GRADATION OF STRUCTURE AMONG ANIMALS. 41 



Mammalia stand highest; and it seems equally evident, 

 that in the main Insects and Crustacea are superior to 

 Worms, Cephalopoda to Gasteropoda and Acephala, and 

 Echinoderms to Acalephs and Polypi. But there are 

 genuine Insects, the superiority of which over many Crus- 

 tacea would be difficult to prove : there are worms which 

 appear in every respect superior to certain Crustacea : the 

 structure of the highest Acephala seems more perfect than 

 that of some Gasteropoda, and that of the Halcyonoid 

 Polyps more perfect than that of many Hydroids. Classes 

 do not therefore seem to be so limited hi the range of 

 their characters, as to justify in every type a complete 

 serial arrangement among them. But, when we come to 

 the orders, it can hardly be doubted that the gradation of 

 these natural divisions among themselves in each class 

 constitutes the very essence of this kind of groups. As a 

 special paragraph is devoted to the consideration of the 

 character of orders in my next chapter, I need not dwell 

 longer upon this point here. 1 It will be sufficient for me 

 to remark now, that the difficulties, with which geologists 

 have met in their attempts to compare the rank of the dif- 

 ferent types of animals and plants with the order of their 

 succession in different geological periods, have chiefly 

 arisen from the circumstance, that they have expected to 

 find a serial gradation, not only among the classes of the 

 same type, where it is only incomplete, but even among 

 the types themselves, between which such a gradation 

 cannot be traced. Had they limited their comparisons to 

 the orders which are really founded upon gradation, the 

 result would have been quite different; but, to do this, 

 requires more familiarity with Comparative Anatomy, with 

 Embryology and with Zoology proper, than can naturally 



3 See Chap. II, Sect. 3. 



