METAMEKIC SEOMENTATJUN AND HO.MOLOGY. 
237 
very simplest cases, aud is quite incapable of accounting for the 
relative shifting of organs accompanied by the unequal variation 
of segments in different regions. Whether we measure an object 
in inches or in centimetres the relative position of its parts 
remains unaltered. Only could there be relative shitting if 
the redivision was unequal along the series. Comparing, for 
instance, the vertebral column of Dromceus with that of 
Struthio, we might say that in the former the cervical i-egion 
has been divided into eighteen and the lumbar region into six 
segments, while in the ostrich the cervical region has been di- 
vided into seventeen and the lumbar into eight segments. Ob- 
viously this would be no explanation at all, but merely a state- 
ment indifferent words of the original problem we set out to solve. 
No doubt such a view avoids the difficulty of zones of growth 
or of reduction ; but no more than the theory of intercalation 
can it be applied to such cases as the apparent suppression of 
the mid-region of the trunk in fishes where the pectoral meets 
the pelvic fin, or the independent shifting of the median and 
paired fins discussed above. Some theory of the redistribu- 
tion of the formative substances to which morphological differ- 
•entiation is due is necessary if we are to explain homology ; 
mere redivision does not help us at all.^ 
If neither migration nor intercalation nor redivision can 
account for the change in position of fins or paired limbs, 
there remains the theory of transposition (12 and 13). Fiir- 
bringer has shown how the nerve-plexus of a limb may become 
more or less extensive by the gradual assimilation of the 
nerves of neighbouring segments (9). Nerves at the anterior 
or posterior end may increase in size, and new nerves from 
adjoining segments may enter into the composition of a plexus ; 
so that by gradual growth a limb originally supplied by, say, 
nerves e f g, may come to be supplied by nerves d e f g h, 
c D E F G H J, and so on. Or, on the contrary, by a similar 
^ It may well be doubted whether “ redi vision ” ever takes place. 
When the total number of segments varies, the variation may better be 
interpreted as due to differences in growth — that is to say, to the 
addition or suppression of segments at the growing end of the series. 
