276 ESSAY ON CLASSIFICATION. 



class; we may rather expect them to be prominent be- 

 tween the orders of closely allied classes, or between the 

 orders of a higher class and the lower classes of the same 

 branch. We find, for instance, a remarkable correspond- 

 ence between the orders of the class of Batrachians, and 

 those of the class of true Reptiles. 1 The same may be 

 said of the order of Cetacea in the class of Mammalia, as 

 compared to the whole class of Fishes, or of the lower 

 order of the Insects (the Myriapods) as compared to the 

 class of Worms, or of the lower order of Acalephs (the 

 Hydroids) as compared to the class of Polyps. 2 An accu- 

 rate knowledge of this kind of analogies is of the utmost 

 importance for the study of the true affinities of animals, 

 since a misapprehension of the real value of their struc- 

 tural features has again and again misled zoologists into 

 combining such groups as if they were truly related. In 

 the beginning of the last century, for instance, the Cetacea 

 were generally united with the Fishes, to which they are 

 only analogous ; and even to this day we see the Hydroids, 

 which are true Acalephs of a lower order, united into one 

 class with the Polypi. 



Family analogies. It requires little familiarity with 

 the animal kingdom to know how strong may be the re- 

 semblance between the forms of animals, even when they 

 belong to entirely different types; but, unless their pat- 

 tern be determined by identical structural features, their 

 form certainly cannot be considered as homologous; and 

 however close the resemblance may be externally, an 

 attempt to distinguish between analogical and homologi- 



1 For further details upon this 2 For further details respecting 



point, see the second part of the first the Hydroids, I must refer to the 



volume of my Contributions to the third volume of my Contributions to 



Nat. Hist, of the Un. St., Sect. Ill, the Nat. Hist, of the Un. St., now in 



p. 252. the press. 



