262 BIOGENESIS AND ABIOGENESIS VIII 



whether we look at it from a practical or from a 

 theoretical point of view. A parasite may be 

 stamped out by destroying its germs, but a patho- 

 logical product can only be annihilated by 

 removing the conditions which give rise to it. 



It appears to me that this great problem will 

 have to be solved for each zymotic disease 

 separately, for analogy cuts two ways. I have 

 dwelt upon the analogy of pathological modifi- 

 cation, which is in favour of the xenogenetic 

 origin of microzymes; but I must now speak 

 of the equally strong analogies in favour of the 

 origin of such pestiferous particles by the ordinary 

 process of the generation of like from like. 



It is, at present, a well-established fact that 

 certain diseases, both of plants and of animals, 

 which have all the characters of contagious and 

 infectious epidemics, are caused by minute organ- 

 isms. The smut of wheat is a well-known instance 

 of such a disease, and it cannot be doubted that 

 the grape-disease and the potato-disease fall 

 under the same category. Among animals, 

 insects are wonderfully liable to the ravages of 

 contagious and infectious diseases caused by 

 microscopic Fungi. 



In autumn, it is not uncommon to see flies 

 motionless upon a window-pane, with a sort of 

 magic circle, in white, drawn round them. On 

 microscopic examination, the magic circle is found 

 to consist of innumerable spores, which have been 



