Note B to Page 295. 447 



alluded in Nature (vol. xli, p. 41) to the difficulty which the 

 apparently exceptional case of gall-formation presents to the 

 theory of natural selection. Therefore I supplied (vol. xli, p. 80) 

 the suggestion given in the text, viz. that although it appears im- 

 possible that the sometimes remarkably elaborate and adaptive 

 structures of galls can be due to natural selection acting directly 

 on the plants themselves seeing that the adaptation has refer- 

 ence to the needs of their parasites it is quite possible that 

 the phenomena may be due to natural selection acting indirectly 

 on the plants, by always preserving those individual insects (and 

 larvae) the character of whose secretions is such as will best in- 

 duce the particular shapes of galls that are required. Several 

 other correspondents took part in the discussion, and most of 

 them accepted the above explanation. Mr. T. D. A. Cockerell, 

 however, advanced another and very ingenious hypothesis, 

 showing that there is certainly one conceivable way in which 

 natural selection might have produced all the phenomena of 

 gall-formation by acting directly on the plants themselves 1 . 

 Subsequently Mr. Cockerell published another paper upon the 

 subject, stating his views at greater length. The following is the 

 substance of his theory as there presented : 



Doubtless there were internal plant-feeding larvae before there 

 were galls : and, indeed, we have geological evidence that boring in- 

 sects date very far back indeed. The primitive internal feeders, then, 

 were miners in the roots, stems, twigs, or leaves, such as occur very 

 commonly at the present day. These miners are excessively harmful 

 to plant-life, and form a class of the most destructive insect-pests 

 known to the farmer : they frequently cause the death of the whole 

 or part of the plant attacked. Now, we may suppose that the secre- 

 tions of certain of these insects caused a swelling to appear where 

 the larvae lived, and on this excrescence the larvae fed. It is easy to 

 see that the greater the excrescence, and the greater the tendency ot 

 the larvae to feed upon it, instead of destroying the vital tissues, the 

 smaller is the amount of harm to the plant. Now the continued life 

 and vitality of the plant is beneficial to the larvae, and the larger or 

 more perfect the gall, the greater the amount of available food. 

 Hence natural selection will have preserved and accumulated the 

 gall-forming tendencies, as not only beneficial to the larvae, but as a 

 means whereby the larvae can feed with least harm to the plant. So 



1 Nature, vol. xli, p. 344. 



