HOMOLOGY OF THE GUT 75 



plants, true foliage leaves occur, they are homo- 

 logous throughout. Sometimes other structures 

 (cladodes and phyllodes) can function as leaves, 

 i.e., as organs of assimilation ; but the special 

 homologies of certain leaf-like structures do not 

 affect the general homology of leaves, which is 

 one of the cardinal points in vegetable mor- 

 phology. The distinction between the special 

 homology of parts and the general homology 

 of the whole requires to be emphasised. 



The special morphology of the alimentary tract 

 of animals is a large subject which demands 

 exhaustive analysis ; but it is distinct from the 

 question of the general homology of the gut. It 

 is probable that no single division of an insect's 

 gut, for example, can be strictly homologised 

 with a corresponding part of a fish's gut, but 

 looked at in their entirety, the general homology 

 of the one with the other is still a question 

 apart, to be answered by an appeal to first prin- 

 ciples. That it must be answered one way or 

 the other is evident from the fact that objections 

 have been raised against the alleged view "that 

 the one organ which is homologous throughout 

 the animal kingdom is the gut," [Gaskell, l.c.~\. 

 This is perhaps an extreme way of putting it, 

 but we may examine the point very briefly, 

 always bearing in mind the distinction between 

 special and general homology. 



Dr Gaskell regards the nervous system as 



