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- 
jitera, and Neuroptera, as mentioned above b , the struc- 
a N. Diet. d'Hisi. Nat. xvi. 313. Comp. i. 420. 
b See above, p. 23. 
