234 THE PLANT WORLD. 



That this is true, it may at first be somewhat difficult to appre- 

 hend. The ashes and maples, it may be said, grow peacefully, and 

 in their tranquil shade there is nothing that smacks of struggle. 

 And when contemplating one of our ash-trees, red maples, or 

 box-elders upon the fertile soil of one of our central states, along 

 the roadside or in the meadow, where any spot, almost, whereon 

 a seed might chance to fall, would furnish the proper conditions 

 for germination and growth, it is hard to believe that in the past 

 the ancestors of these trees were subjected to severe hardships, 

 owing to overcrowding upon small tracts, and through natural 

 selection, were driven to produce the winged fruits by means of 

 which the species might become broadly dispersed over wide 

 regions, and thus escape extinction. It may be held that even a 

 gentle breeze can waft an actually wingless fruit to a suitable 

 spot. But he who argues thus is thinking of present and local 

 conditions only. 



Now let us look at one of these trees under another and a dififer- 

 ent environment. Upon the great Pine Ridge Indian reservation 

 in South Dakota, for example, are numerous streams and " draws " 

 or' coulees (which are nothing more or less than ravines or gullies 

 free of water except in time of flood and rain) which, as a rule, 

 are far below the general surface of the adjacent country, and 

 often miles from one another. Upon the flood-plains of such 

 streams, and in the " draws " grow ash-trees and box-elders in 

 company with trees of a few other species. Nowhere on the 

 high, dry hills do they or any other trees, except pines, grow, 

 although I have often found their wind-tossed samaras there ; and 

 if germination takes place, the seedlings are doomed to perish. 

 Along the draws and other water-courses all the various kinds of 

 trees that grow in such places are crowded together in dense and 

 tangled masses ; wherefore it is to the advantage of a given spe- 

 cies that its seeds shall be carried to a " draw " or creek-plain 

 where the chances of life are more favorable. Now let us suppose 

 the ancestor of our box-elder and maples, on the one hand, or that 

 of our ash-trees on the other, growing in an isolated " draw " amid 

 vast hills barren of deciduous trees and unfavorable to their 

 growth. Its fruits as yet are wingless, but often vary towards 



