40 Mr. E. R. Lankester on the use of 
ments operating on similar materials in the two stocks, the 
Mammalian and Sauropsidan, having produced results in the 
way of structure which have a certain agreement. What, 
exactly, is to be ascribed to homogeny, and what to homoplasy; 
in the relations of this series of structures, is a matter for 
careful consideration. s was remarked above, the right 
ventricle of the bird's heart is not homogenous with the right 
ventricle of the mammal's heart, nor the left with the left; 
but the two cavities in each case are homoplastic—the same 
conditions as regards the maintenance of animal heat and 
other matters belonging to the circulation, which evoked or 
were the cause of the perpetuation of this structure in the one 
case having equally operated in the other. As to the bones of 
the skull, the room for diversity is not very great when the 
homogenous basis is given which all higher Vertebrata have 
inherited from a common ancestor; but there can be no doubt 
that many of the bones in the fish's skull are not homo- 
genous with those of other Vertebrata, whilst they appear to 
be related as homoplasts. That similar forms may arise in 
this way in the skulls of two divergent stocks, and lead to 
close correspondences which are not traceable to homogeny, is 
indicated by the fact that membrane-bones corresponding in 
position and relations in the skulls of one group to cartilage- 
bones in the skulls of another group are observed*. The 
membrane-bone in this case is certainly not homogenous with 
the cartilage-bone ; but it is homoplastic with it; and in the 
same way it is very probable that membrane-bones in two 
skulls are in some cases only homoplasts, though they may 
have been the subject of speculation as to their homology. 
e mammalian malleus and mandible present an homogeny 
of the general region only, when compared with the bones of 
the suspensorium and lower jaw of the fish, the individual 
bones of which, as well as the opercular bones, are not repre- 
sented in the mammalian skull by corresponding individual 
bones. and not even by homoplastic developments. The 
Sauropsidan suspensorium, in being segmented, presents a 
closer homoplastic agreement with that of osseous fish; an 
probably a true homogenetie correspondence is to be admitted 
in the quadrato-articular articulation of Fishes and Saurop- 
sida. 
It may be said that the term “analogy,” already in use, is 
sufficient to indicate what is here termed * homoplasy ;” but 
analogy has had a wider signification given to it, in which it is 
* As en example, the cartilage-bone in the fish's skull, which Mr. 
Parker proposes to call pterotic, oe ^d considered the homologue of 
ay be cited, 
the squamous in mammals, may 
eer eS 
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