302 THE SIGNIFICANCE OF SEXUAL REPRODUCTION 



and it has yet to be proved that all the purposeful reactions men- 

 tioned by Nageli are not similarly secondary characters or adapta- 

 tions, 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 wit- 

 nessed 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 2 have shown that the puncture made by the 

 Cynijos 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 egg. 

 The presence of this small, actively moving, foreign body stimu- 

 lates 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 in- 

 truding larva, either enclosing it by woody tissue devoid of nourish- 

 ment, 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 ; but the growth is not 

 such as to crush the embryo, which remains free in the cavity, the 

 so-called larval chamber, which is formed around it. It would be 

 out of place to discuss here the question as to how we can conceive 

 that the plant is thus compelled to produce a growth which is at 

 any rate indifferent and may be injurious to it ; and which, more- 

 over, is exactly adapted to the needs of its insect-enemy. But it 

 is at all events obvious that this cannot be an example of a self- 



1 Adler, ' Beitrage zur Naturgeschichte der Cynipiden,' Deutsche entom. Zeitschr. 

 XXI., 1877, p. 209; and by the same author, ' Ueber den Generationswecheel der 

 Eichen-Gallwespen,' Zeitschr. f. wiss. Zool., Bd. XXXV. 1880, p. 151. 



a Beyerinck, ' Beobachtungen iiber die ersten Entwicklungsphasen einiger Cy- 

 nipidengallen,' Verhandl. d. Amsterd. Akad. d. Wiss. Bd. XXII. 1883. 



