INTERNAL ANATOMY OF INSECTS. 29 
a 
of building a nest still later.» But we have not the 
slightest ground for believing that these new instincts are 
preceded by any change in the structure of the great 
sympathetic nerve, or of any other portion of the nervous 
system: and the same may be said as to the sexual in- 
stinets developed in quadrupeds some years subsequent 
to their birth. If, then, these remarkable changes in the 
instinctof the higher classes of animals can take place 
independently of any visible change in the nerves, what 
substantial reason can be assigned why they may not 
also in the class of insects ? 
On the whole, I think you will agree with me, that 
there is nothing in Dr. Virey’s hypothesis which should 
lead me to alter the opinion I have already so strongly 
expressed in a former letter?, as to the insufficiency of 
the mechanical theories of instinct hitherto promulgated, 
adequately to explain ail the phenomena; and unless 
they do this they are evidently of small value. Such 
theories as I have there adverted to may often seem to 
be supported by a few insulated facts, but with others, far 
more numerous, they are utterly at variance’ and, to 
omit many other instances, I am strongly inclined to 
doubt the possibility of satisfactorily explaining the va- 
riety of instincts exercised by a bee”, or the extraordi- 
nary development of new ones in particular circum- 
stances only*, on any merely mechanical grounds. 
And after all, even suppose it could be demonstratively 
shown that every instinct is as clearly dependent on se- 
condary causes, as I have formerly admitted that some 
doubtless seem to be, yet what would this teach us as 
to the essential nature of instinct? We have advanced 
* Vor. II. 4th Ed. p. 467.» Ibid p. 499. —* Ibid. p, 509. 
