§ v.] Origin and distribution. 39 



dead animals, and it was found that germs of one or several 

 species of this single group were present in every handful of 

 mud from the bottom of every sheet of water from the sea-level 

 to a height of 2000 metres. The actual presence of the germs 

 may be shown in all these cases by microscopic and experi- 

 mental examination, to the conduct of which we will recur 

 presently. 



As these facts again would lead us to expect, among micro- 

 scopic growths some are rare and some are common, some 

 have a limited and some a very extensive area of distribution. 

 The principle must be the same with these as with the higher 

 and larger organisms ; climatic and other external causes must 

 have a similar effect on the distribution, though for the reason 

 stated above that effect is generally less powerful than in larger 

 and more pretentious forms. The researches into this subject 

 are not yet extensive enough to permit of the production of 

 many details. But we know, for example, that a small Fungus, 

 scarcely visible to the naked eye, Laboulbenia Muscae, which 

 vegetates on the surface of the bodies of living house-flies in 

 Vienna, and which appears to be common in southern Europe, 

 does not occur in the middle and west of Europe ; at all events 

 after careful search it has not yet been found. Instances 

 of the reverse kind are more numerous. Our common species 

 of moulds, Penicillium glaucum, for example, and Eurotium, 

 are spread over all parts of the world and all climates, and the 

 same is the case with other small Fungi and Algae. 



In this point also Bacteria are only special instances of the 

 series of phenomena which have been shown above to occur 

 as a rule in small organisms. Our knowledge of the several 

 species, as appears from preceding lectures, is too imperfect to 

 enable us to make precise statements with respect to the larger 

 number of them ; at the same time we know that some species 

 are comparatively rare, such as Micrococcus prodigiosus and 

 Bacillus Megaterium, while others, like B. subtilis, B. Amy- 

 lobacter, and Micrococcus Ureae, occur in almost every situation 



