If the mechanisms of different things can be described by the mathematical models whose function forms are exactly same, we believe that there is a mechanism similarity among the things. The mechanism similarity may aid our scientific inquiries in mechanistic modelling. The microorganisms of degrading organic chemicals are mechanistically similar to the predator populations in the Volterra model of interacting populations, but the variance of concentration of the organic chemicals is mathematically different to that of density of the prey populations in the Volterra model. The degradation rate of organic chemicals may be described as:
-dx/dt=jx+kxm
where x is the concentration of organic chemical at time t, m is the number of microorganisms capable of degrading the organic chemical at time t, and j and k are the nonbiological (i. e. chemical) and biological degradation rate constants respectively. The contribution of biological factors to degradation rate is greater by far than that of nonbiological factors within the range of ordinary growth temperature. In this case, the equation may also be written as:-dx/dt=kxm. This paper discusses some methods to express m, thereby expressing the degradation rate as a form that can be integrated.