SUCCESSION OF CHARACTERS. 2G5 



mine 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 w T hat are 

 typical, class, ordinal, family, generic, and specific cha- 

 racters, to Le justified in maintaining, that, in the progress 

 of embryonic growth, the features which become suc- 

 cessively prominent correspond to these characters, and 

 occur in the order of their subordination 1 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 develop- 

 ment which may suggest the idea of a progress from a 

 more general typical organization to its ultimate specializ- 

 ation ; 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 into tin's matter. Taking it for granted that what 

 I have said about the characteristics of natural groups 

 in the animal kingdom is correct, that we have, 1st, four 

 great typical branches of the animal kino-dom, character- 



O / .L O 



ized by different plans of structure ; 2nd, classes, cha- 

 racterized by the ways in which, and the means with 

 which these plans of structure are executed ; 3rd, orders, 

 characterized by the degrees of simplicity or complication 

 of that structure ; 4th, families, characterized by differ- 

 ences of form, or by structural peculiarities determin- 

 ing form ; 5th, genera, characterized by ultimate pecu- 

 liarities of structure in the parts of the body ; Gth, species, 



