OEIGIN OF GALLS. 17 



leap from one to two inches ; and when a number are 

 confined in a box, the noise they make by falling on 

 the leaves is like the rain falling on a window. Riley* 

 describes the jumping galls of Andricus saltatorius as 

 presenting a curious spectacle from their keeping up 

 a constant jumping or bouncing movement on the 

 ground. It is obvious that the movement is effected 

 by the larva bringing the head in contact with the 

 anus, thus forming an empty space at either end of 

 the gall ; and when the ends are brought into them 

 again, the gall is set in motion by the sudden expan- 

 sion of the larva. 



It is well understood that the forms, colours, &c, of 

 external feeding caterpillars (see vol. i, p. 46, et seq., 

 for information on this point as regards saw-fly larvse) 

 have been evolved by natural selection for their pro- 

 tection from insects and other enemies. Obviously, as 

 regards the hidden gall larva, its form and texture could 

 not be affected in the same manner. But it is clear 

 that the gall stands in the same relationship to the larva 

 as its skin does to the external feeding caterpillar. 

 The gall larva itself, being hidden, did not afford means 

 for natural selection to act upon ; but the gall did ; 

 and hence we see developed, for the protection of the 

 larva, the various contrivances I have just enumerated. 



But while, to my mind, we can explain, as I have 

 described, the forms of galls, it is not so easy to 

 explain the origin of the galls as galls, without refer- 

 ence to their forms and structures. 



The theories promulgated by the early naturalists, on 

 the origin of galls have practically no scientific value. 

 Some attributed their origin to spontaneous generation. 

 One author concluded that the eggs were drawn up 

 with the sap from the ground to the leaves, &c, on 

 which they were found. Redi,t who had himself suc- 

 cessfully exploded the theory of spontaneous genera- 

 tion, had an equally absurd method of accounting for 



* Johnson's Cyclopaedia, p. 423. 



f Experienze intorno alia Generazione d' Insetti, 1668. 

 VOL. IV. 2 



