There are two principal causes which produce very rapid growth of bacteria. The first of these is the presence in the medium of quantities of material upon which the organisms can feed; the second important factor is the temperature. All organisms, however, do not behave the same when at the same temperature. It has been found that organisms which grow in cold storage plants do not develop rapidly if at all at blood heat. On the other hand, organisms which develop normally at blood heat do not develop rapidly at a freezing temperature. It is believed that all these organisms, rapidly adapt themselves to their environments, so that after a few generations the organism which is normally grown at blood heat may develop at a low temperature, and vice versa.