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and other parasites, and this flora in many instances is constantly subject 
to changes caused by the deposition of other varieties and by dust and 
rain. ven in large bodies of water, like the Manila city supply, the 
bacterial flora changes from time to time, but amoebe are almost always 
present. | 
In all these situations there undoubtedly exists a symbiosis between 
the amoebae and some (but not necessarily all) of the other micro- 
organisms present. ‘This association is not as fixed as it becomes in 
amcebe after cultivation, because the changing flora makes it necessary 
for this symbiosis also frequently to change. Work with cultures has 
shown positively that amocebe are capable of such alterations provided 
they are brought about gradually. 
The proof that a symbiosis with other organisms is present in all 
conditions where amoebe are found outside of certain situations in the 
bodies of the higher animals and that this association is often a chang- 
ing one which necessitates constant readaptation of the amcebe to new 
conditions, is important because of the bearing it will have in the study 
of the higher symbioses. Most of this proof will be furnished when 
symbiosis in cultures is discussed, but a few points may properly be 
emphasized here. 
If pure cultures are made of all the different organisms found in a 
body of water which, for example, contains the usual assortment of 
amoebe and bacteria, amoebe from the original source will grow well in 
some of these cultures, poorly in others, and not at all in the remainder. 
If this experiment be repeated several times with the same cultures 
and the same water, it will sometimes be found that divers bacteria will 
form the best cultures at different times or that amcebe will develop on 
two or more widely different bacteria at the same time. This fact most 
probably indicates the presence of different strains of amcebe—that is, 
amcebe varying in the sense that the symbiosis is different and that 
they may be found in the same body of water at the same time. It is 
further emphasized by the following experiment: 
Plate cultures of all the cultivable bacteria, ete., are made from a body of 
water, which also contains amebe. Four large flasks of the water are now 
taken and sterilized to kill all microérganisms, including amebe. Two of these 
flasks are inoculated with the bacteria originally grown from the water and the 
other two left sterile. After standing a few hours to allow the bacteria to 
develop, the flasks are inoculated as follows: 
One sterile flask and also one of those containing bacteria, with a small quantity 
of water from the original source (which water contains both amabe and bac- 
teria). The other two flasks are inoculated with water from some second source, 
which water also contains ameebe. 
The amoebe from the first water will develop in both flasks in which. 
they have been inoculated, whereas those from the second source will 
often develop only in the sterile flasks. At times, the natural symbiosis 
of the ameebe from both sources is so similar that they will develop in 
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