250 LAKE SUPERIOR. 



to be led by anatomical evidence considered in its absolute results, to 

 combinations strictly opposed to those which an additional acquaint- 

 ance with embryonic development might indicate. 



Guided by his admirable natural feeling of affinities, Cuvier placed 

 in one and the same great division, sharks, skates, and lamprey-eels. 

 Influenced by anatomical investigation, and indeed by the most min- 

 ute and admirable knowledge of their anatomical structure, derived 

 from unparalleled investigations, Joh. Mttller concluded, on the con- 

 trary, that the Cyclostomata were to be separated from the other 

 cartilaginous fishes, and placed by themselves at the other end of 

 the class. Who is right in this case cannot be ascertained by any 

 farther anatomical investigation ; it has thenceforth become a matter 

 of individual appreciation, unless we introduce another principle, by 

 which we can weigh the real value of these remarkable differences. 

 Such a principle, I think, we have in the metamorphosis of embryonic 

 life. Indeed, if it can be shown, that besides the differences which 

 exist in all fishes between their earliest forms and their full-grown 

 state, there are peculiarities in sharks, skates, and lamprey-eels 

 common to all of them, from an early period of development, which 

 remain characteristic throughout life, it must be acknowledged that 

 these families belong to one and the same great group, notwithstand- 

 ing their extreme differences in their full-grown condition. Now, 

 such facts exist. In the first place, it is impossible, without disturb- 

 ing their true affinities, to consider an extraordinary development of 

 pectoral and ventral fins as a standard to appreciate fundamental 

 relations between fishes, as in all fishes, without exception, they are 

 both wanting in earlier life, and as there is scarcely a family in which 

 ventrals at least, are not wanting in some genus or other. We might 

 just as well place Petromyzons among the eels, as their common 

 English name purports, on the ground of the deficiency of their 

 abdominal and thoracic organs of locomotion, as separate them from 

 the other Placoids. Again, the peculiarities in the development of 

 the dorsal, caudal, and anal fins in sharks and skates, and the differ- 

 ences which exist between them and the Petromyzons, indicate in 

 no way their affinity or their difference ; in Petromyzon we have the 

 embryonic condition of vertical fins, where a continuous fold in the 

 skin of the middle line extends, as in aU embryo fishes, from the back 



