362 THE CORRESPONDENCE BETWEEN THE [CHAP. 



form such portions of the general outcome of these researches 

 as appear to be most satisfactorily established, premising, 

 however, that all the statements hereinafter to be made 

 have not yet met with universal assent. 1 



In the first place, it is perfectly obvious that the fore and 

 hind limbs have each a similar division into four main seg- 



O 



ments ; the shoulder girdle, the arm, the fore-arm, and the 

 manus of the one representing respectively the pelvic girdle, 

 the thigh, the leg, and the pes of the other. 



To proceed to further details, it is necessary to place the 

 limbs (at least in imagination) in an exact corresponding 

 position one, in fact, which is often impossible in the adult 

 animal on account of the modifications of the articular sur- 

 faces to suit the posture best adapted for the habits and 

 mode of life of the individual, but which is the position of 

 the limbs of many Mammals, when they first appear as bud- 

 like processes from the side of the body of the embryo. In 

 this position 2 the limbs are extended at right angles to the 



Membres anterieurs et posterieurs clans la serie des Vertebres," (Mont- 

 pellier, 1880); G. Gegenhaur, " Untersuchungen zur vergleich. Ana- 

 tomic; I. Carpus und. Tarsus," (Leipzig, 1864, p. 115 ff. ) ; and "Zur 

 Gliedmassenfrage," (Morphol. Jahrb. v. 1879) ; F. M. Balfour, " A 

 Treatise on Comparative Embryology," (London, iSSi). 



1 For an exposition of the very opposite hypothesis of " Antero- 

 posterior Symmetry," see Jeffries Wyman, " On Symmetry and Horno- 

 logy in Limbs," (Proc. Boston Nat. Hist, June, 1867, p. 277) ; and 

 the elaborate series of papers by Dr. Elliott Coues, published in the 

 " Medical Record" (New York) for 1870. 



This horizontal position is probably not the primary one, and 

 wherever it occurs, as in Selachians and many Mammalia, it is a secondary 

 modification of the original position of the cheiropterygium, in which 

 the long axis of the limb is directed obliquely outwards and backwards 

 towards the tail, the ulna and the fifth finger looking dorsally, and the 

 palmar and plantar surfaces of hand and foot looking towards the sides 

 of the body. This position was that of the limbs of the Enaliosauri, and is 

 still repeated by the Cetacea and by Ceratodus and the embryonic limbs of 



