Chap. 38 



ORGANIC EVOLUTION CONSERVATION 



781 



Fig. 38.2. The gradual clambering on to the land, a restoration of early amphib- 

 ians (labyrinthodonts) of ancient Carboniferous times, the period of coal deposits. 

 The history of plants and animals is the story of increase, and the invasion and 

 filling of habitable space. Painting by F. L. Jaques. (Courtesy, American Museum 

 of Natural History, New York.) 



Conditions and Varieties of Adaptations. Adaptations of one kind shut 

 out others. Australian koalas ("teddy bears") live where eucalyptus trees 

 are abundant and they are adapted to a pure diet of their leaves. They cannot 

 live on anything else. Birds use their bills and feet to manipulate their food but 

 those that are highly efficient tools for one skill are worthless for some other 

 — the beak and talons of a hawk are poor seed pickers (Fig. 36.7). 



An anteater that can poke its snout into an anthill and collect a dozen ants 

 on its sticky tongue could scarcely use it to catch a grasshopper (Fig. 5.3). 

 Such a particular tool is overspecialized, on a byroad, even a dead end. It 

 allows its owner only one very particular kind of food. An anteater must have 

 ants or starve. The zigzag course of evolution is full of byroads and pockets of 

 adjustments so perfectly special that they come to a standstill in their perfec- 

 tion. Among them are the sponges with their elaboration of water tubes and 

 the starfishes with their structures locked to the number five. 



Adaptations of Particular Structures in Different Species. The 

 fore limbs of vertebrates show striking and varied adjustments to use in each 

 of the three basic environments — water, air and land. The relation of the bones 



