238 SENSES OF INSECTS. 
nostrils, which co-exist with them, than they have to the 
eyes or ears. I have on aiormer occasion observed, that 
a gradual change sometimes takes place in the functions 
of particular organs; but still, generally speaking, this 
observation 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 walk- 
ing, 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 in fish are changed to jms. Again, the 
primary use of the heart of animals is the elaboration of 
the nutritive fluid; its secondary, to be the organ of a 
system of circulation, by which that fluid may alternately 
receive and part with oxygen: but in the dorsal vessel 
of insects which is analogous to the heart, the circulation 
ceases, the oxygenation of the blood being effected by 
other means; but still its primary function, the prepara- 
tion of the nutritive fluid, as there is reason to think, is 
discharged by it?. So that it seems a law to which Na- 
ture in most cases adheres. 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. 
* See above, p. 88, 90, note *; comp. p. 115. 
