22 MAN, — THE ANIMAL 



nical term has been used for nearly 200 years, but 

 it remained for these two men to formulate the 

 generalization that was to place the study of 

 Biology upon a scientific basis. All life begins as 

 a single cell and the cell is the smallest vital unit 

 into which a complex living body can be resolved. 

 The trees, the daisy, the bird or man can be 

 analyzed in terms of the cells found in each. This 

 word, cell, is the name for the active, vital sub- 

 stance, protoplasm. This means that such ques- 

 tions as disease, heredity, growth, old age, in 

 short, every biological question, can only be an- 

 swered through a study of cells. The cell, then, 

 is the modern starting point for all biological 

 problems. The law of structure for all living 

 things asserts that they are composed of one or 

 more than one cell. 



7. The Law of Growth. — We can distinguish 

 In living things a number of common parts, for 

 example, all fish, frogs, and men have a heart, 

 stomach, kidneys, a nervous system, skeleton, 

 muscles, etc. Perhaps you have noticed the 

 gradual growth from infancy to maturity of an 

 individual in one of these great groups of animals. 

 The most searching inquiry fails to find any new 

 parts added but rather a turning of the immature 

 structure into a mature state. This is easily illus- 

 trated by examining the wrist of a child and com- 

 paring it with that of an adult. How do the adult 



