GENERAL CHARACTERISTICS 5 



the aorta, which passes over the left branch of the windpipe ; whereas in the other 

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

 metamorphosis 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. 



With the sole exception of the egg-laying Mammals, or Monotremes, 



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

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



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 enor- 

 mously elongated neck of the giraffe, or in the extremely shortened one of the 

 whale, where the vetebrae are reduced to thin plates of bone. 



As a rule, Mammals 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 ap- 

 proximately equal length. In some cases, however, the hind-limbs may be enor- 

 mously 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 Mam- 

 mals 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 



