ANIMALS ON POLISHED SURFACES. 217 
pressure upon the exterior surface, that the animals 
in question are enabled to walk up a polished per- 
pendicular, like the glass in our windows, or with 
their backs downward on a ceiling, without being 
brought to the ground by the weight of their bodies.” 
To dissent from an hypothesis so generally received, 
including among its advocates numerous illustrious 
names, may, perhaps, be deemed presumptuous ; 
nevertheless, as facts absolutely irreconcilable with 
this supposition have been forced upon my attention, 
while engaged in examining the evidence by which it 
is supported, I shall, with every sentiment of respect 
for the high authorities to whom I stand opposed, 
submit my views to the consideration of candid and 
intelligent naturalists. 
Concerning the structure of the instruments by 
means of which flies ascend the vertical sides of 
smooth bodies, various opinions have been promul- 
gated. Some authors compare them to sponges, and 
conjecture that they are designed to conta a glu- 
tinous secretion capable of adhering to well-cleaned 
glass. Dr. Hooke describes them as palms or soles 
beset underneath with small bristles or tenters, like 
the wire teeth of a card for working wool, which he 
conceived give them a strong hold upon objects 
having irregular or yielding surfaces; and he ima- 
gined that there is upon glass a kind of smoky 
substance penetrable by the points of these bristles*. 
* ¢ Micrographia,’ pp. 170, 171. 
