CHAPTER XXXIV. 

 THE DOCILE MAMMALS. 



WE may call the third sub-class docile or teachable mam- 

 mals, because they comprise the domestic animals, such as 

 the sheep, ox, pig, elephant, dog, cat, and monkey, all of 

 which have a high grade of intelligence, while their brains 

 are larger, and more complicated, the nearer they approach 

 man. 



They are also called placental mammals, because the 

 young at birth are of considerable size and nearly perfect in 

 development, being nourished until born by means of a 

 highly vascular mass or thick membrane (placenta) sup- 

 plied with arteries and veins, developed originally from the 

 allaiitoii, which is a temporary embryonic membrane. 



The different groups of placental mammals are wonder- 

 fully adapted to different modes of life, so that though 

 nearly all live on the ground, they have such different hab- 

 its, such varied modes of obtaining their livelihood that 

 there is room for all. We shall see that though all live on 

 milk when young, yet when able to earn their own living 

 they are very dependent upon the structure of their teeth 

 and limbs, tf we pick up a tooth we may know whether it 

 belonged to a grass-eating creature, like an ox, or a flesh- 

 eater, like a lion or cat, so intimate is the relation between 

 the structure and uses to which the different organs are 

 put, all being controlled by a brain and system of nerves so 

 much more perfect than any we have hitherto studied. 



ORDERS OP PLACENTAL MAMMALS. 



1. No incisor teeth; sometimes entirely 



toothless Bruta. Sloth. 



2. Rodents: with large incisor teeth Olires. Rat. 



8. Fore-limbs often adapted for burrow- 

 ing; teeth sharp; feeding on insects. .Insectivora, Moles, 



