HOMOLOGY, ANALOGY AND PLASIS 



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developments with which comparative 

 anatomy continued in complete rapport. 



The adjective homologous is in reality a 

 geometrical term and applies to corre- 

 sponding members of similar figures. For 

 example, in the two similar pentagons 

 a jS 7 8 e and ABCDE the line a 7 is homol- 

 ogous with the line AC. So too are the 

 sides a and AB, $y and BC, y8 and CD, 

 respectively homologous. In "ideal" 

 morphology the use of the word "homol- 

 ogous" in this positional sense is appro- 

 priate and serves a useful purpose. It 

 condenses into one phrase Geoffroy St. 

 Hilaire's famous principle of connexions, 

 according to which parts are comparable 

 when they occupy corresponding posi- 

 tions, i.e., have identical spatial relations, 

 in the organisms selected for comparison. 



Owen's definition of analogy is: "A 

 part or organ in one animal which has the 

 same function as another part or organ in 

 a different animal." Now, in text-books 

 of comparative anatomy we find that it 

 is customary to analyze animals into 

 systems of organs, thus: 



Integumentary, 



Skeletal, 



Muscular, 



Alimentary, 



Excretory, 



Circulatory, 



Respiratory, 



Nervous, 



Sense Organs, 



Reproductive, 



Endocrine. 



The original conception of this device is 

 due to Cuvier, and it will be noted that the 

 names of these systems, which recur 

 almost en bloc in different phyla, are 

 predominantly functional in signification. 

 Systems,, however, are made up of ' 'parts 

 or organs," and as modern comparative 

 anatomists are only too well aware, the 

 fact of analogical resemblance between 

 organs is in itself a thing of extreme 



interest — cf. Willey. The exponents of 

 ideal morphology discarded likenesses of 

 analogy for two reasons : (1) they elected 

 to believe that structure is prior to and 

 determines function, or, as Geoffroy St. 

 Hilaire put it, "animals have no habits 

 but those that result from the structure of 

 their organs; if the latter varies, there vary 

 in the same manner all their springs of 

 action, all their faculties and all their 

 actions;" (z) equivalence in the matter of 

 spatial relationships, being a more precise 

 conception than "functional" equivalence, 

 could be forthwith applied as a criterion 

 of comparison. 



THE QUESTION OF FUNCTIONAL EQUIVALENCE 



Let us consider for a moment some of the 

 complications involved in so-called func- 

 tional equivalence. 



The difficulty of precise definition. For 

 simplicity let us begin with a concrete 

 example. Let us take an obvious action, 

 such as that of a wing, which is used for 

 flying. How do we define flight? Flight 

 involves in the first instance progression 

 through the air, but so does a mere fall as 

 well as a leap. We therefore incorporate 

 as an essential element the idea of support 

 by the air itself. So modified, the defini- 

 tion still includes the drifting of a balloon. 

 We rectify once more and impose the 

 condition that the flying body must be 

 heavier than air. Soaring and parachuting 

 have still to be eliminated. We amend 

 again and stipulate that the energy determin- 

 ing the progression must not be derived from 

 movement of the air itself. With it all we 

 have yet failed to eliminate gliding, as 

 from a precipice or other attained height. 

 So we lay down the further condition that 

 the energy involved shall not necessarily have 

 been imparted before launching into the air. 

 With these several stipulations we finally 

 conclude that the flying body, itself 

 heavier than air, must be a source of energy 



