230 Basic Structure of Vertebrates 



a "pattern" sufficiently definite to mark off distinctly a great group 

 of animals, and yet so elastic as to admit of great diversity of form 

 within the group. This elasticity of the pattern, enabling the animal 

 to adapt itself to a wide range of environments and manners of living, 

 is achieved in a variety of ways. The proportions of parts may be 

 varied; e.g., the long hindlegs of the frog and the kangaroo, and the 

 long forelegs of the giraffe ; exaggeration of teeth to form tusks in the 

 elephant and the walrus. The spatial relations of parts may be sec- 

 ondarily modified, as when a liver, having developed in its typical 

 ventral position, is pushed laterally or dorsally by crowding of neigh- 

 boring organs. By variation of the internal differentiation of an organ, 

 its function may be greatly altered, as when a portion of the gastric 

 region of the digestive tube of a grain-eating bird becomes the power- 

 fully muscular grinding gizzard and relegates the function of chemical 

 digestion of food to a more anterior region. Secondary and accessory 

 structures of a great variety of sorts may be produced — bony scales, 

 horny scales, feathers, and hair, all developed by the skin; glandular 

 structures in great variety, both integumentary and internal; external 

 ears; marsupial pouches. By such means as these, and without violation 

 of the basic vertebrate pattern, have come into existence vertebrates 

 so different as minnow and elephant, mouse and python, hummingbird 

 and whale, toad, turtle, and man (Fig. 194). 



