98 GRADATION AMONG ANIMALS. 
Take as an illustration the class of Echino- 
derms. The first representatives of this. class 
were a sort of Star-Fishes on stems; then were 
introduced animals of the same order without 
stems; in later periods come in the true Star- 
Fighes and Sea-Urchins; and the highest order 
of the class, the Holothurians, are introduced 
only in the present geological epoch. Compare 
now with this the ordinal division of the class as 
it exists to-day. The present representative of 
those earliest Echinoderms on stems is an animal 
that upon structural evidence stands lowest in 
the class; next above it are the Comatule, cor- 
responding to the early Echinoderms without 
stems; next in our classification are the Star- 
Fishes and Sea-Urchins; and -the Holothurians 
stand highest, on account of certain structural 
features that place them at the head of their 
class. The Series of Time and the Series of 
Rank, then, accord perfectly, and investiga- 
tions of the embryological development of these 
animals have shown that the higher Echinoderms 
pass through changes, during their growth, that 
indicate the same kind of gradation, for the young 
in some of them have a stem which is gradually 
dropped, and their successive phases of develop- 
ment recall the adult forms of the lower orders. 
Take as another illustration. the class of Pol- 
yps. First in time among the early Reef-Build- 
