12 



Invcrtebrata. 



portant resemblances are those which depend on 

 common relationship to the types of the class to which 

 they belong. These likenesses are called resemblances 

 of morphological type. Thus if we compare a dog and 

 a crow, we find in both a skeleton, a brain, a skull, 

 four limbs, a heart &c., and we refer them both to the 

 vertebrate type, inasmuch as they both embody the ideas 

 of structure characteristic of vertebrate animals. Each 

 part in one is said to be homologous with the corre- 

 sponding part in the other, the wing with the fore leg, 

 &c. Homology is thus identity of structure irrespec- 

 tive of function, and parts are homologous which 

 represent the same parts in the ideal type of the class. 

 Such resemblances are the bases of classification. 



Analogy. Likenesses of parts may also depend on 

 similarity of function ; thus the wings of insects and the 

 wings of birds are used for 

 the same purpose, and have 

 certain resemblances. These 

 similarities are called resem- 

 blances of analogy, and they 

 tell us nothing as to the nature 

 of the organs compared. 



Organs which are homo- 

 logous consist of homologous 

 parts ; and as this is not the 

 case in organs resembling each 



Leaf insect (Phylhnm sicci- & 



folium} and its egg. other only in function, we 



must be careful to discriminate morphological from 

 physiological likeness. 



In animals which consist of successive segments in 

 a chain, like centipedes or lobsters, each segment is 



FIG. 6. 



