Sec. 5] SUMMARY OF PART III 313 
of the sapling disappears at BC, but there appears in its 
stead the capital-value of the tree. The change from one 
to the other is evidently entirely nominal, and it is possi- 
ble, by drawing any other vertical line than BC, to create 
an “interaction” simply by calling the portion on the two 
sides of this line by different names. 
When, as in Figure 25, a series of curves is constructed 
and superimposed to represent the income from any speci- 
fied group of capital instruments, the sum total of the income 
is evidently represented by the entire series of teeth in the 
  
Fic. 25. 
top curve. These teeth form a physical picture of the 
“outer fringe” of services, which was discussed In 
previous chapters. If in the diagram we omit the upper- 
most layer of capital, the curve remaining immediately 
below this layer will then be the outer fringe for the entire 
series of capital instruments below it. We may pro- 
ceed step by step in either direction, leaving off an tem of 
capital or taking one on. In every case the outer fringe 
of teeth will represent the sum total of income for the group 
of capital represented below it.