| ** . . ** | ** .. .. ** | * . . * | ** . . * | ** .. . ** | ** .. .. ** | ** .. .. ** | ** . .. ** | ** p’ .. ** | **.. .. ** | ..|**p **._p_ | .. | **** | .. | ... |**|** | .. | .. *| | *| .. | ..... *** | | |** . s|...... *** | | | ** .. | *** | | | ** ... | ***** | | | ** ...  S |***** | | | ** .. | | | | *** .._d’_ | | | | *** | | | | ** | | | | **D’ | | | | | | | | 0+--------------------------------------------------------- M’ M m Figure 2

In Fig. 2 we start as before with our demand curve, and supply curve, cutting one another at the point P. We then suppose that some alteration takes place in the conditions of demand; there has been a growth in the general taste for the commodity or service, and the demand, as we say, has increased accordingly.  How is this fact to be represented in the diagram?  Plainly not by taking another point on the curve, DD’, at a further distance from OY.  For this would merely indicate the larger amount that would be taken, if the conditions of demand had remained unaltered but the sellers had reduced their prices.  The correct way of representing the change we have supposed is to construct a new demand curve (in the figure, the dotted curve dd’), lying at every point above the old demand curve.  For this indicates that larger quantities will be purchased at the old prices, which is exactly what we want to represent.  Similiarly if we wish to represent a change in the conditions of supply, such as might result, in the case of a commodity, from a tax imposed on its production, we must draw a new supply curve, ss’, which in the case supposed, must lie everywhere above the old supply curve.  On the other hand, the decrease or increase in demand or supply, resulting from a change in price, is represented simply by a shifting of the equilibrium from one point to another on the same curve.  The striking pictorial contrast between a movement from one curve to another, and a movement along the same curve should help to make vivid to our minds the fundamental distinction between a change in the conditions of demand, arising from new tastes, enhanced purchasing power, etc.; and a mere change in the amount purchased resulting from an alteration in the price which the sellers ask.  Words, as this necessarily cumbrous sentence shows, are a clumsy instrument for the expression of abstract relations; it is not very easy to see which words in a sentence are the significant, commanding ones, and which are performing, as it were, ordinary routine duties.  A diagram is not exposed to similar ambiguities of emphasis.