310 SIGXIFTCANCE OF SEXUAL REPRODUCTION- [V. 



light was proved by Lister \ who showed that bHnd frogs do 

 not possess the power of altering their colour in correspondence 

 with that of their environment. It is quite obvious that in this 

 case we are not dealing with a primary, but with a secondarily 

 produced character ; and it has yet to be proved that all the 

 purposeful reactions mentioned by Niigeli are not similarly 

 secondary characters or adaptations, and thus very far from 

 being primitive qualities of the organic substance of the forms 

 in which they occur. 



I do not by any means doubt that some of the reactions 



witnessed in organisms do not depend upon adaptation, but 



such reactions are not usually purposeful. Curiously enough, 



Nageli mentions the formation of galls in plants among his 



instancesof purposeful reactions under external stimuli. I think, 



however, that it can hardly be maintained that the galls are 



of any use to the plant : on the contrar}?^, they may even be 



very injurious to it. The gall is only useful to the insect which 



it protects and supplies with food. The recent and most 



excellent investigations of Adler^ and of Beyerinck^ have 



shown that the puncture made by the Cynips in depositing its 



eggs is not the stimulus which produces the gall, as was 



formerly believed to be the case, but that such a stimulus is 



provided by the larva which developes from the Gg%. The 



presence of this small, actively moving, foreign body stimulates 



the tissue of the plant in a definite manner, alwaj-s producing a 



result which is advantageous to the larva and not to the plant. 



It would be to the advantage of the latter if it killed the 



intruding larva, either enclosing it by woody tissue devoid of 



nourishment, or poisoning it by some acrid secretion, or simply 



crushing it by the active growth of the surrounding tissues. 



But nothing of the kind occurs : in fact an active growth of cells 



(forming the so-called ' Blastem ' of Beyerinck) takes place 



around the embryo, while it is still enclosed in the egg-capsule; 



* * Philosophical Transactions,' vol. cxlviii. 1858. pp. 627-644. 



^ Adler, ' Beitrage zur Naturgcschichte dcr Cynipidcn," Deutsche 

 entom. Zeitschr. XXI. 1877, P- 209; and by the same author, ' Ueber 

 den Generationsvvechsel dcr Eichen-Gallwespen,* Zeitschr. f. wiss. Zoo!., 

 Bd. XXXV. 1880. p. 151. 



^ Beyerinck, ' Beobachtungen iiber die crsten Entwicklungsphasen 

 einiger Cynipidengallcn,' Veihandl. d. Amsterd. Akad. d. Wiss. Bd. 

 XXII. 1883. 



