METAMERIC SEGMENTATION AND HO.AIOLOGY. 
243 
nerves and other parts may or may not have accompanied that 
of the condyles. 
Thus a consideration of the condyles, like that of the regions 
of the vertebral column or limbs, inevitably leads us to the 
conclusion that homology is independent of ordinal corre- 
spondence in segmental position. Specialised organs or any 
differentiated parts are truly homologous in different animals 
when they are derived from coi responding parts in the common 
ancestor — the animals being compared as wholes and not their 
separate segments. And this is true whether these organs or 
parts are composed of few or of many, of the same or of 
different segments, or are not segmented at all. 
On the other hand, any attempt to define homology as com- 
plete only when including strict segmental correspondence 
defeats its own object. Ic is doubtful whether any organs or 
parts whatsoever could be proved to be completely homologous 
in this sense ; certainly not the paired limbs, nor the regions 
of the vertebral column, nor the occipital condyles. Even 
when homologous organs appear to be composed of the same 
segments, the appearance may be deceptive; for if any form 
■of multiplication or of intercalation of segments can take place, 
the comparison of segment with segment becomes at once 
uncertain. 
To advocate the view that homology must be considered as 
independent of segmental correspondence is the object of this 
paper; but in conclusion a few words may be said concerning 
the possible connection between them. It is a remarkable 
fact that a constant relation often becomes established between 
■certain segments and certain organs or differentiated parts. 
The various cranial nerves, for instance, appear to bear a 
fixed relation to the anterior segments of the head throughout 
the Craniata. The muscles of the eye, the ear, the jaws, the 
gill-slits, etc., are all definitely related to certain segments, 
moi-e or less closely and in different ways. The neck of 
■mammals (with but three exceptions) is formed of seven seg- 
ments, the last lumbar vertebra of the Artiodactyle Ungulates 
is always the twenty-sixth, the vertebral column of the 
