I found in studying the bacteria of dog dung, that the species existing in the _fresh_ dung, which developed in ordinary plate cultures, appeared to belong to four or five species only, mostly bacilli. At the end of two or three weeks, the original species had given place to others, mostly cocci, in a very similar way to the change which takes place in putrefaction. In fact, many of the organisms are identical with those which cause putrefaction. It will be seen, therefore, that no single species produces the complex chemical and physiological changes which take place, or the bodies necessary for the bating of skin, as some observers have supposed; but the various species succeed one another as the medium changes its reaction and composition, until finally the organic portion is resolved into the simplest bodies such as carbon dioxide, ammonia, and hydrogen. There is thus a moment when the dung is at its best so far as the bating action is concerned, and this moment is due to the vital activity of bacteria, and consequently varies according to the temperature and some other influences (electrical condition of the atmosphere, etc.). One may say it is at its best at about fifteen days in summer, and one month or more in winter. Puer which has been dried, is not so powerful in its action as that which is immediately made into a paste with water. It appears to lose its “nature,” partly owing to irreversible dehydration processes, and partly because some of the bacteria are killed. Plate cultures on agar from fresh puer (Fig. 17), and from a puer wheel in use (Fig. 18), show the number of bacteria in the puer wheel to be much greater than in the fresh puer. Whence it is evident, that the bacteria continue to develop in the puer and to produce their various products, enzymes, etc. We have already considered the action of the chemical products, and in Chapter V. we propose to discuss the action of enzymes. [Illustration: Fig. 18.--Plate Culture from Puer Wheel.] [Illustration: Fig. 19.--B. Putrificus.]