PRINCIPLES OF CLASSIFICATION. 27 



points of difference. For the resemblance does not extend much 

 beyond the presence of wings, the breathing of air, the repro- 

 duction by eggs, and the covering of the wings and body (in 

 some Insects) with a kind of down. The points of difference 

 are much more striking. For the body of an Insect has no 

 internal skeleton, but is included within a jointed envelope, of 

 greater or less firmness ; the nervous centres, instead of being 

 united into one continuous mass the brain and spinal marrow, 

 are scattered through the body in distinct ganglia; the air which 

 enters the body, instead of being confined to certain bags or 

 chambers, is carried through the whole, by means of a widely- 

 distributed system of air-tubes ; instead of a heart, there is a long 

 tube, situated in the back, and divided into chambers that cor- 

 respond with the segments or divisions of the body ; the blood is 

 white, and its circulation slow and feeble ; the temperature of 

 the body is usually not much above that of the air around it ; the 

 body is supported, when on the ground, upon six legs ; and 

 the eyes are compound in their structure, each of the dark hemi- 

 spherical protuberances on the head being, in fact, an assemblage 

 or cluster of distinct eyes. All these are characters, of which the 

 union is peculiar to the Insect, distinguishing it from other 

 classes ; and we see how very little real conformity there can be 

 between Insects and Birds ; since the points of difference are so 

 much greater than those of agreement. 



4. The fact is, that these two classes belong to different pri- 

 mary subdivisions of the Animal kingdom ; the Birds to the 

 group of VEHTEBRATA, which is especially distinguished by the 

 possession of an internal skeleton, protecting the nervous centres, 

 and clothed by the muscles which communicate motion to it ; 

 whilst the Insects form part of the group of ARTICULATA, in 

 which the skeleton is external, having the muscles that move it 

 attached to its interior, and not giving more protection to the 

 nervous centres than it does to the rest of the body. But the 

 class of Birds may be regarded as holding the same position in 

 the Vertebrated seiies, which that of Insects does in the Arti- 

 culated. They are adapted for the same mode of life; and 

 notwithstanding the different plans on which their bodies are 



