ADAPTATION OF LAND REPTILES TO LIFE IN WATER 61 



habits. And any improvement on such forms and habits that 

 their higher development permits them to attain will of course be 

 of advantage in their competitive struggles. A fish makes most 

 use of its tail fin for propulsion. It follows that a land animal 

 seeking to compete with it under like conditions must acquire a 

 tail fin or some other organ which subserves its purpose as fully. 

 The body fins are of little use to a fish, save for equilibration, for 

 preserving its position, for stopping quickly, or for changing the 

 direction of its movements quickly — very different functions from 

 those of the corresponding organs, the limbs, of higher vertebrates. 

 There are few better examples of predaceous, fish-eating fishes than 

 the common gar-pike of our rivers, fishes with a slender body 

 covered with very smooth scales, a strong tail, a short neck, and 

 long jaws armed with numerous slender and sharp teeth. Such a 

 fish, darting into a school of smaller fishes, by quick, sudden changes 

 of movement, actively opening and closing its jaws, is sure to seize 

 some of its sought-for prey. In a direct trial of speed with its 

 victims it would most likely be worsted. 



There have been many animals of high and low rank which in 

 the past and present have gone back from a terrestrial existence 

 to a life in the water, finding at last a congenial home away from 

 the shores. Or, perhaps, like the monitor lizards of today, they 

 have found temporary safety in the water when hard pressed by 

 their land enemies, and finally found, not only protection, but 

 an abundant supply of easily obtainable food therein. As in every 

 vocation of life there have been many failures in such attempts, 

 many partial successes only. But not a few have found abounding 

 and enduring success and final prosperity — success that has led 

 possibly to undue adaptation to surroundings, and to the acquire- 

 ment of great size, for that has been the invariable end of water 

 air-breathers of long duration — specializations which finally pre- 

 vented them from meeting new exigencies. It seems to be a law 

 of evolution that no large creatures can give rise to races of 

 smaller creatures; and as we shall see, the largest sea animals 

 have been the final evolution of their respective races. 



There are no better examples of such success today, nor has there 

 been in all the geological ages, so far as we know, more perfect 



