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 paleontological 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 Kutheria. It would, however, be merely 
encumbering zoological science with new names to give them any 
other than the ordinarily known family designations of Ornitho- 
rhynchide and Echidnide. 
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 
