326 SKETCHES OF CUE AT I ON. 



Next, let us recur to the nature of the geological succes 

 sion of organic types. Every tyro in geological science 

 has learned that we have here, viewed as a whole, a grad 

 ually ascending succession of forms. Among other se 

 quences, we find the fish followed in time by the batrachi- 

 an, which in its embryo state is fish-like, and in its adult 

 state is reptile-like. The batrachian was followed by the 

 strictly air-breathing reptile, in which the ventricle is not 

 separated bjt a partition a condition existing in the heart 

 of the embryo mammal. Finally, reptiles were succeeded 

 by quadrupeds and man. 



Again, the earliest crustaceans were Trilobites, followed 

 by Phyllopods. These were followed by long-bodied lob 

 sters (Macrourans), and these, in turn, by the crabs (Brachy- 

 urans). Now this succession of forms is the same as is ex 

 pressed in the embryonic history of the highest form of 

 crustacean. 



Still, again r fishes with cartilaginous skeletons and (so- 

 called) heterocercal unequally-lobed tails predominated 

 in the earlier periods, while our existing waters are ten 

 anted by fishes with bony skeletons and (so-called) homo- 

 cereal tails. It is a curious fact that this order of succes 

 sion is represented by the embryonic stages of the common 

 whitefish of Europe, and corresponds also to the discrim 

 inations of rank which are recognized in the class of fishes. 



Coral animals furnish us with another beautiful illustra 

 tion of these harmonies. It has been shown by Agassiz, 

 who has enjoyed remarkable facilities for the study of all 

 classes of animals, that the polyps, structurally considered, 

 present a gradation which is expressed, in ascending order, 

 by the following arrangement of groups : Actinice, Fun- 

 gidce, Astrceans, Porites, Madrepores, Ilalcyonoids. From 

 the Actinice, whose soft bodies and indefinite multiplication 

 of tentacles mark them lowest in the scale, to the Haley- 



