MECHANISM AND HABITS. 37 



and given out in such small quantities that they are not 

 perceptible to common observers, neither removeable by the 

 ordinary means of cleanliness even in the best-kept apartment." 

 Under this view of extensive uses, for which its structure and 

 habits are alike admirably adapted (as well as for each other), 

 both are well worthy of general observation and in no wise 

 beneath the scrutiny of scientific, and, what is more, of sensible 

 people. The mechanism even of a Fly's foot thus regarded, 

 we shall never be disposed to look back upon a Sir Joseph 

 Banks, a Sir Everard Home, or a pious Derham, when busied 

 in its examination, as upon " children of a larger growth ' 

 curiously pulling toys to pieces ; and then remembering by 

 whom that mechanism was constructed, we shall not be 

 surprised on finding that observers, even such as these, seem 

 after all, to have been at fault as to its true principle. On the 

 credit of their great authority, books without number have 

 explained, and still continue to tell us how the Fly walks 

 against gravity with equal ease upon a surface rough or smooth, 

 upon our windows as upon our walls, upon the ceiling as upon 

 the floor, with back downwards and with back upwards ; and 

 yet as it would appear they have all told us wrong. They have 

 said, and asserted as proved beyond a doubt, that the sole secret 

 of a My's marvellous walk and hold, is a vacuum, the vacuum 

 produced by certain organs called suckers attached to 

 the end of the foot, which either adheres by atmospheric 

 pressure or is left free to rise, as these suckers are alternately 



