BACTERIA. FUXGI. 163 



That numerous diseases affecting men and animals are caused by bacteria is 

 established beyond question. Indeed, the conviction is gradually gaining ground 

 that all infectious illnesses are occasioned by bacteria, and that the contagious 

 matter which used to be called virus or miasma, but as to the nature of which 

 people formerly had only very confused notions, consists of parasitic bacteria. 

 Different phenomena in organisms in which illness has been induced by infection 

 point to differences in the decompositions effected by the bacteria. But a par- 

 ticular kind of parasitic cell can only set up the same decomposition in any 

 given liquid. If, therefore, the products of separation or decomposition vary in one 

 and the same liquid, this can only be attributed to a difference in the impetus 

 causing decomposition, and therefore to a difference in the parasitic cells; in other 

 words, we are justified in assuming that every distinct infectious disease is due to 

 a special kind of parasitic bacterium. This assumption is believed to be warranted 

 even when no difference in the form of the bacteria is to be discovered which is 

 discernible to sight or demonstrable by the expedients of research. 



Most of the parasitic bacteria regarded as causes of diseases in man and beast 

 are moreover capable of being very clearly distinguished from one another by the 

 shape of their cells. The bacterium supposed to be the cause of diphtheria (Micro- 

 coccus dijihthericus) presents itself in the form of minute spherical cells crowded 

 together in close masses. The bacterium which causes anthrax in cattle (Bacterium 

 Anthracis) has straight rod-like stationary cells. In the blood of people suffering 

 from relapsing typhus, infinitesimally fine spiral filaments (Spirochaete Obermeieri) 

 are found during the fever, whilst in the intestines of cholera patients, the comma- 

 Laeilli, so frequently described, occur; and in these cases, likewise, the organisms 

 are brought into causal connection with the illnesses mentioned respectively. The 

 answer to the question as to whether parasitic bacteria are developed and propa- 

 gated in dead bodies also, thus becoming saprophyt ic and, in general, the detailed 

 description of the organisms, which are so important a factor for the weal or 

 woe of humanity, are reserved for another section. 



The second group of parasitic plants, according to the classification above given, 

 includes several thousands of different kinds of moulds, toad-stools, and Dis- 

 comycetes, which, notwithstanding great diversity in the conditions of life, dis- 

 similarity in the history of their development, and endless variety in the form of 

 their fructifications, yet exhibit great uniformity in respect of food-absorption and 

 in their methods of attacking and draining their hosts. Spores, conveyed by 

 currents of air or carried by animals, germinate under the influence of atmospheric 

 moisture wherever they happen to come to rest. Tubular thin-walled cells, called 

 hyphse, emerge from them and endeavour to grow into the stems, branches, leaves, 

 or fruits of the host, sometimes horizontally, sometimes from above downward, 

 sometimes up in the opposite direction. Many select spots where the resistance 

 offered is nil or only very weak: they grope about on the surface of the host until 

 they find a stoma, and then use it as an entrance, and so enter the passages and 

 lacuna;, of which the stomata are the orifices. Others seek out places where the 



