Chap. I. GRAPATIOX OF STRUCTURE AMONG ANIMALS. 29 



Tlic next question would then l)e : Does the animal kintrtlom constitute several, 

 or any nunihor of graduated series? In attempting to ascertain the value of the less 

 comprehensive groups, when compared to one another, tlie difficulties seem to be 

 gradually less and less. It is already possil)le to mark out with toleralile precision, 

 the relative standing between the classes, though even here we do not yet perceive 

 in all the types the same relations. Among Vertebrata, there can be little if any 

 doubt, that the Fishes are lower than the Reptiles, these lower than Birds, and that 

 Manniialia stand highest ; it seems equally evident, that in the main. Insects and 

 Crustacea are superior to Worms, Cephalopods to Gasteropods and Acephala and 

 Eehinodenns to Acalephs and Polypi. But there are genuine Insects, the superiority 

 of which over many Crustacea, would be difficult to prove ; there are Woi'ms which 

 in every respect appear superior to certain Crustacea ; the structui-e of the highest 

 Acephala seems more perfect than that of some Gasteropods, and that of the Halcyo- 

 noid Polyps more perfect than that of many Hvdroids. Classes do, therefore, not 

 seem to be so limited in 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 para- 

 graph is devoted to the consideration of the character of orders in ni}^ next chapter, 

 I need not dwell longer ujoon this point here.^ It will be sufficient for me to 

 remark now, that the difficulties geologists have met with, in their attempis to com- 

 pare the rank of the different types of animals and plants with the order of their 

 succession in different geological periods, has 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 compari- 

 sons to the orders Avhich are really founded upon gradation, the result would have 

 been quite different; but to do this requires more familiarity Avitli Comparative 

 Anatomy, with Embryology and with Zoolog}^ proper, than can naturally be expected 

 of those, the studies of which are chiefly devoted to the investigation of the struct- 

 ure of our glol)e. 



To appreciate fully the importance of this question of the gradation of animals, 

 and to comprehend the whole extent of the difficulties involved in it, a superficial 

 acquaintance with the perplexing question of the order of succession of" animals in 

 past geological ages, is by no means sufficient ; a complete familiarity with the many 

 attempts which have been made to establish a correspondence between the two, and 

 with all the crudities which have been publi.'<hed upon this subject, might dispel 



» See Clwip. TI. 



