5()6 UNITY OF DliSIGN. 



terrestrial quadruped, furnished with limbs for 

 moving on the ground, and with lungs for breathing 

 atmospheric air ; and how the plan of circulation 

 is altered from branchial to pulmonary, in propor- 

 tion as the gills wither and the lungs are developed. 

 If, while this change is going on, and while both 

 sets of organs are together executing the function 

 of aeration, all further developement were pre- 

 vented, we should have an amphibious animal, 

 fitted for maintaining life both in air and in water. 

 It is curious that this precise condition is the per- 

 manent state of the Siren and the Proteus ; animals 

 which thus exemplify one of the forms of transition 

 in the metamorphoses of the Frog. 



In the rudimental form of the feet of serpents, 

 which are so imperfectly developed as to be con- 

 cealed underneath the skin, and to be useless as 

 organs of progressive motion, we have an example 

 of the first stage of that process, which, when car- 

 ried farther in the higher animals, gives rise to the 

 limbs of quadrupeds, and which it would almost 

 seem as if nature had instituted with a prospective 

 view to these more improved constructions. An- 

 other, and a still more remarkable instance of the 

 same kind occurs in the rudimental teeth of the 

 young of the Whale, which are concealed within 

 the lower jaw, and which are afterwards removed, 

 to give place to the curious filtering apparatus, 

 which occupies the roof of the mouth, and which 

 nature has substituted for that of teeth ; as if new 

 objects, superseding those at first pursued, had 

 arisen in the progress of developement. 



Birds, though destined to a very different sphere 

 of action from either fishes or reptiles, are yet ob- 



