1 IiXTKODUCTION. 



responding parts are k:ttered with corresj^onding lettei'S. They per- 

 form, however, totally ditferent fiinctions, the first being an organ of 

 prehension, the second being devoted to terrestrial j)rogression, and 

 the third Ijeing an organ of Hight. 



(Jn the other hand, whenever we find in different animals organs 

 fulfilling the same ]]ur]iose and doing the same work, then we have 

 to deal with a case of iMiulmjii — the organs are analogous, and the 

 one is said to be the aiuilogiu: of the other. In other words, those 

 parts or organs are itiudoyou^ which resemble one another physio- 

 logically and discharge the same functioits, wholly irrespective of 

 what their fundamental structure may be. In most cases the organs 

 which would ordinarily be called "analogous" are such as differ 

 from one another in strnctnre, at the same time that the}' discharge 

 the same duties. Thus the wings of a bird and the wings of an 

 insect are analogous organs, since they are both organs of flight, and 

 serve to sustain their jiossessor in the air. They are, however, in no 

 way similar to one aiKjther excejjt \\hen regarded from this physio- 

 logical point iif view ; and they ditfej' altogether from a morpjho- 

 li.igical asi)et-t, lieiiig in no way formed ijn the same fundamental 

 ]>lan. It often hapjjens, h(jwever, that "analogous" organs have 

 the dee2)er relation to one another of being constructed ujjon the 

 same juorphological jilan, in which case they are both analogous antl 

 homologous. Thus, the leg of man and the hind-leg of a dog are 

 both analogous and homologous, since they are constructed upon the 

 .same jjlan and discharge similar functions. 



It need only be added in this connection that sound classification 

 depenils (in a coii-ect disci'imination between likenesses of homology 

 and likenesses of analogy. Likenes.ses of analogy — that is, like- 

 nesses dejiendent solely upon the possession of organs discharging 

 the same jihysiological function — are to be disregarded. Thus, the 

 Bird and the Butterfly are not to be groujied together simjjy be- 

 cause both jiossess organs of flight. On the other hand, likenesses of 

 homology — that is to say, likenesses dependent on identity of struc- 

 tural plan — are a safe guide to real affinity, enabling us to trace 

 the genuine relationships which may subsist between animals out- 

 wardly very dissimilar, and affording to us the foundation of a 

 common type capaljle of ahno.st endless modification. Whilst the 

 t/ifori/ of philosophical classification is thus clear, it may be further 

 said that great difficulties attend the carrying out of the admitted 

 theory into actual pmrtirr. This arises chiefiy from flic difiicidty 

 which is met with when we come to disentangle the lumiological 

 fnun the merely analogical likenesses of animals ; and it is in over- 

 coniing this dilliculty that a great portion of the laboura of the 

 philosophical zo ilogist consists. 



