350 EDWIN S. GOODHICM. 



and reptiles (11, etc.). As may be seen in the diagram 

 (fig. 27), the alteration in position of a limb is due to the 

 contribution made to the limb-muscles, etc., of certain seg- 

 ments at one end becoming less and finally ceasing altogether, 

 Avhile the contribution made by certain segments at the other 

 end becomes correspondingly large. Thus new segments 

 may be taken in at one end and old segments may drop out 

 at the other, or the number of segments contributing may be 

 merely increased or diminished. 



A limb may in this way undergo change of position without 

 necessarily undergoing any change of form or structure. 

 The only change involved in the process is that the limb, 

 instead of being derived from a certain set of segments in one 

 region, is derived from a similar set of segments farther up or 

 down the trunk. This is Furbringer's principle of imitative 

 homodynamy, or parhomology, accompanying the progressive 

 metameric modification of a plexus. To borrow Professor 

 Lankester's illustration, it may be compared to the trans- 

 position of a tunc from one key to another on the piano. 

 The tune remains the same, but it is played on different 

 notes. 



We conclude, then, that the change of position of limbs is 

 not due to the actual migration of the limb-rudiment, 

 or limb-substance, but to reduction on one side and 

 growth on the other. The migration is apparent, not 

 real. It is, if one may be allowed the expression, the calling 

 forth of the potentiality of the segments, which shifts, passing 

 up or down like a wave. This view might be called "the 

 theory of the transposition of the limbs." 



The same argument applies to tlie girdles. In some 

 Elasmobranchs (Braus 3), for instance, the pectoral girdle is 

 pierced by thirty-six nerves belonging to the limb plexus 

 (Trygon), in others by twenty (Torpedo), or by three 

 (Lasmargus), in Ceratodus by none at all. These diazonal 

 nerves may each pass through separate foramina, or several 

 may pass through the same foramen. It seems probable, 

 thorclorc, that tlic material (scleromcrc) of a varying niimbi'r 



