264 ESSAY ON CLASSIFICATION. 



themselves than others : Polypi and Acalephs, for instance, 

 stand nearer to one another than to Echinoderms ; Crus- 

 tacea and Insects are more closely allied to one another 

 than to Worms, etc. Upon such degrees of relationship 

 between the classes, within their respective branches, the 

 so-called sub-types or sub-branches have been founded, 

 and these differences have occasionally been exaggerated 

 so far as to give rise to the establishment of distinct 

 1 tranches. Upon similar relations between the branches, 

 sub-kingdoms have also been distinguished, but I hardly 

 think that such far-fetched combinations can be considered 

 as natural groups ; they seem to me rather the expression 

 of a relation arising from the weight of their whole 

 organization as compared with that of other groups, than 

 the expression of a definite relationship. 



SECTION VIII. 



SUCCESSIVE DEVELOPMENT OF CHARACTERS. 



It has been repeated again and again, that the cha- 

 racters which distinguish the different types of the animal 

 kingdom are developed in the embryo in the successive 

 order of their importance : first, the structural features 

 of their respective branches ; next, the characters of the 

 class ; next, those of the order ; next, those of the family ; 

 next, those of the genus ; and, finally, those of the species. 

 This assertion has met with no direct opposition ; on the 

 contrary, it seems to have been approved of almost without 

 discussion, and now to be generally taken for granted. 

 The importance of the subject requires, however, a closer 

 scrutiny ; for if Embryology is to lead to great improve- 

 ments in Zoology, it is necessary at the outset to deter- 



