HOMOLOGY AND ITS INTERPRETATION 35 



determining with critical precision its intrinsic value as an 

 argument for the theory of transmutation. 



Homology is a technical term used by the systematists of 

 botany, zoology and comparative anatomy to signify basic 

 structural similarity as distinguished from superficial func- 

 tional similarity, the latter being termed analogy. Organisms 

 are said to exemplify the phenomenon of homology when, be- 

 neath a certain amount of external diversity, they possess in 

 common a group of correlated internal resemblances of such a 

 nature that the organisms possessing them appear to be con- 

 structed upon the same fundamental plan. In cases of this 

 kind, the basic similarity is frequently masked by a veneer of 

 unlikeness, and it is only below this shallow surface of diver- 

 gence that we find evidences of the identical structure or com- 

 mon type. 



Thus organs of different animals are said to be homologous 

 when they are composed of like parts arranged in similar rela- 

 tion to one another. Homologous organs correspond bone for 

 bone and tissue for tissue, so that each component of the one 

 finds its respective counterpart in the other. The organs in 

 question may be functionally specialized and externally differ- 

 entiated for quite different purposes, but the superficial diver- 

 sity serves only to emphasize, by contrast, the underlying 

 identity of structure which persists intact beneath it. Thus, 

 for example, the wing of a pigeon, the flipper of a whale, the 

 foreleg of a cat, and the arm of a man are organs differing 

 widely in function as well as outward appearance, but they are 

 called homologous, none the less, because they all exhibit the 

 same basic plan, being composed of similar bones similarly 

 disposed with respect to one another. 



Organs, on the other hand, are called analogous which, 

 though fundamentally unlike in structure, are, nevertheless, 

 superficially modified and specialized for one and the same 

 function. The wing of a bird and the wing of an insect furnish 

 a trite instance of such analogy. Functionally they subserve 

 the same purpose, but structurally they bear no relation to 



