The Outline of Science 



remarkable locomotor triumph that has ever been achieved. With- 

 out any apparent stroke of the wings, the bird sails for half an 

 hour at a time with the wind and against the wind, around the 

 ship and in majestic spirals in the sky, probably taking advantage 

 of currents of air of different velocities, and continually changing 

 energy of position into energy of motion as it sinks, and energy 

 of motion into energy of position as it rises. It is interesting to 

 know that some dragon-flies are also able to "sail." 



The web-wing of bats involves much more than the fore-arm. 

 The double fold of skin begins on the side of the neck, passes 

 along the front of the arm, skips the thumb, and is continued 

 over the elongated palm-bones and fingers to the sides of the body 

 again, and to the hind-legs, and to the tail if there is a tail. It 

 is interesting to find that the bones of the bat's skeleton tend to be 

 lightly built as in birds, that the breastbone has likewise a keel for 

 the better insertion of the pectoral muscles, and that there is a so- 

 lidifying of the vertebrae of the back, affording as in birds a firm 

 basis for the wing action. Such similar adaptations to similar 

 needs, occurring in animals not nearly related to one another, are 

 called "convergences," and form a very interesting study. In 

 addition to adaptations which the bat shares with the flying bird, 

 it has many of its own. There are so many nerve-endings on the 

 wing, and often also on special skin-leaves about the ears and 

 nose, that the bat flying in the dusk does not knock against 

 branches or other obstacles. Some say that it is helped by the 

 echoes of its high-pitched voice, but there is no doubt as to its 

 exquisite tactility. That it usually produces only a single young 

 one at a time is a clear adaptation to flight, and similarly the 

 sharp, mountain-top-like cusps on the back teeth are adapted in 

 insectivorous bats for crunching insects. 



Whether we think of the triumphant flight of birds, reach- 

 ing a climax in migration, or of the marvel that a creature of 

 the earth as a mammal essentially is should evolve such a 

 mastery of the air as we see in bats, or even of the repeated but 



