Probable Origin of the Black Hills Forests. 225 



than now, and much of the treeless area north and west of the 

 Black Hills was doubtless forest covered. The Rocky Mountain 

 forest extended more or less completely across that strip of 

 relatively high land called the Wyoming Badlands, which stretches 

 from the Bighorn Mountains in Wyoming to the Black Hills. 

 Even to-day a few stunted yellow pines are found scattered 

 across this area. Undoubtedly across this forest isthmus came 

 all the pines of the Hills. 



From the north a tongue of forest may have stretched south- 

 ward along the isolated buttes and ridges lying in what is now 

 eastern Montana and western Dakota. If so, this slender strip 

 of forest is responsible for the presence in the Hills of the 

 white spruce and the paper birch. It may be, however, that these 

 species owe their place to the accidental agencies of birds or 

 winds, as is apparently thought by Dr. Rydberg to be the case 

 with the spruce. 



From the eastern forest long tongues extend westward across 

 the plains following the river banks and smaller water courses 

 almost to the foot of the Rockies. It is probable that most of 

 the hardwoods of the Hills reached there by this "river route," 

 ascending the Missouri and then the Cheyenne and its tributaries 

 which enter the Hills'. 



The transcontinental trees such as aspen and hackberry prob- 

 ably came up the rivers from the east in the course of their 

 journey across the continent. The Red cedar which follows the 

 uplands may have come from the east or the west or both. 



It is a law of island colonization, that of the various forms 

 which reach an island, some are rejected as unfit to cope with 

 the conditions of their new environment, and others are modi- 

 fied to conform to it. Perhaps still others well suited to the 

 insular conditions never reach them. 



These are well illustrated in the Black Hills. Lodgepole and 

 Limber pines have not found suitable conditions, and have been 

 reduced to a subordinate position. In time they will probably 

 disappear from the Hills. It is' possible that other Rocky Moun- 

 tain species crossed the forest isthmus only to be unable to get a 

 foothold or to be later driven back by the unfavorable conditions 

 they encountered in the Hills. 



The trees from the northern forest, which of all the species 

 have probably been longest in the Hills, have been more or less 



