220 THE GERM-PLASM 



to explain their formation as due to a reaction of the plant in 

 response to the stimulus exerted by the animal. If. as was 

 formerly supposed, the gall resulted from the action of a 

 poison which is inserted into the tissues of the plant by the 

 female during oviposition, this explanation would be totally 

 inadequate ; for it is not conceivable that the infusion of a 

 poison, which happens only once, could with such regularity 

 produce a gall which grows slowly, and only gradually attains its 

 definitive and often complex structure. Moreover, several kinds 

 of galls, differing very much from one another, may be produced 

 from the sa7ne substratvnn. such as an oak-leaf, for instance. We 

 know, however, from the researches of Adler and Beyerinck. 

 that the formation of the gall is not due to the sting of the 

 parent animal, but to the activity of the larva after it has been 

 hatched. We must therefore suppose that this peculiar specific 

 proliferation of the tissues of the plant is due, in the first in- 

 stance, to the stimulus produced by the bodies of the larvae 

 when they begin to move about and to feed, the specific secre- 

 tion of their salivary glands then also having an eflfect. The 

 diversity of the galls arising from the same substratum must 

 therefore be due to differences in these factors ; and the con- 

 spicuous adaptations of the galls, which serve to protect, sup- 

 port, and nourish the parasite, must depend on adaptations of 

 the latter as regards its mode of feeding and movement, and 

 the chemical composition of its salivary secretion. We cannot 

 help accepting this interpretation of the facts since no other is 

 forthcoming ; and we must therefore suppose that natural selec- 

 tion has operated so long on these factors, and has gradually 

 effected such an improvement, that the kind of gall which pro- 

 vided the best protection and nourishment for the species was 

 ultimately produced by the larva. 



Beyerinck has, in fact, proved that cells and tissues often 

 occur in galls which very closely resemble those in different 

 parts of the plant, but which do not exist in the substratum {e.f^. 

 a leaf) on which the gall is produced. De Vries infers from 

 this fact that the primary constituents of such tissues must have 

 been contained in the cells of the leaf, although they could not 

 previously be recognised as such. This inference does not 

 seem to me to follow of necessity, for the stimulus produced by 

 the parasite might conceivably have modified the idioplasm of 

 the cells of the leaf so as to result in the formation of cells 



