182 ESSAY ON CLASSIFICATION 



SECTION VIII 

 SUCCESSIVE DEVELOPMENT OF CHARACTERS 



It has been repeated, again and again, that the characters distin- 

 guishing the different types of the animal kingdom were 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 ap- 

 proved almost without discussion and to be generally taken for 

 granted now. The importance of the subject requires, however, a 

 closer scrutiny; for if Embryology 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 charac- 

 ters, 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 will say more: I am sure there is no such understanding about 

 it among them, for if there was, they would 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 organ- 

 ization 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. 



Let us see whether it is not possible to introduce more precision 

 in this matter. Taking for granted that what I have said about the 

 characteristics of the natural groups in the animal kingdom is cor- 

 rect, 'that we have, 1st, four great typical branches of the animal 

 kingdom, characterized by different plans of structure; 2d, classes 

 characterized by the ways in which and the means with which these 



