(a) _Forest._ Two special expenditures attach to this obstacle: First the effort of clearing a way, second the effort of maintenance, and particularly through the effect of wood upon surface. The effort of clearing, always an expense, made the forest in very early times an insuperable obstacle to any great or considerable road. The forest had tracks, but the main road was compelled to skirt the denser woodland, or at the least to take a tortuous trajectory for the advantage of natural clearings. With the development of civilization that difficulty disappeared, and it disappeared early, although I can call to mind no broad primitive track through any dense woodland. The Roman roads hewed their way through forests where it was necessary, and found in the value of the timber felled an economic compensation for the effort made. But even with them, and even with modern roads, it remains true that the forest governs and modifies road construction. There is case after case where a Roman road, and even a modern road, will skirt a forest rather than be at the effort of overcoming the obstacle: for instance, the case of the forest of Mormal in Northern France. Here the main Roman road from the centre of Northern Gaul to the crossings of the Rhine cuts along the edge of the great wood like a knife, with no growth on its western side. Further, cause and effect reacting on one another, the lack of roads preventing clearing, and the lack of clearing keeping down habitation and so ways, there is no great forest possessing a system of roads anywhere in Europe. All considerable stretches of woodland, where agriculture or other economic effort has not cleared them, have a minimum of roadway. In our northern climate, the larger stuff once felled, upkeep is not a grave economic matter. The use of artificial material, which comes in at the very first stages of road-making, renders the problem here even less important. But in other climates, and particularly in the tropics, it becomes the dominating factor. There are whole districts--as, for instance, on the Amazon basin, or, again, in Central and West Africa--where the problem of communication consists not in the cutting of the original track, expensive as that is, but in its maintenance; and in the greater part of those districts even modern civilization, with its immense material advantages, and with its strong economic inducement to the transport of tropical material, has been unable or unwilling to make and maintain forest roads, at any rate for ordinary wheeled traffic.