CLASSIFICATION 85 



but the subject will be more fully developed in treating separately 

 of each division. 



One of the most certain and fundamental points in the classifica- 

 tion of the Mammalia is, that all the animals now composing the 

 class can be grouped primarily into three natural divisions, which, 

 presenting very marked differential characters, and having no exist- 

 ing, or yet certainly demonstrated extinct, intermediate, or trans- 

 itional forms, may be considered as subclasses of equal value, tax- 

 onomically speaking, though very different in the numbers and 

 importance of the animals at present composing them. These three 

 groups are often called by the names originally proposed for them 

 by Blainville (1) Ornithodelphia, (2) Didelphia, (3) Monodelphia 

 the first being equivalent to the order Monotremata, the second 

 to the Marsupialia, and the third including all the remaining 

 members of the class. Although actual palseontological proof is 

 wanting, there is much reason to believe that each of these, as now 

 existing, are survivors of distinct branches to which the earliest 

 forms of mammals have successively given rise, and for which 

 hypothetical branches Professor Huxley has proposed the names of 

 Prototheria, Metatheria, and Eutheria, names which, being far less 

 open to objection than those of Blainville, are here used as equiva- 

 lents of the latter. 



The only known existing PROTOTHERIA, although agreeing in 

 many important characters, evidently represent two very divergent 

 stocks, perhaps as far removed as are the members of some of the 

 accepted orders of the Eutheria. It would, however, be merely 

 encumbering zoological science with new names to give them any 

 other than the ordinarily known family designations of Ornitho- 

 rhynchidce and Echidn-idce. 



Similarly with regard to the METATHERIA, although the great 

 diversity in external form, in anatomical characters, and in mode of 

 life of the various animals of this section might lead to their 

 division into groups equivalent to the orders of the Eutheria, we do 

 not think it advisable to depart from the usual custom of treating 

 them all as forming one order, called Marsupialia, the limits of 

 which are equivalent to those of the subclass. The characters of the 

 six families which compose the group are extremely well marked 

 and easily defined ; and since they form a regular gradation between 

 two extreme types, they can be satisfactorily arranged in a serial 

 order. A marked distinction in the dentition enables us to divide 

 them into primary groups or suborders. 



The remaining mammals are included in the EUTHERIA, PLACEN- 

 TALIA, or MONODELPHIA. Their affinities with one another are so 

 complex that it is impossible to arrange them serially with any 

 regard to natural affinities. Indeed each order is now so isolated 

 that it is almost impossible to say what its affinities are ; and none 



