PROGRESSION ON AND IN THE WATER. 



93 



force itself into it to prevent its being buoyed up to the sur- 

 face. However paradoxical it may seem, lueight is necessary 

 to aerial flight, and levity to subaquatic flight. A bird destined 

 to fly above the water is provided with travelling surfaces, so 

 fashioned and so applied (they strike from above, downwards 

 and forwards), that if it was lighter than the air, they would 

 carry it off into space without the possibility of a return ; in 

 other words, the action of the wings would carry the bird 

 obliquely upwards, and render it quite incapable of flying 

 either in a horizontal or downward direction. In the same 

 way, if a bird destined to fly under the water (auk and pen- 

 guin) was not lighter than the water, such is the configuration 

 and mode of applying its travelling surfaces (they strike from- 

 above, downwards and backwards), they would carry it in the 

 direction of the bottom without any chance of return to the 

 surface. In aerial flight, weight is the power which nature 

 has placed at the disposal of the bird for regulating its alti- 

 tude and horizontal movements, a cessation of the play of its 

 wings, aided by the inertia of its trunk, enabling the bird to 

 approach the earth. In subaquatic flight, levity is a power 

 furnished for a similar but opposite purpose ; . this, combined 

 with the partial slowing or stopping of the wings and feet, 

 enabling the diving bird to regain the surface at any moment. 

 Levity and weight are auxiliary forces, but they are necessary 

 forces when the habits of the aerial and aquatic birds and the 

 form and mode of applying their travelling surfaces are taken 

 into account. If the aerial flying bird was lighter than the air, 

 its wings would require to be tiuisted round to resemble the diving 

 wings of the penguin and auk. If, on the other hand, the diving 

 bird (penguin or auk) v/as heavier than the water, its wings 

 would require to resemble aerial wings, and they would require 

 to strike in an opposite direction to that in which they strike 

 normally. From this it follows that weight is necessary to the 

 bird (as at present constructed) destined to navigate the air, 

 and levity to that destined to navigate the water. If a bird 

 was made very large and very light, it is obvious that the 

 diving force at its disposal would be inadequate to submerge 

 it. If, again, it was made very small and very heavy, it is 

 equally plain that it could not fly. Nature, however, has 



