According to Malthus, populations and food supply expand in different ways. Food supply increases arithmetically, i.e. with every generation food supply increases the same amount, by, for example, bringing new land into cultivation.
This leads to a linear pattern of growth. In contrast, even if the number of children each family has remains the same, the population will grow geometrically because in each generation there will be 6 more people to have children (see Figure 6.1). As a result of these different growth rates, Malthus argued that the human population was doomed unless limits were put on population growth rates.