CRITERIA OF HOMOLOGY 71 



organ, that can be reduced to unity." 1 Even in the PJiilo- 

 sopJiie anatomique he deals rather with parts than with 

 organs ; he deals, for instance, with the elementary parts 

 of the sternum, not with the organ " sternum " in its totality. 

 The functions of the sternum vary, and the primary pro- 

 tective function of the sternum may be assumed by 

 quite other parts, e.g., by the clavicles in fish, which protect 

 the heart. 2 



True homologies can be established between materials 

 of organisation but not always between organs, which may 

 be composed of different " materials." 



Almost as a corollary to this comes the further view that 

 form is of little importance in determining homologies. An 

 organ is essentially an instrument for doing a particular 

 kind of work, and its form is determined by its function. 

 Organs which perform the same function are usually similar 

 in form though the elementary materials composing them 

 may be different. This is seen in many cases of convergence. 

 Organs, therefore, which perform the same function and are 

 similar in external form are not necessary homologous. 

 Conversely, the same complex of materials, say a fore limb, 

 may take on the most varied shapes according as the function 

 of the organ changes but homology remains though form 

 changes. Accordingly, form is one of the least important 

 elements to be considered in determining a homology. 

 " Nature," he wrote in one of his early papers, " tends to 

 repeat the same organs in the same number and in the same 

 relations, and varies to infinity only their form. In 

 accordance with this principle I shall have to draw my 

 conclusions, in the determining the bones of the fish's skull, 

 not from a consideration of their form, but from a considera- 

 tion of their connections." 3 



Again, after comparing a vertebra of the Aurochs with an 

 abdominal segment of the crab, he says, " I have insisted 

 upon an identity which has extended to the least important 

 relation of all, that of form." 4 



1 Etudes progressives d'un Naturalistc, p. 59, f.n., Paris, 1835. 



2 Phil. Anat., i., p. 444. 



3 Ann. Mus. d'Hist. na/., x., p. 344, 1807. 



4 Isis, P- 534, 1820 (2). 



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