==The Reign of Protoplasm.==—­This substance protoplasm became now naturally the centre of biological thought.  The theory of protoplasm arose at about the same time that the doctrine of evolution began to be seriously discussed under the stimulus of Darwin, and naturally these two great conceptions developed side by side.  Evolution was constantly teaching that natural forces are sufficient to account for many of the complex phenomena which had hitherto been regarded as insolvable; and what more natural than the same kind of thinking should be applied to the vital activities manifested by this substance protoplasm.  While the study of plants and animals was showing scientists that natural forces would explain the origin of more complex types from simpler ones through the law of natural selection, here in this conception of protoplasm was a theory which promised to show how the simplest forms may have been derived from the non-living.  For an explanation of the origin of life by natural means appeared now to be a simple matter.

It required now no violent stretch of the imagination to explain the origin of life something as follows:  We know that the chemical elements have certain affinities for each other, and will unite with each other under proper conditions.  We know that the methods of union and the resulting compounds vary with the conditions under which the union takes place.  We know further that the elements carbon, hydrogen, oxygen, and nitrogen have most remarkable properties, and unite to form an almost endless series of remarkable bodies when brought into combination under different conditions.  We know that by varying the conditions the chemist can force these elements to unite into a most extraordinary variety of compounds with an equal variety of properties.  What more natural, then, than the assumption that under certain conditions these same elements would unite in such a way as to form this compound protoplasm; and then, if the ideas concerning protoplasm were correct, this body would show the properties of protoplasm, and therefore be alive.  Certainly such a supposition was not absurd, and viewed in the light of the rapid advance in the manufacture of organic compounds could hardly be called improbable.  Chemists beginning with simple bodies like CO_{2} and H_{2}O were climbing the ladder, each round of which was represented by compounds of higher complexity.  At the top was protoplasm, and each year saw our chemists nearer the top of the ladder, and thus approaching protoplasm as their final goal.  They now began to predict that only a few more years would be required for chemists to discover the proper conditions, and thus make protoplasm.  As late as 1880 the prediction was freely made that the next great discovery would be the manufacture of a bit of protoplasm by artificial means, and thus in the artificial production of life.  The rapid advance in organic chemistry rendered this prediction each year more and more probable.  The ability of chemists to manufacture chemical compounds appeared to be unlimited, and the only question in regard to their ability to make protoplasm thus resolved itself into the question of whether protoplasm is really a chemical compound.