390 



EVOLUTION AND GENETICS 



Fig. 204. — Transition from aquatic to aerial habitat as shown by leaf struc- 

 ture. A, Proserpinaca paluslris, submerged leaf; B, same, aerial leaf; C, 

 Trapa natans, submerged leaf; D, same, aerial leaf. 



The blade-like canines of the sabre-tooth tiger were apparently 

 so easily broken that their excessive development ultimately 

 became a handicap (Fig. 205). In addition to such positive char- 



