of comparative anatomy, as the foundation of the classification of the 

 animal kingdom, has brought to light, must naturally have blinded 

 us to the imperfections and deficiencies which constantly accompany 

 the most important improvements in the natural arrangement of 

 every class. Nevertheless, our confidence in the possibility of ascer- 

 taining the natural relations of all animals has been increased by the 

 growing agreement between the different systems ; and there is no 

 philosophical observer who has not noticed this process of gradual 

 approximation towards a greater uniformity in the view taken by 

 different observers of the natural affinities of animals, however con- 

 cealed this agreement has often been in consequence of changes of 

 name or transposition of the order in which the objects were intro- 

 duced. 



The time has, however, gone by, when the mere translation of 

 family names, or of more general or minor divisions, into another 

 language, could be presented as a new system, and the raising of a 

 secondary division into the rank of a primary group, or the lowering 

 of a primary division into a subordinate position, constituted an 

 improvement in the knowledge of the natural relations of animals. 

 Nothing short of a material addition to the information we possess 

 respecting any group of animals, can now be considered as a real 

 advance in Zoology. 



It should be further considered that our object is not merely or 

 chiefly to ascertain the structural relations of animals, but to know 

 all the various relations which have been established between them, 

 and which they sustain towards the world in which they live. The 

 knowledge of the natural embryonic development of the order of 

 succession in geological times of the geographical distribution upon 

 the surface of our globe and of the habits arising from their natural 

 relations to the elements in which they live all these considerations 

 are of as great importance in our zoological studies as the knowledge 

 of the structure and functions of their organs, to which, of late, more 

 exclusive attention has been paid. 



- n** 



When comparing, in former years, the characters of fossil fishes, 

 especially with a view of ascertaining their natural relations to the 

 living types, I was struck with the fact that those of earlier ages 

 presented many structural peculiarities, -which occur only in the em- 

 bryonic condition of the fishes of our days, and also that the older 

 representatives of any family rank lower in comparison to their living 

 representatives. 





