GENERAL CHARACTERISTICS. 
3 
the Hoofed Animals or Ungulates (pigs, deer, cattle, horses, etc.), and the 
Carnivores (cats, dogs, etc.), which form three distinct orders, all agree with one 
another in that their young are nourished by milk sucked from the mother. Con¬ 
sequently such animals, together with all others showing the same peculiarity, are 
grouped together to form the class of Mammals or Mammalia. 
If, however, a mammal, a bird, a reptile, and a fish be compared together, it 
will be found that although the three latter differ from the former, in that the 
young are not suckled by the female parent, yet all agree in the possession of what 
we commonly call the backbone; this backbone consisting of a column running 
along the back of the animal, and composed of a number of jointed segments, 
which, although usually formed of bone, may be of cartilage. Such joints are 
technically known as vertebrae , and the whole column as the vertebral column; 
while all the classes possessing this vertebral column are grouped together under 
the name of Vertebrates, or Vertebrata, this largest group being known as a sub¬ 
kingdom. 
We accordingly have a scheme of classification like the following:— 
Subkingdom VERTEBRATA, or Vertebrates. 
Class Mammalia, or Mammals. 
Order Rodentia, or Rodents. 
Family Mu RIDAS , or Rats and Voles. 
Genus Mus, Rats and Mice. 
Genus Microtus, Voles. 
structure In saying that the Vertebrates, or highest of all animals, are 
of the characterised by the presence of a backbone or vertebral column, we 
Vertebrates. j mve gi ven only the primary feature of this great group ; and we must 
accordingly say a few words more on the subject of their structure. Now an 
essential feature in the structure of all Vertebrates is that on that side of the back¬ 
bone lying nearest to the back there runs a tube or canal, formed by arches of bone 
or cartilage springing from the bodies of the vertebrae, within which tube is the 
so-called spinal marrow or cord, which is a rope-like structure formed of nerve- 
tissue, and running backwards from the brain to the hinder extremity of the body. 
On the opposite side of the backbone to that occupied by the spinal marrow there 
is a much larger cavity containing the viscera, such as the heart, lungs, stomach, etc. 
In a cross-section of the body of any vertebrate animal we therefore see two 
tubes—a small one containing the nervous system placed above the backbone, and 
a much larger one containing the viscera situated below the backbone. 
Another noteworthy peculiarity of Vertebrates is that the limbs, which never 
exceed four in number, are always directed away from that part of the body which 
contains the nervous system, and towards that enclosing the viscera; whereas in 
nearly all the lower animals, collectively known as the Invertebrates, the reverse is 
the case. Vertebrates are likewise distinguished by the circumstance that the two 
jaws work in a vertical plane, or, in other words, are upper and lower, instead of 
being right and left, as they are in insects. 
Having said thus much as to the general characters of the Vertebrate sub¬ 
kingdom, we come to the consideration of those of its highest class, the Mammals. 
