6 CLASSIFICATION OF THE MAMMALIA. [CHAP. 



III. The Monodelphia, Eutheria, or "placental mammals " 

 (so-called because the foetus is always nourished while 

 within the uterus of the mother by means of an allantoic 

 placenta), include at present by far the greater portion of 

 the class. While the survivors of the other groups have 

 probably been for a long time in a stationary condition, 

 these have, as there is already good evidence to show 

 throughout all the Tertiary geological age, and by inference 

 for some time before, been multiplying in numbers and 

 variations of form, and attaining higher stages of develop- 

 ment and specialisation in various directions. They con- 

 sequently exhibit far greater diversity of external or adaptive 

 modification than is met in either of the other sub-classes 

 some being fitted to live exclusively in the water as 

 fishes, and others to emulate the aerial flight of birds. 



To facilitate the study of the different component members 

 of this large group it is usual to separate them into certain 

 divisions which are called "orders." In the main zoologists 

 are now of accord as to the general number and limits of 

 these divisions among the existing forms, but the affinities 

 and relationships of the orders to one another are far from 

 being understood, and there are very many extinct forms 

 already discovered which do not fit at all satisfactorily into 

 any of the orders as commonly defined. 



Commencing with the most easily distinguished, we may 

 first separate a group called Edentata, composed of several 

 very distinct forms the Sloths, Anteaters, and Armadillos 

 which under great modifications of characters, of limbs, and 

 digestive organs, as well as habits of life, have just enough 

 in common to make it probable that they are the very 

 specialised survivors of an ancient group, most of the 

 members of which are extinct, but which the researches of 

 palaeontology have not yet revealed to us. The characters 



