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 % 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 

 a VOL. II. p. 461. 



