protoplasm to be not one, but more likely a mixture of several substances.  But although it was more complex than any other substance studied, its general characters were so like those of albumen that it was uniformly regarded as a proteid; but one which was of a higher complexity than others, forming perhaps the highest number of a series of complex chemical compounds, of which ordinary proteids, such as albumen, formed lower members.  Thus, within a few years following the discovery of protoplasm there had developed a theory that living phenomena are due to the activities of a definite though complex chemical compound, composed chiefly of the elements carbon, oxygen, hydrogen, and nitrogen, and closely related to ordinary proteids.  This substance was the basis of living activity, and to its modification under different conditions were due the miscellaneous phenomena of life.

(c) Significance of Protoplasm.—­The philosophical significance of this conception was very far-reaching.  The problem of life was so simplified by substituting the simple protoplasm for the complex organism that its solution seemed to be not very difficult.  This idea of a chemical compound as the basis of all living phenomena gave rise in a short time to a chemical theory of life which was at least tenable, and which accounted for the fundamental properties of life.  That theory, the chemical theory of life, may be outlined somewhat as follows: 

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.