INTERNAL ANATOMY OF INSECTS. 27 



a determinate series in the nervous system and the gan- 

 glions of the caterpillar, by which alone she lives, she 

 will act according to a certain sequence of operations > 

 and, so to speak, she will sing the air engraven within 

 her. When she undergoes her metamorphosis into a 

 butterfly, her nervous system being, if I may so express 

 myself, pulled out a notch, like the cylinder, will present 

 the notes of another tune, another series of instinctive 

 operations ; and the animal will even find itself as per- 

 fectly instructed and as capable of employing its new or- 

 gans, as it was to use the old ones. The relations will 

 be the same ; it will always be the play of the instru- 

 ment a ." 



This illustration is doubtless at the first glance very 

 striking and plausible : but a closer examination will, I 

 think, show, that, as in so many other instances in meta- 

 physical reasoning, when fanciful analogies are substi- 

 tuted for a rigid adherence to stubborn facts, it is satis- 

 factory only on a superficial view, and will not stand the 

 test of investigation ; and as this is a question intimately 

 connected with what I have advanced on the subject of 

 instinct in a former letter, I must be permitted to go 

 somewhat into detail in considering it. 



To prove his position, Dr. Virey ought at least to be 

 able to show that, whenever a change takes place in the 

 instincts of insects in their different states of larva and 

 imago, a corresponding change takes place in the exter- 

 nal structure of the nervous chord. But what are the 

 facts? In three whole orders, viz. Orthoptera Hemi- 

 ptera, and Neuroptera, as mentioned above b , the struc- 



* N. Diet. d'Hist. Nat. xvi, 313. Comp. i. 420. 

 b See above, p. 23, 



