If we were to attempt to group the names associated with the study of biology, we would find that in a general way they were connected either with discoveries of a purely scientific nature or with the benefiting of man's condition by the _application_ of the purely scientific discoveries. The first group are necessary in a science in order that the second group may apply their work. It was necessary for men like Charles Darwin or Gregor Mendel to prove their theories before men like Luther Burbank or any of the men now working in the Department of Agriculture could benefit mankind by growing new varieties of plants. The discovery of scientific truths must be achieved before the men of modern medicine can apply these great truths to the cure or prevention of disease. Since we are most interested in discoveries which touch directly upon human life, the men of whom this chapter treats will be those who, directly or indirectly, have benefited mankind. The Discoverers of Living Matter.--The names of a number of men living at different periods are associated with our first knowledge of cells. About the middle of the seventeenth century microscopes came into use. Through their use plant cells were first described and pictured as hollow boxes or "cells." But it was not until 1838 that two German friends, Schleiden and Schwann by name, working on plants and animals, discovered that both of these forms of life contained a jellylike substance that later came to be called _protoplasm_. Another German named Max Schultz in 1861 gave the name protoplasm to _all living matter_, and a little later still Professor Huxley, a famous Englishman, friend and champion of Charles Darwin, called attention to the physical and chemical qualities of protoplasm so that it came to be known as the chemical and physical basis of life. [Illustration: Prof. Tyndall's experiment to show that if air containing germs is kept from organic substances, such substances will not decay. The box is sterilized; likewise the tubes (_t_) containing nutrients. Air is allowed to enter by the tubes (_u_), which are so made that dust is prevented from entering. A thermometer (_th_) records the temperature. The substances in the tubes do not decay, no matter how favorable the temperature.] Life comes from Life.--Another group of men, after years of patient experimentation, worked out the fact that _life comes from other life_. In ancient times it was thought that life arose _spontaneously_; for example, that fish or frogs arose out of the mud of the river bottoms, and that insects came from the dew or rotting meat. It was believed that bacteria arose spontaneously in water, even as late as 1876, when Professor Tyndall proved by experiment the contrary to be true.