LOUIS AGASSIZ. 241 



was obliged to describe a very nnmerons and entirely new faiina. "When 

 he commenced his investigation of fishes, he was not acquainted with 

 any species older than those of the coal strata ; the number of these 

 fossils has immeasurably increased since then, and is now counted by 

 thousands. Agassiz recognized that among the living species a small 

 number only have a heterocercal tail ; that is to say, one in which the 

 upper lobe is formed by the prolongation of the vertebral column. These 

 animals are the last representatives of a type largely diffused in the 

 Devonian and Carboniferous seas. All the other fishes, on the contrary, 

 have a homocercal or symmetrical tail, at the base of which the vertebral 

 column stops, and does not penetrate either of the lobes. He observed 

 that in the embryo of certain fishes the tail is at first heterocercal, as 

 that of the paleozic fishes, and afterward becomes homocercal. This 

 important discovery, in connection with others of a like nature, per- 

 mitted him to establish the law that the " embryo of the fish during its 

 development, the present class of fishes with its numerous families, 

 and the type of fish in its geological history, undergo strictly analogous 

 phases," and in a more general way he applies this law to vertebrates ; 

 " The successive creations have undergone phases of development anal- 

 ogous to those the embryo passes through during its growth, and simi- 

 lar to the gradations the present creation shows us in its ascending 

 series, considered as a whole." Then rising from the consideration of the 

 fishes to more general views of the phases of creation, he writes : " The 

 most incontestable result of modern paleontological research, in the ex- 

 amination of the question which at present occupies us, is the fact, now 

 beyond controversy, of the simultaneous appearance of particular types 

 of all classes of invertebrate animals from the earliest development of life 

 uijon the surface of the globe." The history of this successive develop- 

 ment " shows conclusively the impossibility of referring the first inhabi- 

 tants of the earth to a small number of branches, differentiated from 

 one parent stock by the influence of the modifications of exterior con- 

 ditions of existence."* 



It was in 1844 that Agassiz wrote these lifies, and he all his life re- 

 mained faithful to this opinion. The future will decide whether he was 

 right or wrong ; it is true that for the moment the balance does not seem 

 to lie in his favor. It is difiicult, indeed, to comprehend why the results 

 of these admirable researches, and of those he made later, did not lead 

 him to sustain the theory of transformation of which they seem to be the 

 natural consequence. Still it is impossible to consider his opposition to 

 this theory as resulting from prejudice, or as resting upon other grounds 

 than pure scientific reason. 



Agassiz had a mind too comprehensive, he was too enterprising, to con- 

 fine his attention to a single class of the animal kingdom, and we find 

 him soon producing a series of works upon a variety of subjects. Thus 

 he undertoolv, with JVI. Desor, the study of the Eehinoderms ; and their 



* Introduction to a mouograpli of the fossil fishes of tlie old red sandstone, pp. 4, 13. 

 S. Mis. 59 16 



