HOMOLOGY, ANALOGY AND PLASIS 



J 55 



experimental investigation to be so dis- 

 similar, would doubtless have appeared as 

 functionally equivalent just because both 

 subserve the same use. 



As another example one might adduce 

 those types of articulation which in 

 certain animals permit not only of free 

 hinge movement in various directions but 

 also of torsional movements. In verte- 

 brates these are the ball and socket joints. 

 An articulation permitting similar freedom 

 of movement is the coxo-basal articulation 

 of isopods (Tait, 1917,0). The design of 

 this joint is quite different from that of the 

 bony ball and socket. The hollow 

 cylinder of the basipodite rests by its 

 edge upon a projecting hook-like process 

 of the coxa. Another device designed to 

 give similar results is that present in the 

 Balanidae (Tait and Emmons) . The oper- 

 culum of these sessile cirripedes does not 

 articulate at any fixed point directly with 

 the surrounding shell. It rests upon a 

 closed cushion of fluid, and is bent in any 

 given direction or torsionally rotated by 

 means of appropriate muscles, the under- 

 lying fluid being meantime subjected to 

 high pressure. In these various articula- 

 tions, therefore, we find three different 

 types of physical or physiological mech- 

 anism, all designed to give the same 

 complex result. 



Indications are not wanting that equally 

 striking differences in intrinsic mechanism 

 exist between selected organs of one 

 animal group and their accredited 

 analogues in other groups. 



By way of resume we may note the 

 following. Inj asserting a functional 

 difference between organs, one must dis- 

 tinguish clearly between function in the 

 sense of use (Function A), and function in 

 the sense of intrinsic mechanism (Function 

 B). In the historical separation of like- 

 ness by "analogy" from likeness by 

 "homology," the use of a part (Function 



A) is what was meant by its function. It 

 is particularly to be noted that neither 

 Geoffroy St. Hilaire nor Owen established 

 any case of homology in which function 

 in the sense of intrinsic mechanism (Func- 

 tion E) varies; in all the subsequently 

 accepted cases of homology suggested by 

 them the essential mechanism remains 

 unchanged. At the same time it is 

 possible to point to deep-seated differ- 

 ences of function (i.e. Function B) in 

 organs which they would undoubtedly 

 have slumped together as analogous. 

 It is true that Geoffroy sought to 

 develop a detailed homology between a 

 skeletal segment of an arthropod and a 

 vertebra of the Chordata. Von Baer, 

 also on homological grounds, explained 

 the conformation of a vertebrate as being 

 a compound between articulate and mol- 

 luscan elements, the animal parts (jensu 

 Bichat) of the vertebrate being of the 

 articulate type, the vegetative parts being 

 of the molluscan type. These and many 

 similar erroneous attempts at homological 

 reasoning, perfectly valid according to 

 Geoffroyian criteria, were seen to be 

 grotesque so soon as the doctrine of 

 likeness by descent became prevalent; 

 but it may also be pointed out that such 

 cases of fictitious homology would equally 

 have broken down if tested by the criterion 

 of function in the sense of intrinsic 

 mechanism (Function E). 



owen's "analogy" not a purely 

 functional conception 



In view of an occasional misconception 

 it is well to bear in mind that though 

 Owen's definition of analogy is phrased in 

 purely functional terms, it nevertheless 

 bears an inherent structural connotation. 

 We might illustrate by reference to the 

 so-called reticulo-endothelial system. 

 When foreign particles are injected into 

 the circulation of a mammal they come to 



