45 G ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



question ; and of those which relate especially to the Amer- 

 ican fauna, and thus concern every native naturalist. 



Origin of Vertebrate Limbs. 



Various views have been entertained respecting the gene- 

 sis and development of the limbs of Vertebrates. That most 

 prevalent twenty years ago, or more, was one especially 

 maintained by Professor Owen, namely, that they were di- 

 verging appendages of special arches, the anterior or pec- 

 toral being the " ribs " or " pleurapophyses " of the occipital 

 arch, and the posterior having corresponding relations with 

 the pelvic arch. This, however, was long ago shown to be 

 untenable. Recently several naturalists (e. </., F. Balfour, J. 

 K. Thacher, and St. George Mivart) have become satisfied that 

 the limbs were originally developed from lateral fins of the 

 same nature and parallel with the dorsal and anal, and that, 

 in the words of Balfour, they are " remnants of continuous 

 lateral fins." Balfour was led to this conclusion by the 

 study of the embryonic development of Selachians, while 

 Thacher and Mivart have reached the same result chiefly 

 from the investigation of the scrown forms of the same class, 

 and the likeness and essential similarity of structure of the 

 lateral and median fins. There can be, in fact, very little 

 doubt that the view of these naturalists named is the correct 

 one, and, this being adopted, all the facts of development 

 and modifications of the limbs harmonize. In the lowest of 

 the Vertebrates the Leptocardians, & e., Amphioxus and 

 Epifjonichthys there are lateral folds, one on each side, of 

 the same character as the dorsal and anal folds, designated 

 as fins ; and doubtless the nature of the limbed fins would 

 have been recognized before had not the Marsipobranchiates 

 intervened between the Leptocardians and the Selachians. 

 All living Marsipobranchiates have eel-like forms and are 

 destitute of lateral fins ; and this negative character, which 

 is probably simply the result of the elongated form and an 

 atrophy or loss, has been insensibly taken as a principal 

 feature of the class. In truth, however, the form is no more 

 to be considered as a necessary element of the Marsipobran- 

 chiate type than is the eel-like form of the Teleost, and we 

 must recognize in the living Marsipobranchiates simply 

 modified and divergent derivatives of a type which doubt- 



