332 THE CORRESPONDENCE BETWEEN THE [chap. 



limbs actually go through so many distinct phases in the 

 course of development, as all the various modifications from 

 the primitive to the most adaptive positions may take place 

 gradually and even simultaneously. 



In what may be considered the first stage of modification 

 (Figs, c and d), each segment of the limb is simply bent 

 upon the one above it. The proximal segments (humerus 

 and femur) remain unchanged in position, the dorsal surface 

 still looking upwards, and the ventral surface downwards ; 

 the middle segment is bent downwards, so that its ventral 

 surface faces inwards and its dorsal surface outwards ; and the 

 joints between these segments (elbow and knee) form pro- 

 minent angular projections. The third segment being bent 

 to a greater or less degree in the opposite direction to the 

 middle one, retains much of its primitive position, the dor- 

 sal surface being directed upwards and the ends of the digits 

 pointing outwards. The relations of the preaxial and postaxial 

 borders of the limb are unchanged. No Mammal habitually 

 carries its limbs in this position, although the climbing Galeo- 

 pithecus and the Sloths are not far from it. It is, however, 

 very nearly the normal position of some Reptiles, especially 

 the Tortoises, though it is ill adapted for anything but a 

 very slow and clumsy mode of progression. 



The next change, and one which takes place at a very 

 early period in embryonic life, and which is one of the most 

 essential in giving the characteristic conformation of the 

 extremities of the higher Vertebrates, is a rotation of the 

 whole limb from the proximal end, though in opposite 

 directions in each case. 



The anterior extremity (see Fig. e) is rotated from the 

 shoulder, through nearly a quarter of a circle, backwards, so 

 that the humerus, instead of being at a right angle to the axis 

 of the trunk, is nearly parallel with it, the elbow points back- 



