GENERAL CHARACTERISTICS. 5 



in the other two classes mentioned the aorta crosses either the right branch or both 

 branches of the windpipe. 

 Respiration. ^^ MamrnaLs, whether they live on the land or in the water, 



breathe air by means of lungs suspended in the chest ; and during no 

 period of their life do they ever develop gills; neither do they ever underTro a 

 metamorphosis analogous to that presented by the change of a tadpole into a frof. 

 By these last two negative characters they are, therefore, sharply distinguislied 

 from the Amphibians, with which, as we have seen, they agree in the mode by 

 wliich the skull is articulated to the first joint of the backbone. 



Young-. '^\\X'^ the sole exception of the egg-laying Mammals, or Mono- 



tremes, of Australia and New Guinea, which are the lowest members 

 of the class, the young of Mammals are invariably born in a living condition. 



Vertebra of A remarkable feature in Mannnals is the circumstance that, with 



Neck. oj^iy yjj.gg constant exceptions, the number of joints, or vertebrae, in 



the neck i.s seven ; this number being equally constant in the enormously elongated 



neck of the giraffe, or in the extremely short(>ned one of the whale, where the 



vertebrae are reduced to tliin plates of bone. 



Structure. ^^ '^ ^'"^'^' M^"""*!'** l^^ve the two pairs of limbs characteristic of 



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

 wanting. In a large proportion of specie.s the hind- and fore-limbs are of approxi- 

 mately equal length. In some cases, however, tJie hind-limbs may be enormously 

 elongated at the expen.se of the fore-limb.s, as we .see in the kangaroos and junqiing 

 mice ; and progression is then effected by means of leaps and bounds from these 

 strong hind-limbs. The opposite extreme of iimb-stnicture is shown among the 

 bats, where, while the hinder pair retain their normal structure, the fore-limbs are 

 enormously elongated to afford supjiort to 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 Bird.s. In the wliales and doljjhins, which lead a purely 

 aquatic life, we find the fore-limbs modified into paddles for swinnniug, while the 

 hind ones are, as we have .said, totally wanting. Similar conditions obtain in the 

 dugongs and manatis ; but in the true .seals, which are less completely aquatic, 

 the hind-limbs are still well developed, although directed backwards to form, in 

 connection with the tail, a kind of rudder. The bats are the only Mammals which 

 are wholly adapted for flight, but we meet with certain forms in other groups, such 

 as the flying squirrels among the Rodents, and the flying phalangers among the 

 Pouched Mannnals, which are enabled to take long leap-like flights from tree to 

 tree by means of a kind of a parachute formed of folds of skin running along the 

 .sides of the body from limb to limb. The limbs themselves are not, however, 

 .specially modified ; and true flight, in the sense of propulsion caused by up-and- 

 <lown strokes of the fore-limbs, is not performed by these Mammal.s. We shall 

 have something more to add on the subject of limbs in the paragraphs devoted to 

 the skeleton. 



Almost as great variations are displayed in the modifications and uses of the 

 tail of Mammals. In tlie majority of cases the tail is present and forms a tapering 

 axis, often clothed with long hair, which may considerably exceed the total length 

 of the body. The Mannnal, in which the relative length of the tail is greatest, is 



