346 VICTOR E. SHELFORD. 



large inland body of water like Lake Michigan (Atwood and 

 Goldthwait, '08) or they may be extensive. In both cases the 

 processes proceed in a definite direction. 



In eastern North America, the topography of the Appalachian 

 region is in the main features of importance in this connection, 

 an erosion topography. In eastern Tennessee I found [as de- 

 scribed by Cowles (unpublished)] that the tops of the mountains 

 were frequently covered with conifers, the sides with oak and 

 hickory, and the bases with beech and maple. In Figs. I and 2, 

 the area marked with dashes is conifers, the blank area is oak 

 and hickory, and the stippled area beech and maple. Each was 

 occupied by different animal communities. The beech and maple 

 are at a level at which the whole area will be when the mountain 

 is reduced to a peneplain level (unpublished conclusion of Cowles). 

 Hills with isolated patches of conifers at the top are numerous 

 throughout the Appalachians. Hills covered with oak and 

 hickory without the conifers, and surrounded by beech and 

 maple and the other mesophytic trees that grow with these, 

 were doubtless very common in the foothills of the Appalachians 

 under primeval conditions. 



Turning to Figs. I and 2 we note that as the height of the 

 mountain is reduced the low beech area becomes larger, at the 

 expense of the oak and hickory habitat, and the oak and hickory 

 habitat in turn adds to itself at the expense of the conifers. In 

 Figs. 3 and 4 is shown an hypothetical stage in which the stream 

 has cut through the upland and captured the headwaters of the 

 stream at point d. This has completely isolated the oak-hickory. 

 It is completely surrounded by the beech and maple. The 

 conifer community has disappeared. Peneplainations have taken 

 place completely. For example, remains of ancient peneplains 

 are recognized in the Appalachian region, each corresponding to 

 a relative lowering of the level of the sea. The first was com- 

 plete. The second was sufficient in extent to cause the isolation 

 of numerous uplands and groups of mountains. The erosion 

 processes now in progress have still further dissected the land 

 into uplands of all possible heights, between the upper and lower 

 limits. Each peneplanation was accompanied by changes in 

 physical conditions, in vegetation and by ecological succession 

 of animal communities (Adams, oi). 



