296 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
group, from the time of inception of trenching, had the waters of the 
upper margin to draw on, whereas the lower streams stood in its 
shadow: Its own capacity for trenching on across the plate was de- 
limited by competition with slopeward streams, Nos. 1 and 2 of the 
medial zone, of like grade with itself. In spite of the lateral stream’s 
advantage in fall, the increasing obliquity of its course rapidly reduced 
its slope. ‘Therefore, with like volume of upper sheet flood, its divide 
finally became adjusted to medial streams which possessed the advan- 
tage of direct flow down the slope. The larger rhythm of the three 
groups, A, B, and C, appears to have been propagated down, not up, 
the slope. The rhythm of streams flowing off the opposite or left-hand 
side of the plate is of a different sort, with five major streams of in- 
creasing length from top to bottom of the plate. 
The main medial streams, 1 and 2 (Fig. 2), and the side streams, 
show signs of having either shadowed or beheaded the three subordi- 
nate medial streams 3, 4, and 5. Nos. 3 and 5 appear to be shadowed 
by the right and left side streams, respectively, No. 4 appears to have 
had its right fork beheaded by the left tributary of No. 1, and its left 
fork by the right tributary of No. 2. Considering the side streams as 
tributaries of the nght and left fall lines as though the latter were two 
master stream courses, the two fall lines may be considered to have 
propagated across the plate the rhythmic arrangement observed. 
Shadowing 3 and 5, they left space for 1 and 2: 4, however, in the 
middle, was overshadowed by unequal competition with its two rivals. 
The process of shadowing begins with the first determination of 
drainage areas before distinct channels have been eroded. The proc- 
ess of beheading or capture takes place after channels have deter- 
mined local competitive base levels. The result of competition is 
determined for the same general zone by relative volumes of water. 
Beheading is accomplished by undermining headward. Shadowing 
is accomplished from above mouthward in a very early stage of the 
development of drainage. Probably on a mathematically uniform 
surface with uniform conditions, shadowing is a rhythmic process 
involving many miniature or embryonic beheadings. ‘The theory 
of arborescent stream development is dependent on such a rhythmic 
process. ‘The writer believes a complete statement of the mechan- 
ism of this process has still to be worked out. Like the many proc- 
esses of mottling, rippling, wave motion, and bilateral, concentric, 
and radial symmetry in nature, the development of digitate drain- 
age is a simple group of rhythms in its ideal form, but probably 
never occurs simply in natural examples. Nevertheless, probably the 
