Skc. 5] INCOME ACCOUNTS 129 
Any merchant’s stock that rapidly changes the individual 
elements of which it is constituted is most conveniently 
treated as a whole. To use Professor Clark’s admirable 
simile, it is like Niagara Falls, which remains a waterfall, 
although consisting each day of entirely different drops of 
water. The stock of a butcher, grocer, or fruiterer consists 
of rapidly changing elements, but remains as a whole 
relatively unchanged. Though it would be logically sound, 
it would be foolish and impracticable to keep an income 
and outgo account for each individual leg of mutton or 
box of figs. The tendency to-day, however, is distinctly 
toward a more detailed accounting. Some business firms, 
by means of modern card indices, keep a careful record for 
each separate variety of commodity dealt with, if not for 
each individual article in that variety. The important 
thing to observe is that the net income of the entire group 
is simply the difference between the sums of the incomes 
and outgoes of the elementary units which constitute that 
group. The very item which, for the elementary unit, 
constitutes “capital” cost, and which, for that unit, occurs 
but once, becomes, for the group, the regular cost of re- 
plenishing, and recurs annually. From the explanations 
and illustrations which have been given, it is clear that 
consistency and logic must assign to every cost, whether 
large or small, regular or irregular, a place as an element of 
outgo in the income-and-outgo accounts. 
§5 
Whether or not the irregularities of income from indi- 
vidual articles of wealth are smoothed away in the total, 
the combined income, even from a large group of articles, 
is not necessarily an absolutely steady flow. We usually 
strive to make it so to some extent; but we do not always 
succeed, nor do we even always try. When income does 
vary, the method of measuring which has been given will 
unerringly register that variation automatically. The 
K