228 
EDWIX 8. GOODKIOH. 
the homology of the occipital condyle in the Amphibia and 
the Amniota, we are asked on what segments they occur, it 
being implied, if not expressly stated, that if they are not on 
the same segment they cannot be homologous. 
The whole subject of homology and segmentation is very 
complex, imperfectly understood, and well worthy of further 
study. In the following paper I have endeavoured to put the 
theory clearly before the reader, and to come to some definite 
conclusion. 
Let us consider first of all the paired limbs. Xo one will 
deny that the pectoral limbs are homologous throughout the 
land-vertebrates or Tetrapoda ; and the same may be said of 
the pelvic limbs. By homologous we mean, in this case, that 
these organs can be traced back in an uninterrupted phyletic 
series to some common ancestral form. Yet the limbs do not 
necessarily occupy the same segments. For instance, in the 
Amphibia, while the pectoral limb belongs to segments 2, 3 
and 4, in the Anura, it belongs to segments 2, 3, 4 and 5, in 
the Urodela ; and while the pelvic limb occupies segments 8, 9 
and 10, in the frog, it is in segments 16, 17 and 18 in 
Salamandra, in segments 20, 21 and 22 in Necturus, and still 
farther back in Amphiuma. Moreover, the sacral vertebra is 
the ninth in the frog, the sixteenth in Salamandra, and the 
twentieth in Necturus (see Diagram 1). 
Among the living Reptilia the limbs vary considerably in 
position, especially the pelvic (Diagram 1). For instance, 
while in Chamaeleo the brachial nerve-plexus extends over 
segments 3-6, and the pelvic plexus over segments 18-22, 
ill Lacerta the former is from segment 6 to 9, and the 
latter from segment 26-31. In the limbless reptiles the 
nerve-plexus can still be found (Gadow" Carson (5) ). 
'I'hus Amphisbaena has a brachial plexus in segments 2, 3 and 
4, and a pelvic plexus in segments 97, 98 and 99; Bungarus,. 
a brachial plexus in segments 4 and 5, and a pelvic in segments 
230 and 231 ; Python has a brachial plexus in the third and 
fourth segments, and a pelvic in segments 342-345. The 
sacral vertebrm are the twenty-fifth and twenty-sixth in, 
