XXIV INTRODUCTION. 



which it develops (ccenogenetic characters). In most cases it 

 has hitherto been impossible to distinguish these two sets of 

 characters satisfactorily ; and a final appeal must thus be made 

 to Palaeontology, notwithstanding its imperfections, to determine 

 the laws by which evolution proceeds. 



Origin of the Vertebrata. Perhaps the most disappointing 

 element in palseontological results thus far, is the lack of all 

 information concerning the origin of the great sub-kingdoms or 

 phyla of animals. Even in what might appear to be the most 

 promising case, namely, that of the Vertebrata, there are no 

 known facts distinctly favouring any of the rival theories con- 

 cerning their origin based on embryology. Possibly all the 

 earliest types were destitute of hard parts and thus incapable 

 of fossilization. In any case, the oldest Ostracoderms (p. 3) 

 from the Upper Silurian and Lower Devonian, sometimes 

 claimed as the immediate allies of the Crustacean or Arachnid 

 Merostomata of the same period, are fundamentally different 

 from the latter in every character which admits of detailed 

 comparison ; they are to be regarded merely as an interesting 

 example of mimetic resemblance between organisms of two 

 different grades adapted to live in the same way and under 

 precisely similar conditions. 

 Jfc 



ERRATUM. 



Page 36, lines 5 7. Dr Traquair points out to the author that the 

 appendages of the gill-arches of the Acanthodians are fixed upon their 

 anterior or concave border, and thus cannot be interpreted as in the 



text. 



