ORGANIC EVOLUTION 219 



7. The fitting of a part of the cells to perform special 

 functions follows the universal law of specialization, that 

 special fitness for one thing involves limitations in respect 

 to other things. 



8. The primary differentiation in all the higher organ- 

 isms is that into germ plasm and body plasm, the former 

 appearing in cells of two complemental sorts, eggs and 

 sperms. 



9. Increase in size of the cell complex necessitates sup- 

 porting structures and circulatory apparatus, but these 

 parts in the different plant and animal groups are exceed- 

 ingly different in structure. 



10. Exposure to the air in terrestrial organisms necessi- 

 tates the removal of the organs for intake of oxygen from 

 the surface of the body and the development of epidermal 

 layers to withstand evaporation. 



Besides these matters of general organization, there are 

 many other things pertaining to the functions of organ- 

 isms, to the phenomena of growth and metabolism, to the 

 finer structures of protoplasm and to the behavior of its 

 parts in reproduction, that are common to plants and 

 animals. A few of the better known (cytological) phenom- 

 ena of the behavior of nuclear parts in reproduction, will 

 be briefly noticed in the next chapter. 



The simpler organisms best illustrate the common feat- 

 ures of plant and animal organization. The forms we have 

 been considering in Chapter III illustrate rather a few of 

 the main lines of divergence : but beneath their diversity 

 lie the common features just stated. All living things are 

 composed of one kind of substance, that is organized into 

 equivalent structural units, that increases by one method 

 of growth, and that reproduces successive ganerations from 

 a common starting point. 



