RELATIONS AND SURROUNDINGS. 139 



algae derive their nourishment from the waste matter of 

 the animal tissues, and the animal cells profit by the 

 oxygen and possibly other matter given off by the plants. 

 Many of our flowers and our insects depend upon one 

 another; the flower furnishes pollen and nectar as food 

 to the insects; the insect helps to pollinate the flower 

 and thus secures its fertility (Fig. 28) . 



The structures and instincts which make these various 

 associations possible are adaptations to the existence of 

 other organisms. 



155. Competition Among Animals of Different Species 

 and the Adaptations to it. It is here that the competi- 

 tion in nature is most fierce, and its results are stamped 

 upon the whole nature of the animal kingdom. It has 

 been said that all species of animals have their enemies. 

 There are two principal ways in which this antagonism 

 may show itself : (i) in active preying upon other species 

 or (2) in the underhand kind of preying which we call 

 parasitism. The adaptations for the predaceous habit 

 involve two kinds: the protective adaptations which 

 enable animals to escape their enemies, and the offensive 

 adaptations which enable the latter to capture their prey. 



It is clear that in the long run there must be some 

 balance between these two types of adaptations if both 

 kinds of animals are to continue. If the offensive qualities 

 were too well developed, it would result in the destruction 

 of the prey which would mean also the end of the preda- 

 tory forms. Similarly, if the defensive qualities outrun 

 the offensive the captors would die out from lack of food. 



156. Adaptations for Protection. It is clear that keen- 

 ness of scent, timidity which causes animals to keep in 

 retired places, and fleetness, are qualities which will help 



