172 ESSAY ON CLASSIFICATION. Part I. 



to Ecliinodenns ; Crustacea 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 have been founded, and these differ- 

 ences have occasionally been exaggerated so far as to give rise to the establishment 

 of distinct branches. Upon similar relations between the brauches, 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 characters distinguishing the 

 different types of the animal kingdom were developed in the embryo in the suc- 

 cessive order of their importance : first the structural features of their respective 

 branches, next the charactei-s 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 almost without discussion, and to be generally taken for granted now. 

 The importance of the subject requires, however, a closer scrutiny ; for if Embry- 

 ology is to lead to great improvements in Zoology, it is necessary, at the outset, 

 to determine well what kind of information we may expect it to furnish to its 

 sister science. Now I would ask if, at this day, zoologists know with sufficient 

 precision what are typical, class, ordinal, family, generic, and specific characters, to 

 be justified in maintaining that, in the progress of embryonic growth, the features 

 which become successively prominent correspond to these characters and in the 

 order of their subordination ? I doubt it. I Avill say more : I am sure there is 

 no such understanding about it among them, for if there was, they Avould already 

 have perceived that this assumed coincidence, between the subordination of natural 

 groups among full-grown animals and the successive stages of growth during their 

 embryonic period of life, does not exist in nature. It is true, there are certain 

 features in the embryonic development which may suggest the idea of a progress 

 from a more general typical organization to its ultimate specialization, but it nowhere 

 proceeds in that stereotyped order of succession, nor indeed even in a general way, 

 in the manner thus assumed. 



