78 



THE AMERICAN NATURALIST 



[Vol. XL 



Before taking up the question of the unity of the Gnathostome 

 type, a brief consideration of some of the general features of the 

 vertebrates will clear the way for a better understanding of the 

 arguments and facts which bear on the solution of the problem. 



First of all we may well consider amphioxus as a typical ancestral 

 vertebrate. 



The Vertebrata, as we find them to-day, form a morphological 

 unit in much the same way that the birds form a closed group — 

 and it is only through paleontology that the shading off of the 

 group into lower forms is to be found or proven. That the verte- 

 brates have arisen from a common source is a statement which 

 cannot to-day be questioned, for the present state of our knowl- 

 edge of their anatomy, ontogeny, and physiology is a practical 

 demonstration of the problem. Each increase in our knowledge 

 only makes the demonstration stronger. All three fields of inves- 

 tigation — comparative anatomy, comparative embryology, and 

 paleontology' — unite in one affirmation of this fact. All three show 

 that there has been an orderly progression (to use only living 

 forms as examples) from amphioxus to man. No one can read 

 Hasse's Das naturliche System der Elasmohranchier , to mention 

 only one of many similar investigations, without having the fact 

 of the historical succession of these early vertebrates indelibly 

 impressed upon his mind. The genetic relationships of this 

 procession of Selachians from the earliest geologic ages, when 

 the fossiliferous records of the vertebrate stock were begun, is 

 proven by the comparative morphologic study of the skeletons 

 of extinct and recent forms and this proof is strengthened by the 

 as yet only partial, embryonic record which has been worked out 

 by many investigators. The course of development is marked 

 by the retention in all living forms of all the divergent branches 

 of vertebrates, from amphioxus up to the mammals, of the same 

 fundamental arrangement of organs and systems of organs, and 

 by the same general cellular structure. I1ie variations in arrange- 

 ments of the organs and the dillVrcnccs in tlu- histological details 

 of the tissues are not greater than the variations of the external 

 form or of the shape of the internal organs. These are large facts 

 of fundamental value, and have a greater morphologic worth 

 than the many minor variations can possibly have, even when 



