478 PIONEERS OF SCIENCE IN AMERICA. 



Professor of Natural History. This position he held till his 

 departure for America, although in the meantime invited to 

 Geneva and Lausanne. 



Through the aid and influence of Humboldt, between whom 

 and Agassiz there existed the warmest friendship, he was en- 

 abled to begin the publication of his Poissons Fossiles, a work 

 evincing such careful and profound research, and such a won- 

 derful power of generalization, as to obtain for him a place 

 among the very first naturalists of the day. Cuvier, who had 

 intended to prepare such a work, generously abandoned his 

 plan after perceiving his young friend's capability, and gave 

 Agassiz the use of his material. This work, which appeared in 

 parts, between the years 1833 and 1845, comprises five volumes, 

 of about 1,700 quarto pages, with an atlas of 400 folio plates, 

 and contains descriptions of nearly a thousand species of fossil 

 fishes. Aside from the great number of species, genera, and 

 families established, Agassiz adopted an entirely new system of 

 classification. In the classification proposed by Cuvier, fishes 

 were divided into two orders, according to the nature of the 

 skeleton, viz., cartilaginous and osseous. Agassiz looking 

 upon the external covering of the animal as a reflex of the 

 connection existing between the being and its surroundings, 

 bearing the imprint of all the peculiarities of its existence, and 

 consequently of its organization deemed that the true princi- 

 ple of the classification of fishes was to be found in the scales. 

 In view of this he proposed a division of the families of fishes 

 into four orders, viz., Placoids, in which the scales are rep- 

 resented by plates of enamel, as in the sharks; Ganoids, in 

 which the scales consist of angular bony plates covered with a 

 thick layer of ^enamel, as in the garpikes ; Ctenoids, or fishes 

 with true scales, in which the posterior edges of the laminae 

 are toothed ; and Cycloids, in which the scales are composed of 

 simple laminae with smooth posterior edges. Agassiz found 

 that the study of fossil fishes exhibits a remarkable parallelism 

 between the development of the individual and that of the class 

 in geologic time. During part of the embryonic life of fishes, 

 and even in some adult forms, the dorsal cord exists as a simple 

 gelatinous cylinder, surrounded by a fibrous sheath, in which, 

 after a time, there is found a cartilaginous and then an osseous 

 deposit, which goes to form the vertebrae, the ossification tak- 

 ing place first in the apophyses. This embyronic character 



