232 Professor L. Agassiz on the Principles of 



in a permanent condition, the embryonic state of development 

 of common Gasteropoda, during which the bulk of the yolk 

 passes through the process of repeated divisions. 



This principle — of embryological changes as a foundation of 

 the natural classification in the internal arrangement of all 

 the minor groups in the natural classes of the animal king- 

 dom — applies vt^ith equal success to the Vertebrata. 



We need only contrast the successive changes of tailless 

 Batrachians during their metamorphoses, vv^ith the perma- 

 nent forms of the caudate and branchiate types in that order, 

 to be satisfied that the relative rank of all these genera can 

 in no way be better determined, than by a direct comparison 

 of the permanent forms of the whole group, with the succes- 

 sive changes in the embryonic condition of its higher types ; 

 and a comparison of the metamorphoses themselves, in the 

 different genera, will leave no doubt as to which of them the 

 highest rank should be assigned. 



I have already, on other occasions, alluded to the improve- 

 ments which are likely to be introduced into our classification 

 of birds, upon considerations derived from embryological data. 

 I may be permitted here to add, that even the classification 

 of Mammalia will receive decided improvements upon the 

 consideration of embryological changes. A single instance, 

 even now, will at least shew that the true relative rank of 

 their families can be determined in that way. We need only 

 compare, among true Carnivora, the Plantigrades, the Digiti- 

 grades, and the web-footed Seals, with the transformation of 

 the limbs in the embryo of cats and dogs, to be satisfied that 

 the order in which these animals, as arranged by Cuvier, 

 does not agree with their natural metamorphoses, and that 

 the Plantigrades should rank below the Digitigrades, nearer 

 to the Seals, and the Digitigrades highest ; and the affinity 

 of the Ice Bear to the Seals will further sustain this 

 statement. 



These remarks will, at the same time, shew that no inves- 

 tigations are at present more needed to improve our natural 

 methods in classification, than a thorough study of young 

 animals ; and that an extensive illustration of the young of 

 all the principal representatives of the great natural groups 



