136 FOREST ENTOMOLOGY. 



world. But here I may suggest that although natural selection cannot 

 have acted upon the plant directly, so as to have produced galls ever 

 better and better adapted to the needs of the insect, it may have so 

 acted upon the plants indirectly fhrough the insects. For it may very 

 well have l^een that natural selection would ever tend to preserve 

 those individual insects, the quality of whose emanations tended to 

 produce the form of galls best suited to nourish the insect progeny ; 

 and thus the character of these pathological growths may have become 

 ever better and better adapted to the needs of the insects. Lastly, 

 looking to the enormous number of relations and inter-relations 

 between all organic species, it is scarcely to be Avondered at that 

 even so extraordinary an instance of correlation as this should have 

 arisen thus by accident, and then have been perfected by such an 

 indirect agency of natural selection as is here suggested. 



"Again, quite independently, and still more recently, Mr Mivart 

 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 sug- 

 gestion given in the text^ — viz., that although it appears impossible 

 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 reference to the needs of 

 their parasites, — it is (juite possible that the phenomena may be due 

 to natural selection acting indirectly on the plants, by always preserv- 

 ing those individual insects (and larvfe) the character of whose 

 secretions is such as will best induce the particular shapes of galls 

 that are required. Several other correspondents took part in the 

 discussion, and most of them accepted tlie 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 (vol. xli. 

 p. 344). Subsequently, Mr Cockerell published another paper on 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 

 insects date very far back indeed. The primitive internal feeders, 



