21-6 SENSES OF INSECTS. 
casting dust in our eyes : for though three of these 
are nasal organs, bearing nostrils; the two others have 
no relation to the question, the horn of the rhinoceros 
and the crest of the cock being merely appendages, and 
have no more analogy to the nose and nostrils, which 
co-exist with them, than they have to the eyes or ears. 
I have on a former occasion observed, that a gradual 
change sometimes takes place in the functions of parti- 
cular organs ; but still, generally speaking, this obser- 
vation regards secondary functions — the primary usually 
remaining untouched. We may say, for instance, with 
regard to the primary use of the legs of animals, that it 
is locomotion ; while the secondary is either walking, 
running, jumping, flying, or swimming, according to the 
circumstances and nature of the animal. Thus the fore- 
legs of the Mammalia, in birds become wings, and both 
pair mfsh are changed to fins. Observe, I do not say 
always and invariably, but in most cases, — that analogous 
parts have analogous uses, at least as far as primary uses 
are concerned. When, therefore, we cannot have demon- 
strative evidence concerning the function of an organ 
discoverable in any animal, we may often derive satis- 
factory probable arguments from the analogies observa- 
ble in their structure compared with that of other animals, 
concerning the nature of whose organs we have no doubt. 
In fact, the chief evidence we have with regard to the 
office of the organs of sense in the animals immediately 
below ourselves, is that of analogy ; — because we see 
with our eyes, hear with our ears, &c, we conclude, 
with reason, that they do the same. 
Jn inquiring therefore into what may be the most ge- 
