But there are many other classes of bacteria which do not

exist in a condition approaching isolation in nature. Such are the

vast groups of non-parasitic forms which carry on important

processes in the soil, in the sea, in rivers, etc. For the study of

such organisms it must be allowed that the method of pure cultures

is strikingly artificial and that one must not therefore be disappointed

if in using such a method one fails to elucidate their

natural activity, or to reproduce in the laboratory the phenomena

occurring in the field. Bacteria concerned with important processes

in the soil, for example, have been found, when studied in

pure culture, to yield most unexpected and apparently contradictory

results, which may doubtless be traced to this artificial change

of environment, and it is certain that our knowledge of these

orgauisms would be materially advanced if attempts were also

made to study them when associated with the forms accompanying

them in nature. Hitherto this has rarely been done, but the

following examples may serve to illustrate the point.

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