596 HISTORICAL GEOLOGY. 



central part of the spinal cord (notochord) remains persistent ; 

 they have also a vertebrated tail, like young Grars, as observed by 

 Agassiz. 



6. The species under the early comprehensive types not the lowest species 

 of the group represented. — This follows from the proposition on p. 583. 



7. In the first appearance of a group (as that of Vertebrates, or that of 

 Reptiles,) the species are often from near the junction of its inferior and supe- 

 rior subdivisions, — species from the middle or upper portion of the inferior 

 either occurring alone, or else associated with others from the middle or lower 

 portion of the superior. The former frequently pertain to a comprehensive 

 type, which is intermediate between the two subdivisions, though belonging 

 usually to the inferior: sometimes there is more than one comprehensive type, 

 as in the case of land-plants. — This principle is announced and briefly 

 illustrated on page 396. 



A more systematic exhibition of the principle is here given. 



The following are the two grand subdivisions in some of the higher groups in 

 Nature, — the first mentioned being the inferior, the other the superioi: The 

 latter is also the more typical group, or that in which the idea of the type is 

 more fully represented : — 



a. Life in general. — (1.) Vegetable kingdom; (2.) Animal kingdom. 



b. Vegetable kingdom. — (1.) Cryptogams, or flowerless plants; (2.) Phseno- 

 gams, or flowering plants. 



c. Animal kingdom. — (1.) The flower-like type, including Radiates; (2.) the 

 true Animal type, or cephalized species, that is, those having a head (or ante- 

 rior and posterior polarity with bilateral symmetry), including Mollusks, Arti- 

 culates, and Vertebrates. 



d. Sub-kingdom of Mollusks. — (1.) The flower-like type, including the Bryo- 

 zoans, closely like flowers, the Brachiopods, which also in general were attached 

 below by stems or pectecels, and Ascidians, also, often attached and many 

 radiated exteriorly; (2.) the true Molluscan type, including Acephals, Cepha- 

 lates, and Cephalopods. 



e. Sub-kingdom of Vertebrates. — (1.) Water-vertebrates, including Fishes; (2.) 

 Land-vertebrates, including Reptiles, Birds, and Mammals. 



/. Glass of Crustaceans. — (1.) Entomostracans; (2.) MalacostracaUs. 



g. Class of Reptiles. — (1.) Amphibians; (2.) True Reptiles (p. 344). 



h. Class of Mammals. — (1.) Marsupials, or Semi-oviparans ; (2.) Non-marsu- 

 pials, or typical Mammals (p. 423). 



■\ 1. Life in geneeal. — In the inferior subdivision the earliest species of life 

 were probably the ProtopJiytes, — these and other Algae commencing in the later 

 Azoic. They have the locomotive powers of animals, and may therefore be 

 regarded as an example of one of the comprehensive types, and the first. The 

 Protozoans (Rhizopods, etc.) may have been the associated species of the superior 

 group, as remarked on page 147. The two are alike in extreme simplicity of 

 organization. In Algae the Radiate type of structure, characteristic of the typi- 

 cal plant, is not brought out; and in Protozoans neither- of the four great 

 Animal types appears, — the Radiate, Molluscan, Articulate, or Vertebrate. 



