242 
EDWJN S. GOODIMCH. 
nerve. This nerve, or complex of nerves, belongs with 'the 
various parts it supplies to a region extending in the 
Amphibia behind the occipital condyle to the second spinal 
nerve (the hypoglossal in the frog comes out between the 
first and second vertebras — see Diagi-am 6) — and in front of 
the first vertebra in the Amniota.^ The independent trans- 
position of skeletal and neuro-musciilar elements can alone 
account for such relative displacement. 
Of course the whole question of the homology of the 
condyles in the Amphibia and Amniota may be avoided by 
supposing that they have been independently developed in 
the two groups, are not homologous, and have not altered 
their position since they made their first appearance. But, 
although we are not able to prove with absolute certainty 
that the common ancestor already possessed a skull with 
differentiated occipital region and well-defined condyles, yet 
the evidence points strongly to this conclusion. The reptiles 
and stegocephalous Amphibia merge into each other in 
the Permian and Carboniferous strata, and for my own part, I 
am firmly convinced that they are all derived from some 
common terrestrial ancestor with well-developed condyles. 
However, even supposing- this was not the case, we should 
still be met by tlie difficulty of the hypoglossal. And since 
the common ancestor cannot be held to have been provided 
with several condyles, one of which remains in the amphibian 
and the other in the amniote, the theoiy of transposition 
seems to be the only one applicable in this case, as it has 
already been shown to be in the case of the fins. 
The conclusion to which Ave are driven is that the occipital 
condyles of amphibians, reptiles, birds and mammals are all 
homologous, Avhatever may be the segments on which they are 
developed. They are fully homologous in the only sense in 
which homology can be practically defined, namely in the 
sense that they can be traced back to a common ancestor. In 
the course of evolution the function of condyle-formation, 
originally belonging to segment N of the series, has been trans- 
posed to segments n + 7 or x-7. The transposition of the 
