THE UNITY OF LIFE 211 



This means ( i ) that the capacity for these various functions is 

 present in all protoplasm, and (2) that division of labor with 

 the specialization of functions results from bringing together 

 many units and giving them a chance to live together. 



174. E pluribus unum. In spite of the many kinds of organs 

 that we find in the human body and other well-developed 

 species the organism always acts as a whole. The various func- 

 tions, however different they may appear, are all junctions of 

 protoplasm. We can understand the body, perhaps, only by 

 studying all the parts, but the several parts have no meaning 

 except in relation to the organism as a whole. It is this unity 

 of the organism that makes life both significant and interest- 

 ing : the more complex the organism, the more varied its parts, 

 the more wonderful is the total life in variety and interest. 



Of course the human body does not come from joining to- 

 gether millions of cells that have once been separate. Like 

 other organisms, it develops from a single cell that divides into 

 two, each of which again divides, and so on until millions are 

 formed (see Fig. 107). The many different kinds of cells and the 

 many different organs appear gradually by a process of di^er- 

 entiation, and the different tissues and organs gradually take 

 on specializations in their functions. The organism has been a 

 unity from the first. It is only because we have taken the body 

 apart in our studies that we must go a step farther and ask our- 

 selves how the parts are kept working together. 



175. How unity is maintained in higher animals. We have 

 seen that the food-getting and digesting organs deliver the ma- 

 terial ready for assimilation to the blood (see page 136) ; 

 that the oxygen-getting organs deliver their oxygen to the blood 

 (see section 136) ; and that all the cells of the body take from 

 the blood the materials that they use, and throw back into the 

 blood their wastes (see section 1 50) . The blood system is there- 

 fore in touch with all the other systems and organs, and con- 

 stantly tends to bring about a certain unity of the body, at 

 least in a chemical or nutritional sense. The nutrition of every 

 cell is dependent upon the condition of the blood, the oxygen 



