554 MERISTIC VARIATION. [part i. 



In the most usual forms of extra limbs in vertebrates a more 

 or less amorphous pair of limbs, compounded together for a great 

 part of their length, are attached to a supernumerary piece fitted 

 into some part of the shoulder-girdle, or more often into the 

 pelvic girdle. 



It is important to notice that though, as many (especially 

 Ercolaxi) have shewn, a complete series can be constructed, 

 ranging for instance from the ordinary pygomelian up to com- 

 plete posterior duplicity, yet repetition of limbs may be and often 

 is wholly independent of any axial duplicity, being truly a repe- 

 tition of appendicular parts only. 



The question naturally arises whether there is ever an extra 

 limb placed as a single copy of a normal limb of the same side 

 as that on which it is attached. As to this the evidence is not 

 wholly clear, but I incline to think that no case known to me 

 can properly be so expressed. Perhaps the condition which comes 

 nearest to this is exemplified by a case of a Frog fully described 

 by Kingsley 1 , where a single extra left hind leg is said to have 

 been attached to the left side of the pelvis. It is difficult to 

 question that this was actually the fact, for the figure clearly 

 represents the extra limb as a left leg; but though the muscles 

 are fully described, the bones are not ; and it still seems possible 

 that there was in reality some duplicity in the limb. The leg 

 was admittedly abnormal in its anatomy and the naming of the 

 muscles must in part have been approximate. 



But though perhaps it should not be positively stated that 

 no single extra limb is ever formed in a vertebrate in Succession 

 to the normal limb of the same side of the body, it is certainlv 

 true that in the enormous majority of polymelians the extra 

 repetition consists of parts of a complementary pair. These phe- 

 nomena are thus of interest as bearing upon the morphology of 

 repetitions in Secondary Symmetry, but in all probability are 

 not of the nature of variations in the constitution of the Pri- 

 mary Symmetry. 



A just view of the details of these phenomena can only be gained 

 from the specimens or from numerous drawings. The cases of extra 

 limbs in Batrachia may be conveniently studied as exhibiting most of the 

 different kinds of Secondary Symmetries both in the fore and hind 

 limbs. In all, some fifty cases are recorded. These may be found 

 from the following references. The evidence up to 1865 was put 

 together by Dum^ril, and an abstract of it is given also by Lunel, 

 and by Kingsley. A fuller bibliography is given by Ercolaxi. The 

 best papers on the subject are marked with an asterisk. I have added 

 a few references of less importance not included in the other biblio- 

 graphies. 



* Dumeril, Nouv. Arch. Mus. Paris, 1865, i. p. 309, PI. xx. 



* Lunel, Mem. soc. phys. d'hist. not. de Geneve, 1868, xix. p. 305, PL 



1 Proc. Bost. N.H.S., 1881—2, xxi. p. 169, PI. n. 



