WALKING, SWIMMING, AND FLYING. s 2 9 



the International Series, has given the results of a long course of 

 observations and study upon the subject. 1 



The three modes of progression, apparently so unlike, are never- 

 theless essentially the same. The limbs of the quadruped, the wings 

 of the bird, and the fins of the fish, are built upon the same general 

 plan of structure, and are applied fundamentally to the same uses. 

 They are traveling surfaces, and their wide range of modification is in 

 direct relation to the media in which they are used. The one treads 

 the solid ground, another the water, and another the yielding and 

 elastic air. " But walking merges into swimming, aud swimming into 

 flying, by insensible gradations ; and these modifications result from 

 the fact that the earth affords a greater amount of support than the 

 water, and the water than the air." 



Most terrestrial quadrupeds can swim as well as walk, and some 

 can fly. Many marine animals both walk and swim, and birds and 

 insects walk, fly, and swim, indiscriminately. It is not surprising, 

 therefore, that, between the typical foot, wing, and fin, innumerable 

 modifications in structure and form occur ; indeed, so graduated are 

 they that it may be difficult to determine where one form ends and 

 another begins. 



In Fig. 1 we have several illustrations of the traveling surfaces of 

 » • 



Fig. 1. 



A— Extreme form of compressed foot, as seen in the deer, ox, etc., adapted specially for land 

 transit. 



B— Extreme form of expanded foot, as seen in the ornithorhynchus, etc., adapted more particu- 

 larly for swimming. 



O— Intermediate form of foot, as seen in the otter. 



D — Foot of frog. Here the foot is equally serviceable in and out of the water. 



E — Foot of the seal, which opens and closes in the act of natation. 



animals. The small feet of the quadruped, the webbed feet of the 

 ornithorhynchus, the otter, the walrus, and the triton, indicate with 

 certainty the media to which they are adapted, and perhaps in nothing 

 is modification of structure and form to habits more apparent than in 

 the locomotive appendages of animals. The webbed structure between 

 the toes of animals which live partially on the land, and of some ter- 

 restrial animals, as the water-dog, is wonderfully significant. 



The wing of the penguin, Fig. 2, is scarcely more than a flipper, 

 and the same is true of the auk. 



Sir John Lubbock describes a species of insect whose wings are 



1 " Animal Locomotion ; or, Walking, Swimming, and Flying.' 1 By J. Bell Pettigrew, 

 F. R. S. International Scientific Series. New York : D. Appleton & Co. 

 vol. iv. — 34 



