576 Chronicles of Science. [Oct. 



Morphology. 



Homogeny and Homoplasy. — In the July number of the 'Annals,' 

 Mr. Kay Lankester proposes to use these terms to signify certain 

 relations of parts in organisms which have hitherto been confused 

 under the one head of homology. Homogeny is applied to such 

 structures as owe their identity in arrangement and relation to 

 inheritance from a common ancestor ; parts which are thus rendered 

 similar in two organisms are said to be homogenous one with the 

 other. Many structures present, however, a very close similarity 

 in their relations to surrounding parts in two organisms without 

 being so inherited; the similarity being due merely to a com- 

 munity of external conditions in the two cases, necessitating similar 

 corresponding internal arrangements. These agreements are said 

 to be due to " homoplasy," and are called homoplastic one with 

 another. The fore-limbs of all vertebrata are thus broadly homo- 

 genous, that is, are inherited from a common ancestor. But the 

 four cavities of the heart of the mammal and of the bird are not 

 homogenous each with each. The hearts as a whole are so, but 

 since the common ancestor of birds and mammals had in all proba- 

 bility a heart with three cavities, the four cavities cannot be due 

 to inheritance in the two cases. They are homoplasts; they are 

 due to similar exigencies in the mammalian and ornithosaurian 

 stock after their divergence from a common stock. Various in- 

 stances of homogenetic and homoplastic agreement are distinguished 

 in Mr. Lankester 's paper, and it is pointed out that what are called 

 serial homologies belong to the category of homoplasts ; thus, the 

 fore and hind limb of vertebrata agree in many of their details of 

 structure on account of the mechanical arrangements required in 

 fore and hind limb being to a very great extent identical. In a 

 subsequent number of the same periodical, Mr. St. George Mivart, 

 F.E.S., writing on the use of the term homology, accepts the 

 terms homogeny and homoplasy, though he would retain Professor 

 Owen's word homology, in a wide sense, distinguishing homogenetic 

 homologies and homoplastic homologies. He also proposes to dis- 

 tinguish "ancestral homogeny" and "developmental homogeny." 

 But it appears that "ancestral homogeny" is all that the term 

 homogeny was defined to include. What Mr. Mivart calls "de- 

 velopmental homogeny," when it is not accompanied by ancestral 

 homogeny, falls simply under the category of homoplasy. The 

 subject is a little abstruse, but is of importance, since the doctrine 

 of homology as propounded by Professor Owen has sunk very 

 deeply into the mind of British anatomists ; and now that so many 

 have accepted the doctrine of evolution, and the doctrine of creation 

 by types is no longer in favour, it becomes necessary to remodel 

 our terminology in accordance with new ideas. 



