The Limitations of the Method of Pure Culture. 121 
containing vast groups of organisms morphologically indistinguish¬ 
able from each other, and only to be separated by their behaviour, 
it is necessary to obtain the object of study absolutely alone. Of 
course the same necessity holds for all cases in which the nutrition 
of the organism is the subject of investigation. It is much to be 
desired that the chemical physiology of many of the lower organisms, 
about whose nutrition we have little or no exact knowledge, should 
be worked out in this way. 
The study of bacteria, in so far as it has hitherto proved 
successful, has concerned itself almost entirely with the investigation 
of the group in strictly pure culture, and the enormous amount of 
valuable research, completed in so extraordinarily short a time, is 
largely due to the fact that this modern branch of science was 
attacked, in this respect, in so scientific a spirit. 
At the same time it should be recognised that the pure culture 
of an organism does not always furnish a satisfactory condition for 
the complete elucidation of its physiological activity. The bacteria 
which have been studied with the greatest enthusiasm are certain 
specialised groups possessing pathogenic properties when living as 
parasites upon certain animals; and in many such cases the pure 
culture method seems very adequate. For when developing to 
such an extent as to cause disease in the highly specialised 
medium provided by the host, such organisms are existing in a 
culture that is practically pure ; hence the study of these bacteria 
in pure culture is an approximation in this respect, to the conditions 
obtaining in nature, or at least in that part of their life-history 
of most importance to us. 1 
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 dis¬ 
appointed 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 pro¬ 
cesses 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 
1 Of course the life-history of pathogenic bacteria outside the 
host is also a subject of very great importance, and to this 
different conditions necessarily apply. 
