and their Embryonic Development. 243 



lutely safe guide in the estimation of the relations of animals 

 to each other \ Ciivier, who made the study of comparative 

 anatomy the foundation of classification, carried out this 

 principle in a most remarkable manner, and improved the 

 natural arrangement of animals most surprisingly ; indeed, 

 he made zoology truly a science by it ; but with a tact that 

 characterises genius, he limited the absolute consequences of 

 this law by a true appreciation of the relative value of cha- 

 racters ; introducing at the same time with the principle of 

 classification, according to the structure of animals, that of 

 subordination of characters, without which the first great 

 principle might mislead us, instead of helping to ascertain 

 the true relations of organised beings. Now it seems to me 

 as if zoologists and anatomists had of late insisted too strictly 

 upon the absolute differences which exist between animals, 

 instead of attempting to appreciate the relative value of the 

 differences noticed. Of course, as this latter point rests 

 almost within the limits of individual appreciation, it is more 

 difficult to find the right path here, than in almost any other 

 department of zoological investigations ; but I hope to be 

 able to introduce another great principle of zoological classi- 

 fication, which shall afl[^ord a safe guide to settle such doubts ; 

 I mean the study of embryonic development. 



Let me now show, in the present instance, how I consider 

 it possible to be led by anatomical evidence considered in its 

 absolute results, to combinations strictly opposed to those 

 which an additional acquaintance with embryonic develop- 

 ment might indicate. 



Guided by his admirable natural feeling of affinitiei, 

 Cuvier placed in one and the same great division, sharks, 

 skates, and lamprey eels. Influenced by anatomical investi- 

 gation, and indeed by the most minute and admirable know- 

 ledge of their anatomical structure, derived from unparalleled 

 investigations, Joh. Muller concluded, on the contrary, that 

 the Cyclostomata were to be separated from the other car- 

 tilaginous fishes, and placed by themselves at the other end 

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

 by any further anatomical investigation ; it has thence- 

 forth become a matter of individual appreciation, unless 



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