_The Association of Organisms._ At a later stage we shall have an opportunity of discussing symbiosis and allied conditions. Here it is only necessary to draw attention to a fact that is rapidly becoming of the first importance in bacteriology. When species were first isolated in pure culture it was found that they behaved somewhat differently under differing circumstances. This modification in function has been attributed to differences of environment and physical conditions. Whilst it is true that such external conditions must have a marked effect upon such sensitive units of protoplasm as bacteria, it has recently been proved that one great reason why modification occurs in pure artificial cultures is that the species has been isolated from amongst its colleagues and doomed to a separate existence. One of the most abstruse problems in the immediate future of the science of bacteriology is to learn what intrinsic characters there are in species or individuals which act as a basis for the association of organisms for a specific purpose. Some bacteria appear to be unable to perform their regular function without the aid of others. An example of such association is well illustrated in the case of tetanus, for it has been shown that if the bacilli and spores of tetanus alone obtain entrance to a wound the disease may not follow the same course as when with the specific organism the lactic-acid bacillus or the common organisms of suppuration or putrefaction also gain entrance. There is here evidently something gained by association. Again, the virulence of other bacteria is also increased by means of association. The _Bacillus coli_ is an example, for, in conjunction with other organisms, this bacillus, although normally present in health in the alimentary canal, is able to set up acute intestinal irritation, and various changes in the body of an inflammatory nature. It is not yet possible to say in what way or to what degree the association of bacteria influences their _rôle_. That is a problem for the future. But whilst we have examples of this association in streptococcus and the bacillus of diphtheria, _B. coli_ and yeasts, tetanus and putrefactive bacteria, _Diplococcus pneumoniæ_ and streptococcus, and association amongst the various suppurative organisms, we cannot doubt that there is an explanation to be found here of many hitherto unsolved results of bacterial action. This is the place in which mention should also be made of higher organisms associated for a specific purpose with bacteria. There is some evidence to support the belief that some of the Leptotricheæ (Crenothrix, Beggiatoa, Leptothrix, etc.) and the Cladotricheæ (Cladothrix) perform a preliminary disintegration of organic matter before the decomposing bacteria commence their labours. This occurs apparently in the self-purification of rivers, as well as in polluted soils.