Likenesses ; or. Philosophical Anatomy 26 



^liile in others their homology is a subject of keen discus- 

 sion. 



If it be asked what is meant by parts being ' homologous ' 

 if they are not ' homogenetic,' it may be replied that it means 

 they show a complex likeness, or agreement, as to their 

 relative positions to other surrounding parts. This likeness, 

 or agreement, may be of different kinds, according as we 

 follow different lines of thought. An intellect of a higher 

 order than that of man would probably detect an indefinite 

 number of relations between two animals and between their 

 component parts, which relations escape our observation 

 altogether, though we can readily enough apprehend a con- 

 siderable number of such relations. 



Thus we may enumerate as examples of different kinds of 

 homology : — 



1. Parts which have a similarity of function but differ 

 structurally and in their relations of position to all the other 

 portions of the body — e.g. the legs of a lizard and of a 

 lobster. 



2. Parts which are similar both as to function and 

 relative position — e.g. the wings of a bat and of a bird. 



3. Parts which, upon the hypothesis of evolution, are 

 descendants of some ancient similar structure — e.g. the arm 

 and leg bones of the horse and of the rhinoceros. 



4. Parts which are similar as to their mode of origin in 

 the individuals compared, whatever be their racial genetic 

 relations — e.g. the occipital skull bones of a panther and of a 

 perch. 



5. Parts which are alike but which do not arise similarly 

 in the individuals compared, whether or not they are the 

 descendants of parts in some one common ancestral form — 

 e.g. the legs of different kinds of flies — these insects differing 

 strangely in their modes of attaining their adult structure. 



6. Laterally homologous parts. 



