24 ESSAY ON CLASSIFICATION. Part I. 



of their first appearance upon eartli ; and tliougli some still maintain that Vertebrata 

 originated somewhat later, it is universally conceded that they were already in exist- 

 ence toward the end of the first great epoch in the history of our globe. I think 

 it would not be difficult to show upon physiological gi-ounds that their presence upon 

 earth dates from as early a period as any of the three other great types of the 

 animal kingdom, since fishes exist wherever Radiata, MoUusks, and Articulata are 

 found together, and the plan of structure of these four great types constitutes a 

 system intimately connected in its very essence. Moreover, for the last twenty 

 years, every extensive investigation among the oldest fossiliferous rocks has carried 

 the origin of Vertebrata step by step further back, so that whatever may be the 

 final solution of this vexed question, so much is already established by innumerable 

 facts, that the idea of a gradual succession of Radiata, MoUusks, Articulata, and Ver- 

 tebrata, is for ever out of the question. It is proved beyond doubt, that Radiata, 

 MoUusca, and Articulata are everywhere found together in the oldest geological for- 

 mations, and that very early Vertebrata are associated with them, to continue 

 together through all geological ages to the present time. This shows that even in 

 those early days of the existence of our globe, when its surflice did not yet present 

 those diversified features which it has exhiljited in later periods, and which it exliibits 

 in still greater variety now, animals belonging to all the great types now represented 

 upon earth, were simultaneously called into existence. It shows, further, that unless 

 the physical elements then at work could have devised such plans, and impressed 

 them upon the material world as the pattern upon which Nature was to build for 

 ever afterwards, no such general relations as exist among all animals, of all geo- 

 logical periods, as well as among those now living, could ever have existed. 



This is not all : every class among Radiata, MoUusks, and Articulata, is known 

 to have been represented in those earliest days, with the exception of the Acalephs^ 

 and Insects only. It is, therefore, not only the plan of the four great types which 

 must have been adopted then, the manner in which these plans were to be executed, 

 the systems of form under which these structures were to be clothed, even the ulti- 

 mate details of structure which in different genera bear definite relations to those of 

 other genera; the mode of diflerentiation of sj^ecies, and the nature of their rela- 

 tions to the surrounding media, must likewise have been determined, as the character 

 of the classes is as well defined as that of the four great branches of the animal 

 kingdom, or that of the families, the genera, and the species. Again, the first rep- 

 resentatives of each class stand in definite relations to their successors in later 



* Acalephs have been foiinil in the .Jurassic Lime- softness of their body. Insects are known as early 

 stone of Solenhofeii ; tlieir absence in other forma- as the Carboniferous Formation, and may have ex- 

 tions may be owing simply to the extraordinary isted before. 



