The population of hares evolves due to births and deaths exactly as a bacteria population: $$ \Delta H = a \Delta t H^n $$ However, hares have an additional loss in the population because they are eaten by lynx. All the hares and lynx can form \( H\cdot L \) pairs in total. When such pairs meet during a time interval \( \Delta t \), there is some small probablity that the lynx will eat the hare. So in fraction \( b\Delta t HL \), the lynx eat hares. This loss of hares must be accounted for. Subtracted in the equation for hares: $$ \Delta H = a\Delta t H^n - b \Delta t H^nL^n$$