It has been aptly remarked that this first step was the greatest of all,--"for the transition from the hollow tree to the ship-of-the-line is not so difficult as the transition from nonentity to the hollow tree." The first object for obtaining motion upon the water must evidently have been to enable the navigator to cross a river,--not to ascend or descend it; as it is apparent he would not seek the means of following or stemming its current while the same purpose could be more easily served by walking along the shore. It is not difficult to suppose that the oar was suggested by the legs of a frog or the fins of a fish. The early navigator, seated in his hollow tree, might at first seek to propel himself with his hands, and might then artificially lengthen them by a piece of wood fashioned in imitation of the hand and arm,--a long pole terminating in a thin flat blade. Here was the origin of the modern row-boat, one of the most graceful inventions of man. [Illustration] From the oar to the rudder the transition was easy, for the oar is in itself a rudder, and was for a long time used as one. It must have been observed at an early day that a canoe in motion was diverted from its direct course by plunging an oar into the water and suffering it to remain there. It must have been observed, too, that an oar in or towards the stern was more effective in giving a new direction to the canoe than an oar in any other place. It was a natural suggestion of prudence, then, to assign this duty to one particular oarsman, and to place him altogether at the stern.