INSTINCT 227 



because they are useful. That they might be pre- 

 served if they took place need not be disputed, but 

 the evidence, in my estimation, is totally inadequate 

 to show that such changes occur. 



As I have said elsewhere, it is easy to imagine that 

 any finite existence may originate by starting with 

 nothing and adding atom by atom. So it may be 

 imagined with instincts. 



The Beaver furnishes one of the most remarkable 

 examples of instinct among vertebrates. It lives in 

 communities and constructs dams, sometimes as much 

 as three hundred yards long, across shallow streams 

 of water. These dams are built of sticks of wood, 

 generally about three feet long and six or seven 

 inches in diameter, which the animal cuts with its 

 teeth. They are put in the water and held in posi- 

 tion by means of mud, stones and moss which are 

 placed upon them. The dams are ten or twelve feet 

 thick at the base, and when the streams are wide, 

 instead of extending straight across, they are made 

 to curve upstream against the current, thus enabling 

 the structure to better resist the force of the water. 

 The amount of labor necessary to construct a large 

 dam is enormous, and requires an incredible number 

 of logs of wood and great skill in engineering. 



Near the dam the beavers build their houses. Each 

 house is about seven feet in diameter on the interior 

 and three feet high in the center and the walls are of 

 great thickness. Each lodge is large enough to ac- 

 commodate five or six beavers. 



The outside is plastered with mud and carefully 

 smoothed, and the mud is renewed each year in order 

 to keep the houses in good repair. All the houses of 

 the colony are surrounded by a ditch which contains 

 water, and each lodge is connected by a passage-way 

 with the ditch. 



