INTERNAL ANATOMY OF INSECTS. 29 
when we advert to the higher classes of animals, which 
surely in any investigation of the nature of instinct ought 
to be closely kept in view ; for the faculty, though often 
less perfect in them than in insects, is still of the same 
kind, and may consequently be expected to follow the 
same general laws. In a young swallow, for example, 
all its instincts are not developed at once any more than 
in an insect. The instinct which leads it to migrate 
does not appear for some months after its birth, and that 
of building a nest still later. But we have not the slightest 
ground for believing that these new instincts are pre- 
ceded by any change in the structure of the great sym- 
pathetic nerve, or of any other portion of the nervous 
system : and the same may be said as to the sexual in- 
stincts developed in quadrupeds some years subsequent 
to their birth. If, then, these remarkable changes in the 
instinct of 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 a , as to the insufficiency of 
the mechanical theories of instinct hitherto promulgated, 
adequately to explain all 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 
1 Vol. II. p. 461. 
