The system of training thus worked out had been evolved in the face of many difficulties. There were no text-books, no traditions, no bodies of accepted rules and methods. As finally developed, it was modeled somewhat on the British system, with important modifications and differences. But the passing months saw in it, as it evolved, many and sometimes striking changes. It was constantly in a fluid state, subject to the results of experiment and of observation upon the cadets in training, to the conclusions of instructors and field commanders after comparison of experience, and to the evolving ideas of scores of air service men. And especially was it subject to the information, suggestions and orders that came back from the battle front in France, where air warfare was being shaped daily and weekly by war conditions and demands into new methods and new developments. And the training on these fields four thousand miles away had to be kept closely in touch with these constant developments and imperious needs and its methods and aims changed from day to day, if necessary, to meet the requirements. By experiment, observation, steady thinking at high pressure and comparison of ideas on the part of every instructor, every officer and every cadet at every field, methods of instruction were hammered out for each phase of the work. Each field brought its offering of daily experience and almost every flight contributed something to the accumulation of facts out of which grew, finally, some surety of knowledge. Into the development of methods flowed a steady stream of ideas, discoveries, experiences and experiments, and so day by day the American system of training grew to better results and higher efficiency. Text-books, for the most part, were type-written or mimeographed accounts of results that had been gained the month, or the week, or the day before by following certain methods, with comments and suggestions as to their use.