1902.] NATURAL SCIEXCES OF PHILADELPHIA. 207 



determining genetic relation^^hips, it will be Avell to discnss briefly 

 the kinds of structural relations known as " homologies." 



The working principle of the phylogenist who is studying a line 

 of development represented to-day only by fragments, and who has 

 not the resource of fossil remains of the extinct members of that 

 line, is that similarity in essential particulars denotes genetic affin- 

 ity. This principle would seem the more probable the more com- 

 plex the organisms treated, for it Avould not be probable that two 

 complex forms from very different beginnings could come to resem- 

 ble each other in all particulars. The less complex and more 

 plastic the organization, the more complete could be the convergence 

 of sti'ucture. This principle is a necessary postulate, and though it 

 remains to be thoroughly proved, yet it is allowable so long as the 

 known facts do not contradict it. Without its aid no phylogenetic 

 conclusions would be possible. 



There have been distinguished various kinds and degrees of 

 structural smiilarities or homologies. Owen, in his Lectures on 

 Vertebrata, 1846, defines; "Analogue, a part or organ in one 

 animal which has the same function as another part or organ in a 

 different animal. Homologue, the same organ in different animals 

 under every variety of form and function." This distinction, 

 generally accepted to-day, terms ' ' analogical ' ' a similarity of 

 function, and " homological " a similarity of structure. He distin- 

 guishes further three kinds of homological relations. Special 

 homology, " correspondency of a part or organ, determined by its 

 relative position and connections, with a part or organ in a different 

 animal ; the determination of which homology indicates that such 

 animals are constructed on a common type." Genei'al homology, 

 " a higher relation of homology .... in which a part or series of 



parts stands to the fundamental or general type Thus when 



the basilar process of the human occipital bone is determined to be 

 the 'centrum' or' body' of the last cranial vertebra." Serial 

 homology or homotypy, the relation of segmentally arranged parts of 

 the same organism. " In the instance of serial homology .... 

 the femur, though repeating in its segment the humerus in the more 

 advanced segment, is not its namesake — not properly, therefore, its 

 ' homologue.' I propose, therefore, to call the bones so related 

 serially in the same skeleton ' homotypes ' and to restrict the term 



