322 BULLETIN OF THE 



In the foregoing discussion, I have endeavored to show that there is 

 considerable evidence in favor of the view that the excretory system of 

 Vertebrates has developed from a system of metameric nephridia, such 

 as are present in Annelids. None of the evidence seems to me, how- 

 ever, final. The excretory systems of the two groups are very similar, 

 but we have no means of limiting definitely the part that has been 

 played by physiologically similar needs in moulding the structure of the 

 organs. Nor am I committed to the theory of the Annelidan extraction 

 of A'ertebrates. I fully realize that such a theory can only be estab- 

 lished by investigations which shall include in their scope the entire or- 

 ganization of the two groups. So far as this larger theory has been 

 dealt with in this discussion, it has been with the view of contributing 

 such evidence as the excretory system offers, and I have naturally left 

 untouched the mass of evidence which proceeds from other organs. To 

 this in addition we must appeal for the justification of the broader 

 theory. 



Cambridge, April 25, 1891. 



