115 



enough, too, animals which usually do not develop pashm 

 almost immediately tend to its production when taken to 

 the Tibetan region, as is notably the case with dogs. Less 

 frequently the hair of the body takes the form of stiff 

 bristles, as on the pig; and still more rarely this thicken- 

 ing is carried to such an extent as to produce spines, of 

 which we have the best instances in the porcupine and 

 hedgehog, belonging, it should be borne in mind, to dis- 

 tinct orders. 



"The solid horns of the rhinoceroses, and the hollow 

 horny sheaths of cattle and antelopes are very similar in 

 their nature to hairs, and may indeed be compared to 

 masses of hair welded together into solid structures. 



"Mammals differ from Fish, Amphibeans and Eeptiles 

 in having warm blood which is propelled from a four 

 chambered heart through a double circulating system; 

 one part causing the blood to pass through the lungs to 

 take in a fresh supply of oxygen from the air, and the 

 other serving to supply the freshly oxygenized blood to 

 the various organs and members of the body; the blood 

 for the nourishment of the body being propelled from the 

 heart by a single vessel known as the aorta, which passes 

 over the left branch of the wind pipe. 



"With the exception of the egg laying Slonotremes, 

 Mammals are invariably born in a living condition, and 

 whether they live on the land or in the water breathe air 

 by means of lungs suspended in the chest. As a rule they 

 have the two pair of limbs characteristic of vertebrates, 

 but occasionally, as in the whales, the hinder pair may be 

 wanting. In some cases, like the kangaroos and jumping 

 mice, the hind limbs are enormously elongated and pro- 

 gression is affected by means of leaps and bounds. The 

 opposite extreme of limb structure is shown in the bats; 

 whose hind limbs retain their normal structure, while the 

 fore limbs are enormously elongated to afford support for 

 a leathery wing like structure, by means of which these 

 strangely modified creatures are enabled to fly in the air 

 with the same ease and swiftness as the birds. In the true 

 seals, the hind limbs are directed backward to form with 

 the tail a kind of rudder, while the fore limbs are short- 

 ened so as to form paddles for swimming, and as before 

 stated, in the completely aquatic mammals the hind limbs 

 are entirelj^ wanting. 



