As time progressed, however, outside demands for a book embodying the writer’s soil studies in the humid and arid regions, especially the latter, became so numerous and pressing that the scope of the work has gradually been much enlarged to conform to these demands; and this, rather than completeness of detail, when such detail can be found well given elsewhere, has been the guide in the necessary condensation of the whole. To give the entire subject matter full elucidation, would require several more volumes. It may not be unnecessary to explain at the outset why and how this treatise deviates in many respects from previous publications on the same general topic. From boyhood up it has fallen to the writer’s lot to be almost continuously in more or less direct contact with the conditions and requirements of newly settled regions, as well as with those hardly yet invaded even by the pioneer farmer; where the question of cultural adaptation was yet undetermined or wholly in the dark. Being during his active life constantly called upon in his official capacity to give information and advice to pioneer farmers or intending settlers in regard to the merits and adaptations of virgin soils, the writer’s attention was naturally and forcibly directed toward soil investigation as a possible means of determining, beforehand, the general prospects and special features of agriculture in regions where actual experience was either non-existent or very brief and partial. In the pursuit of these studies he has been favored by exceptional opportunities, extending over a varied climatic area reaching on the south from the Gulf of Mexico to the Ohio, across to the Pacific coast, and to British Columbia on the north. That a systematic investigation of soils over so large an area, covering both humid and arid regions, should lead to some unexpected and novel results, is but natural; and it is the discussion of these results in connection with those obtained elsewhere, and with some of the prevailing views based thereon, that must serve as the justification for the present addition to an already well-stocked branch of literature. From the very beginning of the scientific study of agriculture, the investigation of soils with a view to the _à priori_ determination of their adaptation, permanent value, and best means of cultural improvement, has formed the subject of continuous effort. It is not easy to imagine a subject of higher direct importance to the physical welfare of mankind, whose very existence depends on the yearly returns drawn by cultural labor from the soil. It is certainly remarkable that after all this long-continued effort, even the fundamental principles, and still more the methods by which the object in view is to be attained, are still so far in dispute that a unification of opinion in this respect is not yet in view; and a return to pure empiricism is from time to time brought forward to cut the Gordian knot.