34 ESSAY ON CLASSIFICATION. 



Insects only. It is, therefore, not only the plan of the 

 four great types which must have been adopted then, 

 but also the manner in which these plans were to be exe- 

 cuted; the systems of form under which these structures 

 were to be clothed, and even the ultimate details of struc- 

 ture which in different genera bear definite relations to 

 those of other genera; the mode of differentiation of 

 species, and the nature of their relations to the surround- 

 ing media, must likewise have been determined; for 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 

 representatives of each class stand in definite relations to 

 their successors in later periods, and, as their order of 

 appearance corresponds to the various degrees of compli- 

 cation of their structure, and forms a natural series closely 

 linked together, this natural gradation must have been 

 contemplated from the very beginning. There can be the 

 less doubt upon this point, as man, who comes last, closes 

 in his own cycle a series, the gradation of which points 

 from the very beginning to him as its last term. I think 

 it can be shown by anatomical evidence, that man is not 

 only the last and highest among the living beings of the 

 present period, but that he is the last term of a series, 

 beyond which there is no material progress possible in 

 accordance with the plan upon which the whole animal 

 kingdom is constructed ; and that the only improvement 



dinary softness of their body. Insects the oldest geological periods, since 



are known as early as the Carbonife- representatives of the family of 



rous Formation, and may have ex- Nilleporina occur in the Silurian 



isted before. Since the publication rocks. 



of these remarks I have ascertained It remains only to be ascertained 



that Millepora is not a Polyp, but now whether all the Zoantharia tabu- 



belongs to the Hydroids. It is thus lata are as truly Hydroids as the 



shown that Acalephs have existed iu genuine Milleporina or not. 



