UNITY OF DESIGN. 633 



less as organs of progressive motion, we have an 

 example of the first stage of that process, which, 

 when carried 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. Another, and a still more re- 

 markable 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 observed to pass, in the embryonic stage of 

 their existence, through forms of transition, which 

 successively resemble these inferior classes. 

 The brain presents, in its earliest formation, a 

 series of tubercles, placed longitudinally, like 

 those of fishes, and only assuming its proper 

 character at a later period. The respiratory 

 organs are at first branchiae, placed, like those of 

 fishes, in the neck, where there are also found 

 branchial apertures similar to those of the lam- 

 prey and the shark ; and the heart and great 

 vessels are constructed like those of the tadpole. 



