CHAP PER Xx 
THE POSITION OF CERTAIN FHCAL ORGANISMS IN NATURE. 
THE previous three chapters have shown that in the 
group of lactose fermenting organisms there is considerable 
difference between the various members of that group in the 
power which they possess to resist sunlight. At one time 
in the course of our work it was hoped that we might be 
able to place each organism in a graded list according to its 
power of resistance. We have come to the conclusion during 
the past three years that this is impossible, because amongst 
intestinal organisms generally, many appear to be about equal 
in this respect, or in other words, in Class II (the medium 
resistant group) the difference, if it exists, is so slight as to 
be of no practical use. It is, therefore, only such organisms 
as show very marked differences in their powers of resistance 
that are likely to be of any use to the water analyst. We 
shall now discuss these organisms and say anything about 
their position in nature that has been ascertained up to this 
time. We shall take susceptible organisms first. 
Colt Communis of Escherich. 
By far the most important of these susceptible organisms 
is the coli communis of Escherich, bacillus Schafferi and 
certain other varieties (which only differ from each other by 
the positive or negative reactions of indol and motility). 
They are without doubt one of the most important group in 
human and animal feces, and, as far as we know, this 
organism is found in the feeces of practically all animals. In 
tropical countries, however, it is distinctly a rare organism, 
but the frequency with which it is isolated varies very 
much; speaking generally it is much commoner in the 
rains than in the hot weather. This little group appears 
to be very susceptible to adverse influences. In our sunlight 
experiments we have never yet found that they survive many 
hours. In our work on naturally polluted lakes and in artifi- 
