CHAPTER XII 

 THE ANCESTRY OF THE VERTEBRATES 



" Ainsi la plus ancienne couche fossilifere connue nous 

 montre des representants de presque toutes les classes 

 d'Invertebres. Cela demontre 1'existence d'une longue 

 periode anterieure a celle sur laquelle la Paleontologie 

 peut nous fournir des renseignements et dans laquelle 

 ont pris naissance presque tous les types actuels. 

 Parmi ces etres, dont les formes resteront tou jours un 

 mystere, devaient se trouver les ancetres sans squelette 

 des Vertebres actuels." 



DELAGE ET HEROUARD, Les Procordes, 1898, p. 357, 



PREVIOUS to the establishment of the modern theory of evo- 

 lution, which removed each animal and plant from an isolated 

 position unrelated to the rest, and assigned to it a place in a 

 connected chain of organic beings, morphological speculation 

 was limited to ingenious comparisons with little or no logical 

 basis, conjured up to explain real or fancied resemblances. 

 Thus Lorenz Oken, having conceived the idea that the head 

 must possess parts corresponding to those of the trunk, consid- 

 ered the nasal cavity, the cephalic thorax ; the mouth cavity, the 

 cephalic abdomen ; and the palate, the cephalic diaphragm ; to 

 him the halves of the upper and lower jaws represented re- 

 spectively the anterior and posterior limbs, in which the teeth 

 were the digits. Thus Geoffrey St. Hilaire compared insects 

 with vertebrates, making the exoskeletal rings the equivalent of 

 the vertebrae, and the jointed legs that of the ribs. The rela- 

 tion of nerve cord, intestine, and main blood-vessels was made 

 the same by placing the insect upon its back. 



Others, like Goethe and Cuvier, sought to base the compari- 

 son between different forms upon the assumption of an arche- 

 type (Goethe's " Urbild "), of which a certain related group of 

 animals might he-considered as so many various modifications. 

 // such an archetype had been considered to have or to have had 



506 



