INTERNAL ANATOMY OF INSECTS. 27 
rations; and the animal will even find itself as perfectly 
instructed and as capable of employing its new organs, as 
it was to use the old ones. The relations will be the 
same; it will always be the play of the instrument?.” 
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®, the struc- 
ture of the nervous chord is not changed; and yet we 
know that many tribes of these orders acquire instincts 
in their imago state altogether different from those which 
directed them in their state of larvae. A perfect Locust, 
for instance, acquires the new instincts of using its wings ; 
of undertaking those distant migrations of which so many 
remarkable instances were laid before you in a former 
letter®; and, if a female, of depositing its eggs in an 
* N. Dict. a’ Hist. Nat. xvi. 313. Comp. i. 420. 
» See above, p. 23. * Vou. I. 4th Ed. 220—. 
