7S JIICRO-ORGANISMS FROM THE BRISBANE AIR. 



observation. Watching these from day to day they will be seen 

 to increase in size ; new specks will appear in other parts of 

 the plate ; and differences between the colonies soon manifest 

 tliemselves. Every day increases their diversity, and at the end 

 of a week or longer the plate displays the variegated condition 

 of those now exhibited. Every germ, where it has fallen, has 

 given rise to its own characteristic growth, and we get as a 

 result a small garden of various lowly organised vegetable 

 forms, as diversified as the weeds which spring up in a fallow 

 field. 



The larger growths on these plates are readily recognised 

 as those of the filamentous fungi commonly known as moulds. 

 These occur in considerable variety, but as they are not the 

 object of the present research, no attempt was made to study 

 them minutely, although they are certainly well worthy of 

 investigation. 



We will now turn our attention to smaller growths on our 

 plate which, even by the naked eye, are readily distinguishable 

 from those of the mould fungi. These are circular, sometimes 

 irregularly lobed or crenated at the circumference, more or less 

 raised from the surface, and very variable in colour. They are 

 colonies, each consisting of an immense number of a particular 

 species of bacterium developed from parent organisms which 

 have fallen on to our plate from the air. Some of these growths 

 are whitish, some yellow, some more or less vivid orange, some 

 pink. We can readily infer that they are of various species. 

 But for the investigation of these special care is necessary, 

 for we are here dealing with the most minute forms of life 

 known to exist, and it is quite impossible to differentiate 

 the species by morj)hological characters only. So simple are 

 they in structure that it will need the most thorough 

 examination by various methods of cultivation to decide whether 

 any two colonies we may pick out belong to the same or different 

 species. Our first object must be to obtain a pure culture of 

 each sort. For this it might seem sufficient to at once inoculate 

 our various culture media from the different growths. But, 

 though this is sometimes successful, I have found it a very 

 uncertain and untrustworthy method. It not unfrequently 

 happens that two or more parent bacteria of different species 



