14 THE HISTORY OF CEEATION. 



covered by membrane, so that no ray of light can enter, 

 and they can never see. Such eyes, without the function 

 of sight, are found in several species of moles and mice which 

 live underground, in serpents and lizards, in amphibious 

 animals (Proteus, Csecilia), and in fishes ; also in numerous 

 invertebrate animals, which pass their lives in the dark, 

 as do many beetles, crabs, snails, worms, etc. 



An abundance of the most interesting examples of rudi- 

 mentary organs is furnished by Comparative Osteology, or 

 the study of the skeletons of vertebrate animals, one of the 

 most attractive branches of Comparative Anatomy. In most 

 of the vertebrate animals we find two pairs of limbs on the 

 body, a pair of fore-legs and a pair of hind-legs. Very often, 

 however, one or the other pair is imperfect; it is seldom 

 that both are, as in the case of serpents and some varieties of 

 eel-like fish. But some serpents, viz. the giant serpents (Boa, 

 Python), have stiR in the hinder portion of the body some 

 useless little bones, which are the remains of lost hind-legs. 



In like manner the mammals of the whale tribe (Cetacea), 

 which have only fore-legs fully developed (breast-fins j, have 

 further back in their body another pair of utterly superfluous 

 bones, which are remnants of undeveloped hind-legs. The 

 same thing occurs in many genuine fishes, in which the 

 hind-leo^s have in like manner been lost. 



Again, in our slow- worm (Anguis), and in some other 

 lizards, no fore-legs exist, although they have a perfect 

 shoulder apparatus within their bodies, which should serve 

 as a means of afiixing the legs. Moreover, in various ver- 

 tebrate animals, the single bones of both pairs of legs are 

 found in all the difierent stages of imperfection, and often 

 the decfenerate bones and those muscles belonging to them 



