At a certain time in the earth’s history, after the hard outer crust had been formed, it became possible for living materials to arise and for simple primitive creatures to exist.  Thus began the process of organic evolution—­the natural history of living things—­with which we are concerned in this and later addresses.  Organic evolution is thus a part of the greater cosmic process.  As such it does not deal with the origin of life, but it begins with life, and concerns itself with the evolution of living things.  And while the investigator is inevitably brought to consider the fundamental question as to the way the first life began, as a student of organic forms he takes life for granted and studies only the relationships and characteristics of animals and plants, and their origins.

But even as a preliminary definition, the statement that organic evolution means natural change does not satisfy us.  We need a fuller statement of what it is and what it involves, and I think that it would be best to begin, not with the human being in which we are so directly interested, nor even with one of the lower creatures, but with something, as an analogy, which will make it possible for us to understand immediately what is meant by the evolution of a man, or of a horse, or of an oak tree.  The first steam locomotive that we know about, like that of Stephenson, was a crude mechanism with a primitive boiler and steam-chest and drive-wheels, and as a whole it had but a low degree of efficiency measured by our modern standard; but as time went on inventive genius changed one little part after another until greater and greater efficiency was obtained, and at the present time we find many varied products of locomotive evolution.  The great freight locomotive of the transcontinental lines, the swift engine of the express trains, the little coughing switch engine of the railroad yards, and the now extinct type that used to run so recently on the elevated railroads, are all in a true sense the descendants of a common ancestor, namely the locomotive of Stephenson.  Each one has evolved by transformations of its various parts, and in its evolution it has become adapted or fitted to peculiar circumstances.  We do not expect the freight locomotive with its eight or ten powerful drive-wheels to carry the light loads of suburban traffic, nor do we expect to see a little switch engine attempt to draw “the Twentieth Century Limited” to Chicago.  In the evolution, then, of modern locomotives, differences have come about, even though the common ancestor is one single type; and these differences have an adaptive value to certain specific conditions.  A second illustration will be useful.  Fulton’s steamboat of just a century ago was in a certain true sense the ancestor of the “Lusitania,” with its deep keel and screw propellers, of the side-wheel steamship for river and harbor traffic like the “Priscilla,” of the stern-wheel flat-bottom boats of the Mississippi, and of the battleship, and the tug boat.  As in the first instance, we know that each modern type has developed through the accumulation of changes, which changes are likewise adjustments to different conditions.  The diversity of modern types of steamships may be attributed therefore to adaptation.