i 5 z 



THE QUARTERLY REVIEW OF BIOLOGY 



be covered in a short article, but the 

 matter may be partially handled by dis- 

 cussing afresh the vexed question of 

 homology and analogy. The line of 

 treatment here presented, of necessity 

 somewhat formal, owes its origin to a 

 suggestive communication by Keith Lucas 

 on The Evolution of Animal Function — cf. 

 also E. S. Russell (1916) and Tait (1917^). 

 Lucas correctly traced the ultimate com- 

 plete separation between physiology and 

 comparative anatomy to the outcome of 

 the famous discussion that arose between 

 Cuvier and Geoffroy St. Hilaire as to the 

 criteria of comparison of animals for 

 purposes of classification. In this dis- 

 cussion, as we know, Geoffroy prevailed 

 over Cuvier. 



HOMOLOGY AND ANALOGY 



Let us go back to the time of Cuvier and 

 Geoffroy. Many of the characteristic 

 parts of animals, head, eyes, legs, 

 wings, etc., had long received at the 

 hands of mankind distinctive names indi- 

 cating equivalence or correspondence. 

 A child can scarcely fail to recognize the 

 essential similarity between, say, the 

 legs of a fly and the legs of a cat. The 

 comparative anatomists, having recog- 

 nized a certain number of different types 

 or patterns of bodily structure, were now 

 faced with a difficulty of a novel nature. 

 An insect and a bird — to select a particular 

 pair of animal forms — belong to two differ- 

 ent types of organization. In what sense 

 are the wings and the legs of the insect 

 equivalent to those of the bird? The 

 penetrating mind of Geoffroy St. Hilaire 

 seized upon a way out of this difficulty. 

 The wings of the insect resemble those of 

 the bird only in so far as they subserve the 

 same function. Though the bird, in 

 virtue of its power of flight, may seem to 

 occupy an anomalous position in the 

 vertebrate series, in essential structure it 



thoroughly conforms to type, and com- 

 parison shows that the bird's wing is, 

 from a structural point of view, the 

 equivalent of the fore-limb of any other 

 land vertebrate. The insect's wing, 

 which is not a modified limb, is obviously 

 a different article. 



The conclusion was that, for a decision 

 on questions pertaining to the arrange- 

 ment or classification of animals, we 

 should close our eyes to similarities 

 dependent on mere equivalence of function, 

 and concentrate solely upon intrinsic 

 structural features. Owen later codified 

 the terminology by which these two 

 forms of resemblance are distinguished. 

 Parts or organs of one animal which 

 have the same function as parts or organs 

 in a different animal he termed analogous. 

 Parts or organs structurally identical 

 under every variety of external form and 

 of function he termed homologous. Biolo- 

 gists thereupon decided to discount analo- 

 gies, the determination of homologies i 

 being considered as rendering better service : 

 for their purpose. 



The introduction of this distinction : 

 between analogy and homology, valuable : 

 and indispensable as it was for its special 

 purpose, reacted upon the outlook of 1 

 physiologists and of anatomists alike. 

 It appeared as if a verdict had been pro- ■ 

 nounced against the right of physiology , 

 to participate in the larger problems of 

 structure, and the effect was to produce a 

 disunion of effort between the two : 

 colleagues in what had previously been 

 considered common, or at least closely 

 contiguous, territory. Medical demands 

 being urgent, the physiologists fell back 1 

 almost entirely upon study of the mech- 

 anism of the human body, and it is curious 

 to note how their science remained 

 thereafter unresponsive to and untouched 

 by some of the broader and more liberal 

 developments of biological investigation, 



