The case of plant-galls is thus of great theoretical interest because we can exclude all preparation of the plant-cells for the stimuli exercised by the animal, since the gall is quite useless for the plant, though many have endeavoured to discover some utility. We have therefore here a clear case of modification due to the effect, exercised once only, of external influences, an adaptation of the animal to the mode of reaction of particular plant-tissues. It might be supposed that if any inheritance of somatogenic modifications, any transmission of the acquirements of the personal part to the germinal part, were possible at all, it would occur in this case, for many species of gall-insects attack plants, particularly oaks, in great numbers every year. It has actually been maintained that galls may arise spontaneously, that is without the presence of a gall-insect. But no proof of this has ever been found, and the fact that no one has paid any attention to the assertion probably implies an unconscious condemnation of the hypothesis of the transmissibility of acquired characters. It has been proved by Nägeli's often discussed experiments on hawkweeds (_Hieracium_) that much less specialized external influences can give rise to changes which are not hereditary. The Alpine species of hawkweed varied considerably in their whole habit in the rich soil of the Botanic Gardens at Munich, but their descendants, when transferred to a poor flinty soil, returned to the habit of the Alpine species. The changes which occurred in garden soil were therefore somatic and, as I have called them, 'transient,' and they did not depend upon variations of the germ-plasm. It may be objected in regard to these experiments that they were not continued long enough to prove that hereditary variations would not also have cropped up in consequence of the altered conditions. But in any case they prove that marked changes in the whole body of the plant may occur without any obvious variation of the germ-plasm. This does not mean, however, that the possibility of variations of the germ-plasm through such direct external influences is disputed. We must assume the occurrence of these on _a priori_ grounds, if we refer--as we have done--individual hereditary variation to fluctuations in the nutrition of the individual determinants of the germ-plasm. It is probable that many general nutritive variations or climatic factors affect the germ-plasm as well as the soma, and it is by no means inconceivable that it is not all, but only certain definite determinants that are caused to vary.