240 LOUIS AGASSIZ. 



The Introduction presents, as it were, the 

 prelude to this vast chapter of natural history 

 in the simultaneous appearance of the four 

 great types of the animal kingdom : Radiates, 

 Mollusks, Articulates, and Vertebrates. Then 

 comes the orderly development of the class by 

 which the vertebrate plan was first expressed, 

 namely, the fishes. Underlying all its divis- 

 ions and subdivisions, is the average expression 

 of the type in the past and present ; the Pla- 

 coids and Ganoids, with their combination of 

 reptilian and fishlike features, characterizing 

 the earlier geological epochs, while in the later 

 the simple bony fishes, the Cycloids and Cte- 

 noids, take the ascendency. Here, for the first 

 time, Agassiz presents his " synthetic or pro- 

 phetic types," namely, early types embracing, 

 as it were, in one large outline, features after- 

 ward individualized in special groups, and 

 never again reunited. No less striking than 

 these general views of structural relations are 

 the clearness and simplicity with which the dis- 

 tribution of the whole class of fishes in rela- 

 tion to the geological .formations, or, in other 

 words, to the physical history of the earth, is 

 shown. In reading this introductory chapter, 

 one familiar with Agassiz as a public teacher 

 will almost hear his voice marshaling the long 



