CHAPTER X 



CONVERGENCE IN MINUTE STRUCTURE 

 (HISTOGENETIC CONVERGENCE) 



IN the preceding chapter we have seen that 

 histogenetic convergence gives no clue to affinity, 

 and that histogenetic divergence is no proof of 

 want of affinity. We may now go further than 

 this and add that where we do find actual histo- 

 logical identity, as between members of different 

 phyla, it seems certain that we are in the presence 

 of true convergence in the sense in which that 

 term is employed here ; and, in the light of facts 

 which are now available, it even begins to appear 

 strange, although only a matter of a few years 

 or months ago, that histological identity should 

 ever have been insisted upon as a criterion of 

 homology except within well-defined limits. 



To my thinking, one of the most remarkable 

 examples of histogenetic convergence is that of 

 the excretory organs of Amphioxus as com- 

 pared with those of certain Annelid worms. 

 The excretory tubules of Amphioxus were dis- 

 covered independently by Weiss and Boveri 

 in 1890 and were described in detail by the 



