_On the Manner and on the Period of Action of the Causes which induce Variability._--This is an extremely obscure subject, and we need here only briefly consider, firstly, whether inherited variations are caused by the organisation being directly acted on, or indirectly through the reproductive system; and secondly, at what period of life or growth they are primarily caused. We shall see in the two following chapters that various agencies, such as an abundant supply of food, exposure to a different climate, increased use or disuse of parts, &c., prolonged during several generations, certainly modify either the whole organisation or certain organs. This direct action of changed conditions perhaps comes into play much more frequently than can be proved, and it is at least clear that in all cases of {268} bud-variation the action cannot have been through the reproductive system. With respect to the part which the reproductive system takes in causing variability, we have seen in the eighteenth chapter that even slight changes in the conditions of life have a remarkable power in causing a greater or less degree of sterility. Hence it seems not improbable that being generated though a system so easily affected should themselves be affected, or should fail to inherit, or inherit in excess, characters proper to their parents. We know that certain groups of organic beings, but with exceptions in each group, have their reproductive systems much more easily affected by changed conditions than other groups; for instance, carnivorous birds more readily than carnivorous mammals, and parrots more readily than pigeons; and this fact harmonizes with the apparently capricious manner and degree in which various groups of animals and plants vary under domestication.