MONEY 
117 
in general demand. It need hardly be 
remarked in addition that they tended to 
be commodities adapted by nature for 
the purpose of effecting exchanges ; and that 
a process of natural selection, or substitu 
tion, as it is called in economics, took place 
whereby the most suitable of these things 
displaced the less suitable. The chief proper 
ties of a suitable medium of exchange may be 
enumerated. It must be divisible without 
losing in value, to enable small purchases to 
be made with it ; comparatively durable, so 
that it does not depreciate in value when it 
is held for a time ; moderately constant in 
value for the same reason ; and moderately 
precious at the time when it is used so as to 
contain a convenient value in a convenient 
bulk and weight. 
Let us argue out the theory of money on the 
simple supposition that every time an exchange 
is effected some money commodity to the 
value of the exchange passes hands ; that is 
to say, let us provisionally ignore credit 
money, the peculiarities of which will be 
treated later. It has been alleged that the 
problem of the value of money even under 
these simple conditions is inherently alien to 
the problem of the value of things, because