THE INTERDEPENDENCE OF LIVING THINGS i:, 



ing over the world of plants and animals an unescajDabie dcixMidenro 

 of one form of life upon another is found in the food relationship 

 by which green plants supply animals with food and in the shelter 

 relationship, by which animals find safety in the protection given 

 by plants. Reducing this search for food and shelter to its ultimate, 

 we find that all animals are dependent upon green plants. 



But does the green plant get anything from the animal ? At first 

 sight it would seem as though it were all give and no take. As we 

 study the situation more closely, however, we find that food-making 

 is dependent upon certain raw materials, some of which, such as 

 nitrogenous wastes, can only be supplied from the dead bodies of 

 organisms or their excreta. Moreover, another important raw 

 material, carbon dioxide, used by green plants in starch-making, is 

 given off as a respiratory by-product by animals, and in this same 

 process oxygen is released. 



All of these facts suggest certain problems. Why, for example, 

 when some animals produce enormous numbers of eggs and others 

 only a few, do not the former outnumber the latter? Of what 

 significance is the mutual aid so frequently observed in nature? 

 What is symbiosis and why is it significant? What is the \'alue of 

 pollination by insects as compared with pollination by other means? 

 What part do bacteria play in the fives of plants and animals? 

 What is the reason for parasitism ? Can the oft-repeated statement 

 that green plants make food for the world be proved ? A start on the 

 answers to some of these questions will be made in the pages that follow. 



Relations between Members of the Same Species 



Many examples of helpful relationships can be .seen between ani- 

 mals of the same species, especially in the care of young. Although 

 in low forms, such as sponges, coelenterat(>s. echinoderms, and a good 

 many fishes, large numbers of eggs are laid and given little or no 

 parental care, the production by the male of immense numbers of 

 sperm cells in the vicinity of the eggs insures chance fertilization and 

 continuity of the species. For example, Norman ' reports that a cod 

 w^hich weighed 21^ pounds produced over 6,650,000 eggs. At tiie 

 time of egg laying each male of the above .species throws billions of 

 sperm cells into the water near the eggs. Higher in the animal scale 

 we find greater provision for care of the young correlatcnl with a re- 

 duction in the number of eggs laid. Many insects lay their eggs on 



1 Norman, J. R., A History of Fishes. Stokes, 1931. 



