_A. External causes--I. Non-living environment: soil, atmosphere, temperature--II. Living environment: plants, animals--Complex interactions--Predisposing causes--No one factor works alone--Tangled problems of natural selection involved. B. So-called internal causes._ It is customary to classify the causes of disease in plants into two principal groups--(1) those due to the action of the non-living environment--soil, atmosphere, physical conditions such as temperature, light, etc.; and (2) those brought about by the activities of living organisms--plants and animals of various species. Before passing to further subdivisions under these two heads, however, it is necessary to observe that no disease can be efficiently caused by an organism alone, since its powers for injury as a parasite, or otherwise, are affected by its non-living environment as well as by the host-plant. For instance, the spores of a parasitic fungus which would infect and rapidly destroy a potato plant in moist warm weather may be showered on to such a plant with impunity if the air remains dry and cool--or on to a cabbage under any circumstances as far as we know. Again, probably no one factor of the non-living environment ever suffices to induce a disease, possibly because no such thing as only one change at a time ever occurs. For instance, it is difficult to say, when a soil becomes sodden with water, whether the excess of water and dissolved matters, the want of air displaced by the water, the lowering of the temperature, or the accumulation of foul products, etc., is the principal factor in causing the damage which results, and we have to determine by the balance of experimental evidence which is the dominant factor in all such cases. The study of aetiology of disease is in fact only a particular case of that of aetiology in general. Plants at high altitudes in the Alps acquire very different characteristics from the same species in the plains. Is this due to the low temperature, the rarer atmosphere, the more intense illumination, the changes in moisture, etc., etc.? The question is more difficult than it appears at first sight, and we must remember that, complex as are the factors working on the host, they are equally complex in their actions on a parasite attacking the host, whence the resulting disease becomes indeed a tangled problem of natural selection.