364 ON THE ORDERS 



These various considerations have led me to imagine, how 

 tmly I have not yet been able to determine, that the test 

 of a relation of affinity is its forming part of a transition 

 continued from one structure to another by nearly equal 

 intervals, and that the test of a relation of analogy is barely 

 an evident similarity in some one or two remarkable 

 points of formation, which at first sight give a character 

 to the animal and distinguish it from its affinities. As a 

 relation of analogy must always depend on some marked 

 property or point of structure, and as that of affinity which 

 connects two groups becomes weaker and less visible as 

 these are more general, it is not at all surprising that what 

 is only an analogical correspondence in one or two im- 

 portant particulars, should often have been mistaken for 

 a general affinity. That the efiects, nevertheless, of this 

 common mistake are by no means trifling, I shall now at- 

 tempt to show in the case of the Winged insects ; and I 

 trust the reader will feel, that because my acquaintance 

 with Zoology may not be extensive enough to enable me 

 to detect the consequences of this error in other places, he 

 ought not therefore to believe that in them it can have had 

 a less baneful influence towards retarding the knowledge 

 of the natural system. 



First, It is a fact, I believe, universally acknowledged by 

 those who have paid any attention to Hexapod insects, that 

 a resemblance in certain important parts of their construc- 

 tion may be traced between the Cimicida and some of the 

 Orthoptera. Nay, on account of this similarity being so 

 striking, Linnasus even united them into one order, He- 

 miptera, to which he assigned the following characters : 

 '* Os Rostrumque hrflexum versus pectus. Alee hemely- 

 tratee ; superioribus semicoriaceis per suturam rectam mi- 



