Note B to Page 295. 447 
alluded in Mature (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 
naiural selection might have produced all the phenomena of 
gall-formation by acting directly on the plants themselves‘. 
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 of 
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 greatcr 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. 
