HOMOLOGY OF THE LIMBS. 305 



As Comparative Anatomy thus shows that the skeleton 

 of the limbs in Man is composed of the same bones, and in 

 the same manner as the skeleton in the four higher verte- 

 brate classes, we may justly infer their common descent 

 from a single parent-form. This parent-form was the most 

 ancient Amphibian possessing five digits both on the fore 

 and on the hind limbs. The outermost part of the limbs 

 has, indeed, been very much modified by adaptation to 

 various conditions of life. The diversities in this point 

 within the mammalian class are enormous. The slender 

 limbs of the swift Deer and the strong, springy legs of the 

 Kangaroo, the climbing feet of the Sloth and the digging 

 paws of the Mole, the fins of the Whale and the wings of 

 the Bat, are all instances. It wiU, of course, be admitted by 

 aU that these organs of locomotion are as diverse as possible 

 in point of size, form, and special function. And yet the 

 internal bony skeleton is substantially the same in them all. 

 In all these difi'erent forms of limbs the same characteristic 

 bones are always represented in essentially the same strongly 

 inherited combination ; and here we have a weighty confirm- 

 ation of the theory of descent, such as is hardly afibrded by 

 the Comparative Anatomy of ahy other organ. (Cf. Plate 

 IV. p. 34, vol. ii. of " History of Creation.") True, in the 

 limbs of the difi'erent Mammals, the skeleton is subject to 

 various arrests of development and reversions, in addition 

 to those due to special adaptation (Fig. 273). Thus, in the 

 fore foot (or hand) of the Dog the first digit, or thumb, is 

 aborted (Fig. 273 II.). In the Pig (III.) and the Tapir (V.) 

 this digit has entirely disappeared. So, too, in the Rumi- 

 nants {e.g., the Ox, Fig. IV.) the second and fifth digits are 

 also aborted, and only the third and fourth are well deve- 



