INTRODUCTION 5 
in the plants. The first of these, constituting what is usu- 
ally known as the lean meat, is a relatively complex system 
of organs, differing widely according to the work performed. 
In the higher animals—the squirrel, for example—there are 
not less than five hundred muscles, which are under the 
control of the nervous system. 
The nervous system consists of the brain and spinal 
cord, which in the squirrel are concealed and protected 
Fic. 3.—Skeleton of squirrel, showing its relation to the body. 
within the skull and back-bone. From them many nerves 
pass outward to the muscles, and as many pass inward from 
the eye, ear, nose, tongue, or skin. By the action of these 
sense-organs the animal determines the nature of its sur- 
roundings, detects its food, recognizes the presence of its 
enemies, and is thus able to direct its movements to the 
greatest advantage. 
5. Multiplication of animals.—The organs thus far con- 
sidered serve to perpetuate the animal as an individual; 
but some provision must also be made for the continuance 
of the race. In the economy of nature each animal before 
