2 Mammals of Eastern Asia 
upon a dorsal axis or backbone, from which muscles move the 
extremities. The entire association of organisms with this kind 
of skeletal arrangement, in contrast to the external hard skeleton 
of crabs and insects, is termed the vertebrates, from the Latin, 
vertebra, "backbone." 
The methods by which vertebrate animals obtain their food 
and grow, by which they make sensory contact with their sur- 
roundings, by which they protect themselves from unfavorable 
conditions, and by which they perpetuate their kinds, provide 
further basic facts for their classification. Their classification is 
based upon the forms and functions of their physical parts, of 
their nervous, respiratory, digestive, glandular, excretory, and 
reproductive systems. It involves their methods of locomotion, 
their senses, and their instincts. 
The most easily observed of these in the mammals are those 
that pertain to their shape and external structure and to move- 
ment. The kind of food eaten can often be determined by the 
forms of the teeth. The climatic habitat of an animal may be 
indicated by qualities of its fur and skin. The degree of acuity 
of its senses of smell, hearing, and sight may be suggested by 
the characters of the muzzle, ears, and eyes. Whether it runs, 
hops, climbs, digs, swims, or flies can usually be deduced from 
the form of its limbs and tail. The limbs, tail, and teeth prob- 
ably furnish the most reliable and simple index for preliminary 
broad classification into orders and families. 
Limbs. The characters of the limbs of a mammal bear a defi- 
nite relationship to its way of moving from place to place and 
its method of seizing food. Probably the most generalized type 
of limb may be seen in the Common Shrew. Shrews' limbs are 
moderately tapered structures that hold the body off the ground, 
permit the animal to walk or run, and have five toes on both 
front and hind feet, each toe provided with a moderately sharp 
claw. The generalized pattern of Shrews' limbs becomes altered 
in a variety of ways. To function as digging tools, the limbs, 
