DEFINITION OF THE TERM INSECT. 4S 
that at the limits of classes and of every other natural 
group, the characters begin to change, those peculiar to 
the one group beginning gradually to disappear, and 
those of the other to show themselves; so that it is im- 
possible almost to draw up aset of characters so precise 
as exactly in every respect to suit all the members of any 
natural group. 
Whichever way we turn our eyes on the objects of 
creation, above—below—athwart, analogies meet us in 
every direction, and it appears clear, that the Book of 
Nature is a Book of Symbols, in which one thing repre- 
sents another in endless alternation. And not only does 
one animal, &c. symbolize another, but even between the 
parts and organs of one set of animals there is often an 
analogy as to their situation and use, when there is little 
or no affinity as to their structure—or again, the analogy 
is in their situation, without affinity in either structure 
or use. ‘Thus certain parts in one tribe represent other 
certain parts of another tribe, though as to their structure 
there is often a striking disagreement. ‘This is particu- 
larly observable between the vertebrate and invertebrate 
animals. I shall therefore, in my remarks on the ge- 
neral and particular structure of insects, contrast it in its 
most important points with that of the first-mentioned 
tribe. 
The first thing that strikes us when we look at an 
insect is its outside covering, or the case that incloses its 
muscles and internal organs. If we examine it attentively, 
we find that it is not like the skin of quadrupeds and 
other Vertebrata, covering the whole external surface of 
the body; but that in the large majority it consists of 
several pieces or joints, in this respect resembling the 
