1. In the absence of foxes, the rabbit population grows at a rate proportional to its current population; thus when with .

2. In the absence of rabbits, the foxes die out; thus when with .

3. The number of encounters between the species is proportional to the product of their populations. Each encounter tends to increase and decrease . Thus the growth rate of includes a term of the form and that of includes a term of the form , where and positive. The parameters , , , and are independent of . These assumptions lead to the equations:

,

.