362 Comparative Anatomy — Its History, Aim, and Method 



time, but in a very loose way. He offered the following definitions: "A 

 'homologue' is the same part or organ in different animals under every 

 variety of form and function." And again, a " 'homologue' is a part or 

 organ in one organism so answering to that in another as to require the 

 same name." Elsewhere he defines homology in one word, "namesake- 

 ism." It is noteworthy that the idea of genetic relation does not enter 

 into his definitions, but near the end of his long preface appears the 

 statement, "The most intelligible idea of homologous parts ... is 

 that they are due to inheritance." 



Gegenbaur further clarified the concept of homology. He — as did 

 Charles Darwin and Haeckel — recognized inheritance as the essential 

 basis of homologies. Owen had applied the term "special homology" 

 to the correspondence between organs of different animals and "general 

 homology" to correspondence of similar parts within the body of an 

 individual animal. Gegenbaur elaborated on Owen's distinction and 

 defined several types of "special" and "general" homology as follows: 



Special Homology. 



Complete: when all of the parts of an organ in one animal are 

 represented in the homologous organ of another animal. 



Incomplete : when not all of the parts of the one organ are repre- 

 sented in its homolog. An "incomplete homology" is "defective" 

 when, in the course of evolution, the original organ loses some of its 

 parts. The foot of the modern horse has only one functional toe, in 

 contrast to the five of the foot of the supposed ancestral mammals. 

 The incomplete homology is "augmentative" when, in the course of 

 evolution, an organ acquires more parts. The heart of a mammal cor- 

 responds to the heart of a fish plus the venous sinus, a receiving cham- 

 ber lying immediately behind the heart of the fish. 



General Homology. 



Homotypy: bilateral correspondence. Right eye and left eye are 

 "homotypic" organs. 



Homodynamy : correspondence of parts arranged in a linear series 

 along the main axis of the body— i.e., serial homology. Vertebrae of the 

 several regions of the vertebral column have the same general struc- 

 ture. The neural arch of one corresponds to the neural arch of another — 

 they are "homodynamic" parts. 



Homonomy: the relation of similar multiple parts which lie in 

 an axis transverse to some long axis. The several fingers are "homo- 

 nomic" parts. 



Homonymy : the relation of similar parts lying in some minor long 

 axis. The several segments (phalanges) of any one finger are "honio- 

 nymic" parts. 



