A growing body of knowledge as to what kind of terrain can stand dense development, and what kind cannot, and how streams and woods and wildlife and even farms can be physically retained among urban populations, and why they ought to be, is becoming available. Its principles are more or less ecological, which simply means that they seek to maintain right uses of different elements of the landscape under urban conditions, in order that these elements may function with a reasonable degree of naturalness, remain compatible with one another and with human purposes, and be available for people's enjoyment. Flood plains make good hay fields or parks, for instance, but poor sites for homes or shopping centers. Porous areas that recharge aquifers ought to be kept as much as possible under vegetation rather than pavements or buildings, if people are to have streams later and not capricious drains that are better off covered over. Steep slopes, if carved severely, usually exact a later revenge. House clusters and townhouses and apartments rightly spaced and located can let the country function even while settling on it numbers of people equivalent to those who would be there if it were hacked into a solid expanse of tiny lots. And so on. Much remains to be learned if the application of these principles is to be ideal. For example, urban hydrology--precisely what happens to the water cycle during various kinds of development, and how it might be adjusted--is a relatively new branch of study and still needs much research. But even with present knowledge, great improvement over present patterns is possible. In the hands of a few emerging experts, planning which pays attention to soils and topography and climate and special landscape features and values can be a subtle art, prescribing villages and farms and factories in the right places, making the most of native vegetation and views and places where George Washington slept and the breezes of July. Its form on a map tends toward curved lines rather than the orderly straight ones abhorred by nature.