1. Living things have a need for various substances, such as vitamins and minerals, which they use to produce energy for growth, mobility, and life itself. All living things require food. Plants make their own food, while animals eat plants and other animals. Food is the source of materials for building protoplasm in the cells, and some foods contain substances that regulate the chemical changes taking place in cells.
Populations of animals require a continuing food supply. The supply is the amount of food in a given environment, while the demand is the amount of food needed by the population. When demand exceeds supply, famine may result. So long as the supply does not exceed demand and can be replenished on a schedule that matches demand, the population will flourish.
The body has certain nutritional needs, meaning requirements for the compounds that are present in food. Nutrients are those substances that nourish the body in one of three ways: 1) they provide the body with fuel for energy; 2) they supply materials for the building of body tissues; and 3) they furnish the materials that help the work of cells. The nutrients needed by the body come in different groups--carbohydrates, fats, proteins, vitamins, and minerals.
These are the nutrients needed by animal populations, and the question of supply and demand applies to each of these nutrients. Organisms may know instinctively which specific foods supply the needed materials to the body and seek these foods out, and so long as the supply matches the demand, a population will be able to sustain itself. The lack of certain nutrients can lead to deficiency disease, indicating that the body has a need that has not been met.
2. Malthus is noting the fact that population is a matter of multiplication while subsistence is a matter of arithmetic, or addition. By this he means that the food supply will grow in an arithmetic fashion, with the growth proceedin...