SAND DUNES AND SALT MARSHES 



all pointed to an inherited instinct, an instinct, 

 however, that is largely if not entirely lost in 

 adult life. This young heron had never prac- 

 tised the art of swimming before— it had prob- 

 ably never left the nesting tree, which was on 

 a marsh island some distance from even the 

 highest tides. Adult herons show their swim- 

 ming ancestry by a distinct web between the 

 middle and outer toes. 



The use of the wings under water in some 

 diving birds, and the significance of this fact, 

 I have already discussed in another place.^ 



One is apt to think of evolution as a thing 

 of the past, an accomplished fact, and to for- 

 get that at the present period of time this 

 great law is still as existent as it has been 

 since the world began. With change in habits, 

 habitat or food, there comes, through natural 

 selection, acting on slight variations and 

 occasional mutations, a change in the struc- 

 ture to fit the new environment, and in time 

 a new species is developed. As new species 

 arose in the past, so they must be in various 

 stages of formation at the present time. The 

 great group of American warblers are for the 



> " A Labrador Spring," Boston, 1910, pp. 180-206. 



292 



