340 GENERAL PRINCIPLES 



poses of securing food or escaping enemies as much as to 

 facilitate mating and the carrying of the species to new and 

 favorable localities. 



724. Differentiation of Germinal and Somatic Tissues. 

 Reproduction in the protozoa and most unicellular plants (all 

 unicellular organisms are some times grouped together under 

 the name Protista) means merely a division of the body of the 

 organism. The two halves resulting from division reorganize 

 themselves, and after a period of growth, each has attained to 

 the condition of the parent cell before division. This process 

 continues indefinitely, and apparently there is no inherent 

 reason why the substance of the Protist should not continue 

 thus indefinitely; that is, the Protist organism does not naturally 

 end in death. With the metazoan the case is entirely differ- 

 ent. At a certain stage of development the body is divided 

 into two classes of cells. A relatively small portion consists 

 of cells destined to develop into gametes and, therefore, to con- 

 tinue on in the succeeding generation. All the remaining cells 

 of the body come to an end with the death of the individual. 

 These two types of cells are distinguished as germ cells and 

 somatic cells. This distinction rests on the principle of divi- 

 sion of labor. Here, as everywhere else in the biological world, 

 the chief end is the perpetuation of the species. This is 

 secured more certainly if by unity of action but division of 

 labor the life functions are performed in the most perfect 

 manner. This seems to be the reason for the existence of the 

 multicellular organism. 



727. Division of Labor and Differentiation. All the various 

 types of aggregation of biological units are attempts to solve 

 the same problem. The colonies of Protists, the hydroid 

 colonies with polymorphism, the polymorphic societies of ants 

 and bees, the various types of alternation of generation found 

 among plants and animals are all devices to secure the most 

 perfect functioning by dividing the functions. The meta- 



