PARTS CONCERNED IN HITUODUCTION. 240 



from tliose to Avliicli the expression was originally applied 

 by Professor Owen. In comparative anatomy, the tenn 

 homologous is employed to indicate parts which occupy 

 corresponding positions in that general plan of conformation 

 traceable throughout large groups of animals. Such part.s 

 have no farther community of office than arises from tlieir 

 physical adaptation. The work which they are naturally 

 adapted to perform, in the more typical species, they must 

 be suited for in all, in some degree, so long as there is no 

 wide departure, on the one hand, from the ordinaiy fonn of 

 the organ, or, on the other, from the ordinary habitat of 

 the gi'oup of animals. But, when this does occur, we 

 have numerous examples of their being turned to totally 

 different uses, or left in a rudimentary state, without any 

 office to perform at all. It is sufficient to cite, in illustra- 

 tion, the rudimentary condition of the pelvis of the whale, 

 or the transference of the respiratory function in the fish 

 from the lung to the gill, the rudimentary homologue of the 

 former organ being left to do duty as a swimming bladder. 

 But, in the correspondences which have been under con- 

 sideration in this chapter, the function or olhce to which 

 the parts minister is a point of prime importance in esti- 

 mating their relations. The spermatic and germinal organs, 

 for instance, have throughout been assumed as the corre- 

 sponding parts in the economy of the two sexes. Yet, there 

 is reason to believe, they are not always really structural 

 homologues ;* and all that is meant by instituting a com- 

 parison between them, is to indicate that they occupy 

 corresponding or equivalent positions in the reproductive 

 process, in as much as they are similarly related to the 

 orthomorphic phase of development on the one lianil, and 

 to the ultimate spermatic and germinal corpusculcs on the 

 other. Hence, it may appear that the term analocjoa* 



*■ See Sanderson in Supplement to Cyclop. Anat. and Physical " Voget. 

 Eeproduct.," p. 252. 



M 3 



