THE HOOFED MAMMALS. 249 



turn our attention to the history of the different orders 

 of Placental animals, whose pedigree can often be very 

 accurately established in detail. 



We must, as already remarked, consider the order of 

 Hoofed animals (Ungulata) as the primary group of the 

 Indeciduata, or Tuffc-placentals ; the two other orders, 

 Whales and Toothless animals, developed out of them, as 

 two diverging groups, probably only at a later period, by 

 adaptation to very different modes of life. But it is also 

 possible that the animals poor in teeth (Edentata) may be 

 of quite a different origin. 



Hoofed animals are in many respects among the most 

 important and the most interesting Mammals. They dis- 

 tinctly show that a true understanding of the natural 

 relationship of animals can never be revealed to us mei'ely 

 by the study of living forms, but in all cases only by an 

 equal consideration of their extinct and fossil blood-relations 

 and ancestors. If, as is usually done, only the living Hoofed 

 animals are taken into consideration, it seems quite natural 

 to divide them into three entirely distinct orders, namely: 



(1) Horses, or Single-hoofed animals (Solidungula,or Equina); 



(2) Ruminating animals, or Double-hoofed (Bisulca, or Rumi- 

 nantia); and (3) Thick-skinned, or Many -hoofed (Multungula, 

 or Pachyderma). But as soon as the extinct Hoofed animals 

 ,of the tertiary period are taken into consideration — of which 

 animals we possess very numerous and important remains 

 — it is seen that this division, but more especially the 

 limitation of the Thick-skinned animals, is completely arti- 

 ficial, and that these three groups are merely top branches 

 lopped from the pedigree of Hoofed animals, which are most 

 closely connected by extinct intermediate forms. The one 



