THE LIVING ORGANISM 11 



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 rail- 

 roads, 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 Twen- 

 tieth 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 "Lusi- 

 tania, " 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 steam- 

 ships may be attributed therefore to adaptation. 



