146 Lectures on Bacteria. [^ xiii. 



persons, and therefore to part again for an unlimited number of 

 times with the same minute portion of contagiura which he 

 himself received. But if we are forced to recognise the charac- 

 teristic qualities of living beings in these contagia, there is no 

 good reason why we should not regard them as real living 

 beings, parasites. For the only general distinction between 

 their mode of appearance and operation and that of parasites is, 

 that the parasites with which we are acquainted have been seen 

 and the contagia have not. That this may be due to imperfect 

 observation is shown by the experiments on the itch in 1840, 

 in which the contagium, the itch-mite, though almost visible 

 without magnifying power, was long at least misunderstood. 

 It was only a short time before that the microscopic Fungus, 

 Achorion, which causes favus, was unexpectedly discovered, as 

 well as the Fungus which gives rise to the infectious disease in 

 the caterpillar of the silkworm known as muscardine. Other and 

 similar cases occurred at a later time, and among them that of 

 the discovery of the Trichinae between 1850 and i860, a very 

 remarkable instance of a contagious parasite long overlooked. 

 Henle repeated his statements in 1853 in his 'Rationelle Patho- 

 logic,' but for reasons which it is not our business to examine 

 here, his views on animal pathology met with Uttle attention or 

 approval. 



It was in connection with plant-pathology that Henle's views 

 were first destined to further development, and obtained a firmer 

 footing. It is true that the botanists who occupied themselves 

 with the diseases of plants knew nothing of Henle's pathological 

 writings, but made independent efibrts to carry on some first 

 attempts which had been made with distinguished success in 

 the beginning of the century. But they did in fact strike upon 

 the path indicated by Henle, and the constant advance made 

 after, about the year 1850, resulted not only in the tracing back 

 of all infectious diseases in plants to parasites as their exciting 

 cause, but in proving that most of the diseases of plants are due 

 to parasitic infection. It may now certainly be admitted that 



