THE ARGUMENTS FROM MORPHOLOGY. 473 



during the development of embryos, there often arise organs 

 which disappear on being replaced by other organs dis- 

 charging the same functions . in better ways ; and that in 

 some cases, organs develop to certain points and are then 

 re-absorbed without performing any functions. Very gene- 

 rally, however, the partially-developed organs are retained 

 throughout life. 



The osteology of the higher Vertebrata supplies abundant 

 examples. Vertebral processes which, in one tribe, are fully 

 formed and ossified from independent centres, are, in other 

 tribes, mere tubercles not having independent centres of 

 ossification. While in the tail of this animal the vertebras 

 are severally composed of centrum and appendages, in the 

 tail of that animal they are simple osseous masses without 

 any appendages; and in another animal they have lost their 

 individualities by coalescence with neighbouring vertebra? 

 into a rudimentary tail. From the structures of the limbs 

 analogous facts are cited by comparative anatomists. The 

 undeveloped state of certain metacarpal bones, characterizes 

 whole groups of mammals. In one case we find the normal 

 number of digits; and, in another case, a smaller number 

 with an atrophied digit to make out the complement. Here is 

 a digit with its full number of phalanges ; and there a digit of 

 which one phalange has been arrested in its growth. Still 

 more remarkable are the instances of entire limbs being rudi- 

 mentary; as in certain snakes, which have hind legs hidden 

 beneath the integument. So, too, is it with dermal 



appendages. Some of the smooth-skinned amphibia have 

 scales buried in the skin. The seal, which is a mammal con- 

 siderably modified in adaptation to an aquatic life, and which 

 uses its feet mainly as paddles, has toes that still bear ex- 

 ternal nails; but the manatee, which is a much more trans- 

 formed mammal, has nailless paddles which, when the skin is 

 removed, are said, by Humboldt, to display rudimentary nails 

 at the ends of the imbedded digits. Nearly all birds are 

 covered with developed feathers, severally composed of a shaft 



