Unity and Interdependence in Living World 655 



process of photosynthesis. Photosynthesis not only supplies many of the 

 foods but also materials used for shelters, clothing, fuels, etc. Refer to 

 the detailed discussion of photosynthesis elsewhere in the text. 



II. WEB OF LIFE AND BALANCE IN NATURE 



All life in the world is so interdependent and closely related that it 

 may be viewed as a web (Fig. 326) composed of various individuals and 

 species of animals and plants which are more or less intimately associ- 

 ated together into a living unit. This web or unity of life may be con- 

 stantly changing from time to time as far as the individuals who com- 

 pose it are concerned, yet there seems to be more or less of a constancy 

 in any given area. Probably no living organism lives unto itself alone, 

 but each organism affects other living organisms and in turn is affected 

 by one or more organisms. The more we study biologic phenomena, 

 the more we realize and appreciate the interdependence of all living 

 things. In any particular area of life, each organism contributes some- 

 thing, either large or small, to the total life of that area. In the web 

 there may appear many struggles and antagonisms among the inhabi- 

 tants, but in spite of them there actually exists a balance in Nature — 

 all these struggles somehow counteract and balance each other so that 

 the number of species in a given area remains about the same. If one 

 group of organisms in a locality is eliminated, another group may take 

 its place or the remaining organisms may expand sufficiently to fill the 

 vacancy which was created. 



All living organisms may be considered as links in a chain, all con- 

 tributing their part so that the chain is an endless one. For example, 

 certain bacteria of the soil change free nitrogen of the air into nitrates 

 which help to build plant tissues. The latter are eaten by animals 

 which may be consumed by other animals. Even if the latter die, their 

 bodies are decomposed by other species of bacteria and molds, thus 

 returning the ingredients to the soil where they are again available for 

 future generations of plants. ' 



III. PLANT AND ANIMAL MIGRATIONS (DISPERSAL) 



Because of a lack of means of locomotion, most plants are not sub- 

 ject to the true migrations found in many animals such as birds, fishes, 

 mammals, etc. However, plants do disperse by slow, gradual spreading 

 by means of seeds, spores, or vegetative (propagative) units, such as 

 parts of stems, roots, etc. 



