FUNDAMENTAL RELATIONS OF ANIMALS 33 



prove; there are Worms which in every respect appear superior to 

 certain Crustacea; the structure of the highest Acephala seems more 

 perfect than that of some Gasteropods, and that of the Halcyonoid 

 Polyps more perfect than that of many Hydroids. Classes do, there- 

 fore, 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 group. As a special para- 

 graph is devoted to the consideration of the character of orders in 

 my next chapter, I need not dwell longer upon this point here.^^ 

 It will be sufficient for me to remark now that the difficulties geolo- 

 gists have met with in their attempts to compare the rank of the 

 different types of animals and plants with the order of their suc- 

 cession 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 grada- 

 tion 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 be expected of those, the studies of which 

 are chiefly devoted to the investigation of the structure of our globe. 



To appreciate fully the importance of this question of the grada- 

 tion of animals and to comprehend the whole extent of the difficul- 

 ties 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. On the other hand, a complete fa- 

 miliarity with the many attempts which have been made to establish 

 a correspondence between the two and with all the crudities which 

 have been published upon this subject might dispel every hope to 

 arrive at any satisfactory result upon this subject, did it not appear 

 now that the inquiry to be conducted upon its true ground must be 

 circumscribed within different limits. The results at which I have 

 already arrived, since I have perceived the mistake under which in- 

 vestigators have been laboring thus far in this respect, satisfy me 



*^See Chap. II, Sect. in. 



