THE ORIGIN OF MAN 11 



hended when many cells live together and form 

 the unit or individual that is in the plants and 

 animals as usually understood. All these forms 

 are said to have a body consisting of unit masses 

 or cells, which in the higher forms of plants run 

 into countless numbers. It is impossible for these 

 cells to remain the same, for as they increase in 

 number they become diversely related to the 

 outer world, to food, to one another, and so on. 

 Division of labor, consequent on diversity of con- 

 ditions, is thus established in the organism. In 

 some cells one kind of activity predominates, in 

 others a second, in still others a third, and this 

 division of labor is associated with that complica- 

 tion of structure which is called differentiation. 



The functions, cellular structure, and develop- 

 ment in plants and animals are essentially alike, 

 and there is no absolute distinction between them. 

 But as these two groups are organisms differing 

 in their totality of organization and in the detail 

 of their functions, they have developed along two 



