236 THE PLANT WORLD. 



the wind until it finds a damp place, when it uncurls, the pods open 

 and sow the seeds." In this plant the contrivance through which 

 dispersal is effected differs from that of the maples, but the end 

 to be attained is identical in both. 



Easy conditions of life cannot have impelled the ashes and 

 maples to develop key-fruits. Great difficulties have in the past 

 been encountered, and the trees that were enabled to establish 

 means of dissemination survived in the struggle for existence. 

 But the barriers to be passed over may not have been in all, or 

 even most, cases hills. Sir John Lubbock finds that the only trees 

 that bear winged fruits are forest-trees, which fact would seem 

 to indicate that such fruits in many instances have been evolved 

 in order to be carried over vast tracts of dense woodland. But 

 the theory here set forth remains unshaken, and is really thus sup- 

 ported, for the principle is the same, namely, that there were 

 areas so conditioned that germination and growth thereon was 

 impossible or unusual, and these unproductive tracts must be tra- 

 versed so that at length the seeds might find a resting-place in 

 propitious and fertile soil. 



To sum up : It is certain that key-fruits were developed in a 

 country where they became of far greater service to the trees 

 which bore them than they appear to be to the ash-trees and 

 maples in many regions of our eastern United States and else- 

 where. But I do not wish to insist that the barriers to be sur- 

 mounted were necessarily hills. They may have been broad dry 

 plains, or forest-growths of other kinds of trees, or even bodies 

 of water — it all depends upon the nature of the region where the 

 changing form first grew. 



