The study of the chemical nature of substances derived from living organisms has developed into what has been called organic chemistry. Organic chemistry has shown that it is possible to manufacture artificially many of the compounds which are called organic, and which had been hitherto regarded as produced only by living organisms. At the beginning of the century, it was supposed to be impossible to manufacture by artificial means any of the compounds which animals and plants produce as the result of their life. But chemists were not long in showing that this position is untenable. Many of the organic products were soon shown capable of production by artificial means in the chemist's laboratory. These organic compounds form a series beginning with such simple bodies as carbonic acid (CO_{2}), water (H_{2}O), and ammonia (NH_{3}), and passing up through a large number of members of greater and greater complexity, all composed, however, chiefly of the elements carbon, oxygen, hydrogen, and nitrogen. Our chemists found that starting with simple substances they could, by proper means, combine them into molecules of greater complexity, and in so doing could make many of the compounds that had hitherto been produced only as a result of living activities. For example, urea, formic acid, indigo, and many other bodies, hitherto produced only by animals and plants, were easily produced by the chemist by purely chemical methods. Now when protoplasm had been discovered as the "physical basis of life," and, when it was further conceived that this substance is a proteid related to albumens, it was inevitable that a theory should arise which found the explanation of life in accordance with simple chemical laws.