The enormous increase in our national income and our national capital is doubted or denied by no one. Now let us express the increase in income as a supposed increase in the average height of the rooms inhabited by the population. According, then, to the figures given by me, we might say in this case that at the beginning of the century the average house was _seven feet_ high—only high enough for tall men to stand up in; and that now houses have been so improved that the average height of a living-room is _seventeen feet_. If any one, dwelling on the fact of such a change as this, were inquiring into its causes, and were basing arguments on its assumed reality, what difference would it make if some opponent were to prove triumphantly that the height of the average room now was not _seventeen feet_, but _sixteen feet six inches_, and that four generations ago it had been _six feet_ instead of _seven_? The difference in the estimates of our national income during the past ninety or a hundred years are not more important for the purpose of any general argument than the difference just supposed with regard to the height of two living-rooms; and readers may rest assured that the round numbers given by me with regard to the growth of the national income and the national capital are so near the admitted and indisputable truth of things, that no possible correction of them would substantially alter any one of the arguments which they are here quoted to illustrate. _September 1894._ PREFACE TO FIRST EDITION