10 IKTKODUCTION. 



responding partii are lettered with corresponding lettex-s. They per- 

 form, however, totally different functions, the first being an organ of 

 prehension, the second being devoted to terrestrial progression, and 

 the third being an organ of flight. 



On the other hand, whenever we find in different animals^ organs 

 fidfiUing the same purpose and doing the same work, then we have 

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

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

 parts or organs are analogous which resemble one another physio- 

 logically and discharge the same functions, wholly irrespective of 

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

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

 from one another in strxicture, at the same time that they 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 svistain their possessor in the air. They are, however, in no 

 way similar to one another except when regarded from this physio- 

 logical point of view ; and they differ altogether from a morpho- 

 logical aspect, being in no way formed on the same fundamental 

 plan. It often happens, however, that " analogous " organs have 

 the deeper relation to one another of being constructed upon the 

 same morphological plan, in which case they are hoth analogous and 

 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 plan and discharge similar functions. 



It need only be added in this connection that sound classification 

 depends on a coi'i-ect discrimination between likenesses of homology 

 ^nd likenesses of analogy. ; Likenesses of analogy — that is, like- 

 nesses dependent solely upon the possession of organs discharging 

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

 Bird and the Butterfly are not to be grouped together simply be- 

 cause both possess 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 capable of almost endless modification. ) Whilst the 

 thrarii of philosophical classification is thus clear, it may be further 

 said that great difficulties attend the carrying out of the admitted 

 theory into actual jv-actici: This arises chiefly from the difficulty 

 which is met with when we come to disentangle the homological 

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

 coming this difficulty that a gi-eat portion of the labours of the 

 ]3hilosophical zoologist consists. 



