GENERAL CHARACTERISTICS. 
5 
Respiration. 
Young. 
Structure. 
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 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 
Neck. 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 
vertebrae 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 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 
