The most wonderful thing about the Beaver is the manner in which he builds dams, to make ponds deep enough for his timber-floating operations, and to afford him a submarine passage to his house. Give him a valley and a stream of water, and he will gladly make a pond out of whatever raw materials are at hand. He uses the four-foot sticks from which he has eaten the bark for food, and with these, and an abundance of mud, he will raise a good strong dam to a height of four feet, and a width on the ground of ten feet or more. The mud used is dug out of the bottom and sides of his pond, and carried, while swimming, between his paws, with his front feet holding it against his breast. The sticks used in the dam are thrust endwise into the mud on top of the dam, and the mud used is patted down with his fore feet. The tail is not used as a trowel, but in swimming it is the Beaver’s propeller. In captivity the Beaver is not wholly a satisfactory animal. Like some human craftsmen, he positively declines to work under observation, and performs nearly all his tasks at night. He thinks nothing of gnawing down a tree a foot in diameter, and cutting its limbs into pieces which he can handle while swimming. If he can secure enough food-wood of kinds of his liking, he eats little else. Besides building dams to create ponds in which he can take refuge when hard pressed, he constructs canals, and houses for winter use. He also digs burrows into high banks; but his entrances to his various homes always are under water. THE ECONOMIC RODENT-REPTILE COLLECTION. As an answer to a frequent question—“Of what use are reptiles?”—a new collection has been installed in the Reptile House. It is supplied with descriptive labels, and serves several purposes. In the first place there is presented an elaborate series of the small gnawing animals, or rodents, a number of the species of which are highly injurious to the interests of agriculturists; secondly, the exhibition contains a large series of those species of snakes that prey upon the destructive rodents,—thus presenting for observation the serpents of marked economic value. Finally, the entire series stands as a clear demonstration of the perfect logic of Nature in which the production of all animal life is carefully balanced; for a part of the exhibition is composed of representative species of mammals that, in turn, prey upon the serpents, thus keeping even the destroyers of the smaller injurious creatures within bounds! [Illustration: BRAZILIAN TREE PORCUPINE.] During the past two years the collection of rodents has been steadily growing in importance. The following is a list of the species on exhibition at the close of the year 1912: RODENTS IN THE ZOOLOGICAL PARK. HARES AND RABBITS (_Leporidae_)