CLASSIFICATION OF THE MAMMALIA 57 



It must be admitted that no method, yet devised, of apply- 

 ing the Linnaean scheme to the fossils is altogether satisfactory, 

 and indeed it is only the breaks and gaps in the palseontological 

 record which makes possible any use of the scheme. Could 

 we obtain approximately complete series of all the animals 

 that have ever lived upon the earth, it would be necessary to 

 invent some entirely new scheme of classification in order to 

 express their mutual relationships. 



In the present state of knowledge, classification can be made 

 only in a preliminary and tentative sort of way and no doubt 

 differs widely from that which will eventually be reached. 

 So far as the mammals are concerned, part of the problem would 

 seem to be quite easy and part altogether uncertain. Some 

 mammalian groups appear to be well defined and entirely 

 natural assemblages of related forms, while others are plainly 

 heterogeneous and artificial, yet there is no better way of 

 dealing with them until their history has been ascertained. 

 The mutual relations of the grand groups, or orders, are still 

 very largely obscure. 



The class Mammalia is first of all divided into two sub- 

 classes of very unequal size. Of these, the first, PROTO- 

 THERIA, is represented in the modern world by few forms, 

 the so-called Duck-billed Mole {Ornithorhynchus paradoxus) 

 and Spiny Anteaters (Echidna) of Australia. They are 

 the lowest and most primitive of the mammals and retain 

 several structural characters of the lower vertebrates. Their 

 most striking characteristic is that the young are not brought 

 forth alive, but are hatched from eggs, as in the reptiles, birds 

 and lower vertebrates generally. 



The second subclass, EUTHERIA, which includes all 

 other mammals, is again divided into two very unequal groups 

 or infraclasses. One of these, Didelphia, contains but a single 

 order, the Marsupialia, or pouched mammals, now in existence, 

 and is also very primitive in many respects, though far more 

 advanced than the Prototheria. The young, though born alive, 



