310 SIGNIFICANCE OF SEXUAL REPRODUCTION [V. 



light was proved by Lister \ who showed that blind 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 Nageli 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 

 instances of 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 contrary, 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 ^g%. The 

 presence of this small, actively moving, foreign body stimulates 

 the tissue of the plant in a definite manner, always 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 b}^ 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 Naturgeschichte der Cynipiden,' Deutsche 

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

 den Generationswechsel der Eichen-Gallwespen/ Zeitschr. f. wiss. Zool., 

 Bd. XXXV. 1880. p. 151. 



2 Beyerinck, ' Beobachtungen iiber die ersten Entwicklungsphasen 

 einiger Cynipidengallen,' Verhandl. d. Amsterd. Akad. d. Wiss. Bd. 

 XXII. 1883. 



