No.424] LATENT OR POTENTIAL HOMOLOGY. 263 
DISTINCTION BETWEEN HomoGENous AND HÓMOPLASTIC 
ORGANS. 
In the strictest sense, special or genetic homology, the 
*homogeny" of Lankester, is the only absolute homology. 
For example, in all four-limbed vertebrates, or Tetrapoda 
(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 an 
embryology, and all 
others are derivatives. 
But suppose we should 
discover that these two 
phalanges had originated Ps. 2.— Ideal embryonic ground plan of rhinoceros molar, 
independently in several s d r 
different classes of vertebrates, and were not derivatives ; 
should ad then be considered analogous or homologous? 
* Again," says Lankester (70), “it may perhaps be admitted 
that the common ancestors of the Osseous Fishes and Mam- 
malia had a skull of decidedly undifferentiated character, with 
a much less amount of 
differentiation than is 
observed in the skulls 
of either of these 
groups. It is only in 
so far as they have 
parts represented in 
the common ancestor 
that we can trace Zo»o- 
re iae het ei Mem nme rey i these routs; 
‘and yet the homology 
of a vast number of bones in the skull of the two is discussed 
and pointed out." Suppose, accordingly, that in the formation 
of dermal roofing bones in different orders of fishes a pair of 
bones corresponding in position to the parietals should arise 



