PREFACE ix 



the time of Linnaeus has consisted largely of 

 rectifications in the systematic positions origin- 

 ally assigned to animals which were thought 

 to resemble others in certain superficial traits 

 of their organisation, and hence were classed 

 together, e.g., the Foraminifera and the Nauti- 

 loidea, the Cephalopoda and Pteropoda, the 

 Cirripedia, Tunicata and Mollusca. It is only 

 after such groups are relegated to their proper 

 position in the zoological system that the 

 phenomena of convergence begin to appear in 

 their true light with an independent interest 

 of their own. The limitations of convergence 

 coincide with those of homology, and the criteria 

 of the one are inversely those of the other. 

 Its importance in morphology is therefore clear 

 enough, and no apology would be required for 

 the repeated discussion of it, were the treatment 

 adequate. 



Up to a certain point the gist of what I have 

 set down in these pages may be regarded as 

 an attempt at a reply to a recent "earthquake 

 hypothesis " concerning the origin of Vertebrates, 

 which has been published in a remarkable volume 

 by a very eminent physiologist, Dr W. H. 

 Gaskell. I can only hope that Dr Gaskell and 

 others will accept it in the spirit in which it is 

 offered. Besides this I have endeavoured to 



