86 GRADATION AMONG ANIMALS. 
the Nautili may be drawn completely within the 
shell. If this view be correct, these two types 
occupy an intermediate position between the 
highest and the lowest divisions,of the Animal 
Kingdom, but stand on equal ground when com- 
pared with each other. 
Another, though a less direct, evidence that 
there is no absolute structural superiority or in- 
feriority between these two types as a whole 
may be found in the fact that the most pro- 
found naturalists who have attempted a serial 
arrangement of the whole Animal Kingdom have 
differed in their estimate of these two divisions, 
some placing the Mollusks highest, while others 
have given the ascendency to the Articulates. 
But is there a transition from Radiates to 
Mollusks, or from Articulates t§ Vertebrates, or 
from any one of these divisions into any other ? 
Let us first consider the classes as they stand 
within their divisions. We have seen that there 
are three classes of Radiates, — Polyps, Acalephs, 
and KEchinoderms; three classes of Mollusks, — 
Acephala, Gasteropoda, and Cephalopoda; three 
classes of Articulates,— Worms, Crustacea, and 
Insects ; and, according to the usually accepted 
classification, four classes of Vertebrates, — 
Fishes, Reptiles, Birds, and Mammalia. If there 
is indeed a transition between all these classes, 
it must become clear to us, when we have accu- 
rately interpreted their relative standing. 
