CHAPTER XXIII. 



SECONDARY SYMMETRY IN VERTEBRATES. 



t 



REMARKS ON THE SIGNIFICANCE OF REPETITIONS IN SECONDARY 

 SYMMETRY: UNITS OF REPETITION. 



THE evidence as to repetition of appendages in vertebrates 

 is of great extent and has been studied by many, but in the 

 morphology of these repetitions there is still much that is ob- 

 scure. Speaking generally, the phenomena are similar to those 

 seen in Arthropods, but there is no approach to the same regu- 

 larity. Nevertheless when two extra limbs are present, it is 

 usually possible to recognize that they are together a comple- 

 mentary pair; and if the extra part is apparently a single limb 

 it is, I believe, never a normal limb and may very often be 

 shewn to contain parts of a pair of limbs. The fact that the 

 geometrical relations of the parts are less regular than they are 

 in Arthropods may probably be ascribed in some measure to the 

 circumstance that the surfaces of the vertebrate limbs do not 

 maintain their original relations but are more or less rotated in 

 the course of their development. 



In Insects it appeared that repetition of the peripheral parts in 

 Secondary Symmetry was not much more common than repetitions 

 of whole limbs, but apparently this is not the case in vertebrates. 

 Perhaps it would be more true to say that in vertebrates it is 

 only in those extensive repetitions which include the greater 

 part of the limbs beginning from the girdles, that the parts 

 are clearly in Secondary Symmetry. From this circumstance 

 doubt suggests itself whether some of the phenomena of poly- 

 dactylism, at present regarded as repetitions of digits in Series, 

 may not really be of the nature of Repetitions in Secondary 

 Symmetry (see p. 378). But however this may be, there are, 

 with the exception of some Artiodactyle cases, no examples of 

 paired repetitious of digits or phalanges at all suggesting a 

 comparison with the double extra tarsi &c. of Insects, or the 

 double extra dactylopodites of Crustacea. 



