An admirable short account of soil in its relation to root-hairs is given in Sachs' _Lectures_, XV.; but for a more exhaustive treatment of the subject of soil the reader is referred to King, _The Soil_ (Wisconsin, 1895), or Warrington, _Lectures on the Physical Properties of Soil_ (Oxford, 1900); Larbalétrier, _L'Agriculture_ (Paris, 1888), chapters II. and III. There is also a very good account in Bailey, _The Principles of Agriculture_ (London, 1898), chapters I.-III. With reference to the organisms in soils and the decompositions they bring about, the student should consult Kramer, _Die Bakteriologie in ihren Beziehungen zur Landwirthschaft_ (Wien, 1890), and Lafar, _Technical Mycology_ (Engl. edition, 1898), sections V., VIII., and IX. CHAPTER VIII. HYBRIDISATION AND SELECTION. _The crossing of varieties of wheat, etc.--The essentials of fertilisation--Rimpau's experiments--Hybrids and selected varieties._ In the more hopeful view of the case which the new agriculture will have to take, it will recognise the physiological truth that since the living plant is the important and variable machine which constructs the produce looked for, and since that machine will work best in proportion as its needs are properly satisfied; therefore in cases where the needs of a given type of the machine cannot be efficiently provided for, it will be well to select some other type which will take what supplies and conditions can be offered. Of course, this is already recognised to a certain extent, as is implied in the practices of "rotation of crops," selection of "pedigree wheats" and mixtures of "pasture grasses," and in decisions as to the quality of land according to the kinds of weeds found on it, and so forth; but I am convinced that the agriculturist of the future--and the same applies to the horticulturist, planter and forester--will have to concern himself more systematically with the working and the variability of the plant, and particularly with what Darwin termed Variation under Domestication, than has always been the custom in the past. The subject of the plasticity of cultivated plants, and especially of hybrids, is in one sense an old one; but much work is being done which proves, as such work is apt to do, that very much more may be done by well-planned experiments on the selection of new varieties raised by hybridising and cultivation. In illustration of this point, a short summary of some of the results of crossing different species of wheat, barley, oats, peas, beet, etc., may serve to show what has been gained and what may be hoped for in these directions. It should be stated that much has been done and is being done in this country as well as abroad, as witness English varieties of corn, peas, and potatoes, and the recent experiments on crossing various kinds of maize in America.