Few cities have been laid out with sufficient foresight to provide for such diagonals. It has usually happened that at first a small city area has been developed, in which the need of diagonal thoroughfares was not felt; and then as the city expanded subdivision after subdivision has been added, wherein the original street system has been followed, with no care or thought for the increased traffic which growth begets. The one idea of those who make new subdivisions is to secure the utmost space to sell immediately, leaving the future to take care of itself. Hence it happens that, as a rule, when diagonal streets become of prime necessity they must be created at large expense, and with great temporary inconvenience. Yet whatever the expense, such thoroughfares must be opened; and the city itself is the gainer in each instance, not alone by the saving of time, but also in the increased valuations for taxation which such improvements inevitably bring about. Fortunately for Chicago a considerable number of diagonals already exist, and the large part which they play in promoting circulation offers the best argument for their extension and completion. Blue Island and Milwaukee avenues, the happy survivals of old country roads, now carry great streams of traffic, while Ogden and Archer avenues and other lesser diagonals are of large utility as time-savers. [Illustration: XCI. CHICAGO. EXISTING AND (IN RED) PROPOSED DIAGONAL ARTERIES. COPYRIGHT, 1909, BY COMMERCIAL CLUB OF CHICAGO ] The third class of thoroughfares are the boulevards properly so called; the streets from which all heavier traffic is excluded; the streets lined with commodious and even fine dwellings; the streets where grass and shrubs and trees assert themselves, and where there may well be continuous playgrounds for the children of the neighborhood, such as many Chicago boulevards now provide. If in certain sections buildings for light manufactures abut upon these thoroughfares, the working people will then enjoy a maximum of fresh air and light; and so will work with greatest effectiveness. The boulevard also affords appropriate sites for statues and fountains, and all other forms of adornment pleasing to the eye, making attractive the city. The smaller parks may well be adjacent to the boulevards, or may be expansions of them, thus providing for larger playgrounds, for places of assembly, and for displays of plants and flowers, and rare and beautiful trees, which appeal to the almost universal love of nature. The principle governing the grouping of boulevards and avenues is the establishment of through connection, so that one thoroughfare shall lead into another, and that circulation shall be everywhere promoted but never impeded. [Illustration: XCII. PARIS. THE AVENUE DU BOIS DE BOULOGNE, LOOKING TOWARDS THE ARC DE TRIOMPHE.] [Illustration: XCIII. PARIS. THE TUILERIES GARDENS, AND CHAMPS ÉLYSEÉS BEYOND, FORMING THE MAIN AXIS OF THE CITY.]