chap, xxiil] SIGNIFICANCE OF SECONDABT? SYMMETRY. 55 



* Kingsley, Proc. Boston N.H.S., 1881—2, xxi. p. 169, PL n. 



* Cavanna, G., Pubbl. del J!. 1st. di Studi super, in , 1879, p. 8, Tav. i. 

 Four important cases; one, jig. 2, apparently resembling Km in somen -p- 



* Mazza, Atti Soc. ital. sci. nat., 1888, xxxi. p. I 15, PI. i. 



TUCKERMAN, JOW. Alldt. I'ln/s., 1886, p. 517, PI. X\ I. 



Cat. Terat. Ser. Coll. Surg. Mus., 1872, No. 23. 

 Heron-Koyer, Bull. soc. Zool. France, 1884, ix. p. 165. 

 Bergendal, Bihang k. svensk. vet. Ah., 1889, xiv. Aid. iv. PI. i. 



* Ercolaxi, Mem. Ace. Bologna, 1881, iv. p. 810, PI. iv. Four important ca 

 and very good bibliography. 



Sutton, Trans. Path. Soc., 1889, xe. p. 161, fig. 



[Three cases in Newts: Triton cristatus, Jackei,, /.<»,!. Cart., lssl. xxn. p. l.'.f,. 

 Triton tceniatus, Landois, H., ibid., 1884, xxv. p. ( J4; Cabiebano, Atti Soc. ital. 

 nat., 1882, xxv]. 



From these Batrachian cases most of the chief features of the 

 phenomena may be learnt. To those wishing to get a general view of 

 the subject of repetition of Vertebrate limbs in a comparatively small 

 compass the valuable memoir of Ercolaxi quoted above is especially 

 recommended. 



Before proceeding to a consideration of the significance of the 

 phenomenon of Repetition in Secondary Symmetry it must be 

 expressly stated that there are in vertebrates a certain number 

 of cases, perhaps even classes of cases, which it is likely differ 

 widely from the rest ; but as was said above, the chief difference 

 between the Vertebrate and Arthropod cases lies in the com- 

 parative simplicity of the latter. It may be stated further that 

 this greater simplicity of the Arthropod cases consists especially 

 in the maintenance of the relation between the extra pair and 

 some normal limb. 



Remembering always the existence of unconformable cases we 

 may, I think, safely gather up from the simple cases several 

 points relating to the problems of Natural History at large. I 

 only propose here to make allusion to those considerations which 

 are not developed in the ordinary teratological treatise-. 



Of the fact that any regularity can be discerned in thes< 

 strange departures from normal structure, and of the bearings 

 of this fact on current conceptions of the causes determining the 

 forms of animals it is now hardly necessary to speak further. 

 Other points not before noticed remain. 



In the Arthropod cases that were spoken of as 'regular' it 

 was seen that the polarity of the Secondary Symmetries has a 

 definite relation to that of the body which bears them. This 

 is quite in harmony with the supposition that they are related 

 to the normal body somewhat as buds are related to a colony, 

 for in most colonial forms the morphological axes and planes 

 of the buds are definitely related to those of the stock. 



But in the Vertebrate cases though there is generally a re- 

 lation of images between the extra pair, a definite geometrical 

 relation between them and a normal limb is seen more rarely. 



