THE ARGUMENTS FROM MORPHOLOGY. b85 



during the development of embryos, there often arise organs 

 which disappear on being replaced by other organs dis- 

 charging the same functions in different ways ; and that in 

 some cases, organs develop to certain points, and are then- 

 re-absorbed without performing any functions. But very 

 generally, the partially-developed organs are retained 

 throughout life. 



The osteology of the higher Verlebraia, 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 vertebrae 

 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 vertebrae 

 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 the dermal ap- 

 pendages. Some of the smooth-skinned amphibia have scales 

 buried in the skin. The seal, which is a mammal considerably 

 modified in adaptation to an aquatic life, and which uses its 

 feet mainly as paddles, has toes that still bear external nails ; 

 but the manatee, which is a much more transformed mam- 

 mal, has nailless paddles, which, when the skin is re- 

 moved, 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 



