THE INFLUENCES OF ENVIRONMENT 271 



pigments, and so on, whieli 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 

 tlie 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 move- 

 ments made by the larva in feeding, and not least the different 

 secretions emanating from the salivary glands of the animal, wliich 

 must contain some substances capable of acting as stimuli and probaljly 

 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 tliat 

 through the ordered succession and exact balancing of these different 

 cell-stimuli the really marvellous structure of the gall may be brouglit 

 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 wliich can act even in minute quantities as specific cell- 

 stimuli, which w^e recpire to make in this attempt to explain galls, 

 is no longer without corroboration since we find analogies in the 

 lodothyrin 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 on]y known l;)y their 

 effects. 



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 eftect, 

 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 



