172 THE GENESIS OF SPECIES. [CHAP. 



between the wing of the bat and that of the flying-dragon, 

 for the latter is formed of certain ribs, and not of limb- 

 bones. 



Homology may be further distinguished into (1) a rela- 

 tionship which, on evolutionary principles, would be due to 

 descent from a common ancestor, as the homological rela- 

 tion between the arm-bone of the horse and that of the ox, 

 or between the singular ankle-bones of the two lemurine 



SKELETON OF THE FLttNG-DBAGOV. 



(Showing the elongated ribs which support the flitting organ.) 



genera, cheirogaleus and galago, and which relation has 

 been termed by Mr. Ray Lankester " homogeny ; 3 and (2) 

 a relationship induced, not derived such as exists between 

 parts closely similar in relative position, but with no 

 genetic affinity, or only a remote one, as the homological 

 relation between the chambers of the heart of a bat and 

 those of a bird, or the similar teeth of the thylacine and 



2 See Ann. and Mag. of Nat. Hist., July, 1870. 



