GENERAL CHARACTERISTICS. 5 



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

 branches of the windpipe. 



All Mammals, 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 undergo a 

 jiirtamorphosis analogous to that presented by the change of a tadpole into a frog. 

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

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

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



Youn With 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. 



vertebrae of A remarkable feature in Mammals is the circumstance that, with 



only three constant exceptions, the number of joints, or vertebrae, in 

 the neck is seven ; this number being equally constant in the enormously elongated 

 neck of the giraffe, or in the extremely shortened one of the whale, where the 

 A'ertebrse are reduced to thin plates of bone. 



structure ^ S a ru ^ 6 ' ^ amma l s have 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 species the hind- and fore-limbs are of approxi- 

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

 elongated at the expense of the fore-limbs, as we see in the kangaroos and jumping 

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

 strong hind-limbs. The opposite extreme of limb-structure is shown among the 

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

 enormously elongated to afford support 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 Birds. In the whales and dolphins, which lead a purely 

 aquatic life, we find the fore-limbs modified into paddles for swimming, 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 Mammals, 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- 

 down strokes of the fore-limbs, is not performed by these Mammals. 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 the 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 Mammal, in which the relative length of the tail is greatest, is 



