208 PROCEEDINGS OF THE ACADEMY OF [April, 



'homologue' to the corresponding bones in different species.® And 

 in his paper On the Archetyjye and Homologies of the Vertebrate 

 Skeleton, in 1848, he states: " Homological relationships .... 

 are mainly, if not wholly, determined by the relatiye position and 

 connection of the parts, and may exist independently of form, 

 proportion, substance, function and similarity of deyelopment. " 



Owen implied by " liomology " an essential agreement in struc- 

 ture and connections, a relation of essential structural agreement. 

 But, as Cu\der before and L. Agassiz after him, he held the view of 

 the occurrence of diffei'ent plans or types of structm-e among ani- 

 mals which could not intergrade and were not gradually evolved. 

 Under the present dominance of the theory of descent, the idea of 

 separately created ' ' types " or " plans ' ' has been relinquished, so 

 that while we retain the term " homology," we define it as a rela- 

 tion of genetic descent. 



Of the kinds of homologies distinguished by Owen, the general 

 and serial homologies are of not the same direct value to the phylo- 

 genist as the special. Kelations of two sides of the same body to 

 each other, of anterior to posterior end, of one antimere or meta- 

 mere to another, of the right arm to the left arm, or of an arm to 

 a leg, etc., are all relations between the parts of one organism. In 

 phylogeny we are immediately concerned with the relations of differ- 

 ent organisms, and with general homologies, including under that 

 term also serial homologies, only in so far as they serve to explain 

 the former. With the treatment of structural characters we shall 

 have to consider general homologies, and now simply mention some 

 kinds of special homologies. 



Under the idea of ' ' special homology ' ' is nov/ generally under- 

 stood a relation of genetic afiinity. " Homologie neunen wir eiue 

 Uebereinstimmung, die auf gemeinsamer Abstammung der betreffen- 

 den Organismen beruht. Der Ausdruck Homologie wird demnach 

 voUkommen im Siune der Homophylie gebraucht. " '' And so we 

 must employ it, with the working theory of " unity of develop- 

 ment ' ' in the place of the earlier ' ' unity of plan. ' ' In order to 

 understand the relations of two organisms as wholes, it is generally 

 first necessary to commence by comparing them part by part ; the 



* Vicq d'Azyr was the first recognizer of serial homology : Parallele 

 des OS qui comjwsent les extremites, Mem. Acad. Sci., 1774. 

 ' Hatschek, Lehrbuch, 1888. 



