Plant-galls afford striking illustration of the extraordinary changes that may be brought about in an organism or in its parts by external influence in the course of the individual life. All possibility of adaptation on the part of the plant is excluded in this case. The gall can only depend upon the direct influence of a stimulus, which is exercised by the young animal, the larva, upon the cells which surround it; and yet these cells vary to a considerable extent, become filled with starch or form a woody layer, secrete special substances, such as tannic acid, in large quantities, or develop hairs, moss-like growths, pigments, and so on, which do not otherwise occur in that particular part of the plant. Since Adler and Beyerinck have proved that it is not a poison conveyed by the mother animal into the leaf or bud when laying the eggs, which gives rise to the gall-formation, the matter has become rather clearer. We can now understand that different stimuli in succession affect the cells which enclose the larva, and that the ordered succession of these and the exactly graded stimulation incite the cells to activity in various ways, whether to mere growth and multiplication in a given direction, or to the secretion of tannic acid, or to the formation of wood, or to the deposition of reserve material, and so on. Even the feeble movements of the young larva may form a stimulus that increases with its growth; then the movements made by the larva in feeding, and not least the different secretions emanating from the salivary glands of the animal, which must contain some substances capable of acting as stimuli and probably changing in character as time goes on. All these factors must act as specific stimuli to the plant-cells, influencing and modifying their processes of growth and metabolism in one direction or another. In principle at least, if not in detail, we understand the possibility that through the ordered succession and exact balancing of these different cell-stimuli the really marvellous structure of the gall may be brought about as the product of the direct influence, exercised only once, of the gall-insect upon the plant's parts. But the animal's power of exercising such a succession of finely graded stimuli upon the plant must be referred to long-continued processes of selection, and the structure of the gall, which is adapted to its purpose down to the minutest details, can thus be understood. The assumption of substances which can act even in minute quantities as specific cell-stimuli, which we require to make in this attempt to explain galls, is no longer without corroboration since we find analogies in the Iodothyrin of Baumann, the specific secretions of the thymus and the supra-renal bodies in the higher animals, not to speak of the 'anti-toxins' of the pathogenic bacteria, which are only known by their effects.