The earliest schemes for classifying living things consisted of two groups or kingdoms – plants and animals. Organisms were assigned to one of the two groups based on motility (the ability to move spontaneously and independently) and sources of food. In the 4th century, two Greek philosophers, Aristotle and Theophrastus, studied these two groups. Theophrastus grouped plants according to stem structure and identified over 500 kinds of plants. This classification system worked only for a relatively small number of organisms. With the discovery of the microscope in the 1500s, scientists found organisms (one-celled) that had some characteristics of each group. As the number of identified organisms increased, the need for a more effective classification system was evident.