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- 

 Fishes 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 Comatulae, 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- 



