DEFINITION OF THE TERM INSECT. 43 



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 a set 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 



