Page:Popular Science Monthly Volume 4.djvu/547

Rh the International Series, has given the results of a long course of observations and study upon the subject.

The three modes of progression, apparently so unlike, are nevertheless 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, and 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



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 terrestrial 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