Xlll 



tremely limited, he has adopted without modification the classifi- 

 cation of Bronn (who has availed himself of all the information 

 published up to his time), except for the Phylactolsemata, for 

 which he has followed Prof. Hyatt, who has since thoroughly- 

 studied that order. 



The details of classification of the families are yet too unsettled 

 to warrant the retention of the many sub-families which have 

 been proposed, and while the necessity for the adoption of such 

 subordinate groups is readily foreseen and admitted, so few 

 have been characterized in a manner which could be maintained 

 against criticism or justified by valid arguments, that only in 

 exceptional cases have any been admitted. 



GENERAL CONSIDERATIONS. 



In this connection it may be remarked that there is no scientific 

 basis for an a priori assumption that because the modifications 

 of an organ are of a certain importance in one branch or class 

 of animals, they are so in others. While such hints may perhaps 

 be of some use, the value requires to be verified in each instance. 

 Because the modifications in structure of the heart in mammals, 

 birds, and reptiles are of prime importance, it does not follow 

 that they are equally so in batrachians and fishes, and such a view- 

 is, indeed, opposed to facts. Still less foundation exists for the 

 a priori application of such ideas to the classification of the 

 mollusks ; and their distribution into two series, distinguished by 

 the bilocular (Monotocardian) and trilocular (Diocorclian) par- 

 tition of the heart, certainly seems to be opposed by the indica- 

 tions furnished by the sum total of the organization. 



And in like manner, because the modifications of a certain part 

 are the best indexes of affinity in one group of a class, it does 

 not follow that even in the same class, in another group, analo- 

 gous modifications are of like value. The dentition, for example, 

 is quite characteristic in the mammalian orders Carnivores, 

 Ungulates, and Rodents ; but in the Implacentals the value of 

 analogous modifications is very much less, and, within the range 

 of the same order (Marsupials), superficial differences, apparently 

 at least, as great as those between the cited orders of Placen- 

 tals are found. If, therefore, the modifications of the dentition 

 are used for the distinction of orders in one case, it is not because 



