in cold-blooded 

 as well as in warm-blooded animals, than is at present ascer- 

 tained. What we do know at present is chiefly connected with 

 insects (76). 



The foul brood in bees, which may in a short time destroy 

 the hives of a whole district, is the work of an (endosporous) 

 Bacillus, B. melittophthorus, Cohn, in all probability the same 

 species as B. alvei, which has been carefiilly studied by Cheshire 

 and Cheyne. 



The disease in silkworms known as flacherie is caused, 



