346 



THE HISTOKY AND SCOPE OF ZOOLOGY 



IX 



Mollusca. 



Echinoderma. 



Arthropoda. 



Vertebrata. 



In representing pictorially the groups of the animal 

 kingdom as the branches of a tree, it becomes obvious 

 that a distinction may be drawn, not merely between 

 the individual main branches, but further as to the 

 level at which they are given off from the main stem, 

 so that one branch or set of branches may be marked 

 off as belonging to an earlier or lower level than 

 another set of branches ; and the same plan may be 

 adopted with regard to the cladi, classes, and smaller 

 branches. The term "grade" was introduced by 

 myself 1 to indicate this giving off of branches at a 

 higher or lower, i.e. a later or earlier, level of a main 

 stem. The mechanism for the statement of the genea- 



1 " Notes on Embryology and Classification," in Quart. Journ. Micr. 



Sci. 1877. 



