crsi- I;K<TI<;I;.\I>ATIOXS 



231 



he presents the matter in terms which may lie briefly analyzed with 

 the usages of other authors, as follows: 



I. HOMOLOGY, GENERAL : as of vertebrae and limits. 



1. IJOMOTYPY : as of opposite limbs, eyes, kidneys, etc. 



2. HOMODYNAMV : (in part the "general," in part the "serial," homology of 



Owen ; the " meristic " homology of Bateson). Corresponding limbs, 

 parts, segments (e.g., the humerns and femur) on the same side of the 

 body. 



?,. HMMOXOMY : parts which are in the same transverse axis of the body, or 

 on only one section of the longitudinal axis ; e.g., the rays of the fins 

 of fishes, the single fingers and toes of the higher vertebrates are 

 homouomous organs. 



II. HOMOLOGY, SPECIAL: (the "homogeny" of Lankester). 



1. COMPLETE HOMOLOGY of elements which have retained their relations un- 

 changed, as of single bones from the Amphibia to the Mammalia. 

 2. INCOMPLETE HOMOLOGY, as of organs which have either gained new parts 



or lost certain of their parts. 

 . (li'fective, as in comparison of fins of teleosts and of selachians. 



b. augmentative, as in the heart of cyclostomes and of the higher vertebrates. 



c. imitative, as where difFereut vertebrae connect with the ilium and become 



sacral. 



III. HOMOMORPHY (Fu'rbringer) : from these homologies certain structures are to 

 be distinguished as homomorphic which are more or less similar to each 

 other but stand in no phylogenetic connection. 1 



Homomorphy comes nearest, as we understand it, to the " homoplasy " of 

 Lankester, but the latter term has the priority of definition. 



metacone- 



DlSTINCTION BETWEEN HOMOGENOUS AND HOMOPLASTIC ORGANS. 



Iii the strictest sense, special or genetic homology, the " homogeny 

 of Lankester, is the only absolute homology. For example, in all 

 four-limbed vertebrates, or Tetra- 

 poda (Credner), the first and 

 second phalanges of the tibial 

 digit or hallux are homogenous; 

 the earliest tetrapods had such 

 phalanges, so far as we can judge 

 from both paleontology and em- metaconule . 

 bryology, and all others are deri- 

 vatives. 



But suppose we should dis- 

 cover that these tWO phalanges Km. ill. Ideal embryonic ground plan of rhinoceros 



' molar, showing relation of primitive t-usps to the folds 



had originated independently in and crests, 

 several different classes of verte- 

 brates, and were not derivatives; should they then be considered analogous 

 or homologous? "Again," says Lankester ('70), "it may perhaps be 



1 Literally translated from Gegenbaur. 



hypocone- 



-protoconule 

 protocone 



