146 
so-called ‘‘ true coli’’ not infrequently misses the mark. 
Many other instances could be quoted, but these will 
suffice. 
Therefore we think it will be conceded that it is not satis- 
factory to arrange different organisms into groups under one 
name, and that it is still more dangerous to make statements 
relative to the significance of the group as a whole, because 
these statements frequently prove incorrect for the individual 
members. 
The division of organisms into groups according to fer- 
mentation reactions is a necessary part of laboratory routine ; 
but it should not be forgotten, that this method of classifi- 
cation tells us nothing of the many characteristics of the 
organisms. Thus taking the group of lactose fermenters as a 
whole, they may be split up into sub-groups innumerable, 
but it is none the less necessary to separate species and to 
study their natural characteristics. The danger in general- 
izing, with the fermentation method as a basis of classifi- 
cation, applies to all groups of organisms, including those 
suggested by MacConkey. 
It may reasonably be asked how does it come about that 
English methods have given such satisfactory results as to 
lead to their general adoption. The answer is that in waters 
in England practically any trace of pollution is dangerous. As. 
we have pointed out in the first chapter, there is not time 
for any very elaborate self-purification to go on in most 
natural sources, and a more detailed examination of the 
process of storage on the lines suggested by us would probably 
show that the results obtained in England resembled those 
in this country. Houston’s figures quoted in Chapters 
VI—X, undoubtedly confirm this statement. At any rate it 
may be stated that the problem is so entirely different in the 
two countries that it would be ridiculous to suppose that 
English standards are suited to the tropical conditions. The 
table given below gives the results in three quarters of what 
would occur if English standards were applied to the waters 
in this country. 
