232 



EVOLUTION AND GENETICS 



may be more certainly accomplished. Pages could be filled with a 

 mere listing of these known associations. 



The Nature of Organic Relations. A primary reason for the 

 association of organisms is the securing of food, but since this in- 

 volves the destruction or injury of other organisms except in the 

 case of the green plants, there must always be adaptations for the 

 protection of those subject to attack. Finally there are associa- 

 tions of convenience, wherein animals of the same or different 



Fig. 139. — Pterodactyl, Rhamphorhynchus phyllurus. (From Lull.) 



species find it possible to meet the requirements of life better 

 through the assistance of others. 



Food- Securing. The Structures Involved. In securing food the 

 adaptation of the mouth is necessarily important. We have 

 already noted a striking example in the evolution of the teeth in 

 elephants and horses, and another in the heterodont dentition 

 of the primates in connection with omnivorous habits. The lips 

 of animals also show adaptation. Grazing species have a prehen- 

 sile upper lip which is useful in gathering tufts of grass into the 

 mouth, while rabbits nibble at leaves and fruits without the neces- 

 sity for securing a quantity of pieces at one mouthful. Whatever 

 may be the food, the teeth at least are formed so that it may be 

 effectively chewed. The appendages are often modified as acces- 

 sory structures. 



Anteaters. Such animals as the anteaters are extreme adapta- 

 tions to a limited diet. The great ant-bear has strong, hooked 

 claws with which it tears open the nests of ants (Fig. 140). Its 

 snout is slender and elongate, its tongue long and sticky, and it is 



