INTRODUCTION. 



5 



described by the stationary and progressive waves in fluids, 

 and likewise with the waves of sound. This coincidence 

 would seem to argue an intimate relation between the instru- 

 ment and the medium on which it is destined to operate — 

 the wing acting in those very curves into which the atmo- 

 sphere is naturally thrown in the transmission of sound. Can 

 it be that the animate and inanimate world reciprocate, and 

 that animal bodies are made to impress the inanimate in pre- 

 cisely the same manner as the inanimate impress each other 1 

 This much seems certain : — The wind communicates to the 

 water similar impulses to those communicated to it by the 

 fish in swimming ; and the wing in its vibrations impinges 

 upon the air as an ordinary sound does. The extremities of 

 quadrupeds, moreover, describe waved tracks on the land 

 when walking and running ; so that one great law apparently 

 determines the course of the insect in the air, the fish in the 

 water, and the quadruped on the land. 



We are, unfortunately, not taught to regard the travelling 

 surfaces and movements of animals as correlated in any 

 way to surrounding media, and, as a consequence, are apt 

 to consider walking as distinct from swimming, and walk- 

 ing and swimming as distinct from flying, than which there 

 can be no greater mistake. Walking, swimming, and flying- 

 are in reality only modifications of each other. Walk- 

 ing merges into swimming, and swimming into flying, by 

 insensible gradations. The modifications which result in 

 walking, swimming, and flying are necessitated by the fact 

 that the earth affords a greater amount of support than the 

 water, and the water than the air. 



That walking, swimming, and flying represent integral 

 parts of the same problem is proved by the fact that most 

 quadrupeds swim as well as walk, and some even fly ; while 

 many marine animals walk as well as swim, and birds and 

 insects walk, swim, and fly indiscriminately. When the land 

 animals, properly so called, are in the habit of taking to the 

 water or the air; or the inhabitants of the water are constantly 

 taking to the land or the air ; or the insects and birds which 

 are more peculiarly organized for flight, spend much of their 

 time on the land and in the water; their organs of locomo- 



