NATURAL HISTORY 



LETTER XXXVII. 



TO THE SAME. 

 DEAR SIR, SELBORNE, 1771. 



ON the twelfth of July I had a fair opportunity of con- 

 templating the motions of the Caprimulgus, or fern-owl, 

 as it was playing round a large oak that swarmed with 



structed. The lateral membranes perform the functions of wings, and 

 serve to propel the body through the air ; while the interfemoral mem- 

 brane acts, by its expansion, as a parachute, and prevents the bat from 

 rapidly falling to the ground. 



But although the larger membranes belong chiefly to locomotion, they 

 contribute also to extend the means by which the animal is enabled to 

 acquire a knowledge of the circumstances in which it is moving. The 

 actions of the bat are confined to the darkness of the night, or at best to 

 the uncertain glimmering of the dusky twilight ; and the sense of vision 

 is consequently comparatively inadequate to guide it in its flights and in 

 the pursuit of its prey. To compensate for the imperfection of its vision, 

 other senses should be rendered more acute ; and this is effected by the 

 exposure of a large extent of naked skin, and by the developement of 

 processes adapted to direct the impulses of the air on the several organs 

 which are destined to appreciate them. 



Destitute almost entirely of hair, the flying membranes of the bats 

 become organs of touch ; and the great surface which they expose to 

 atmospheric impulses must necessarily render them highly susceptible of 

 the finest impressions to which that sense is liable. The perfection of 

 the sense of smell also is, in many cases, aided by a peculiar arrange- 

 ment ; a membrane being frequently developed on the nose, which, by 

 directing the air towards the nostrils, renders more assured the affecting 

 of the olfactory organs by the scents with which the atmosphere may be 

 impregnated. A somewhat similar arrangement adds to the efficiency of 

 the sense of hearing : for the great expansion of the external ear which 

 often occurs in bats, is equally adapted for directing towards the auditory 

 passages the air charged with sounds ; and even in those cases in which 

 the external ears are not disproportionally large, the nakedness of these 

 organs, qualifying them to act also as organs of touch, renders them so 

 susceptible to the finer impulses of the atmosphere as to cause them 

 quickly to assume the state of tension most fitted for directing sound. It 

 would seem, indeed, that the quantity of sound forced occasionally into 

 the ears of bats was so great as to render it necessary to provide the 

 power of closing the auditory passage, by the folding down over it of a 

 kind of internal or second ear; itself, like the outer or ordinary ear, a 

 naked and membranous expansion of the skin, and of course equally 



