442 UNITY OF DESIGN. 



as the gills wither and the lungs are developed. If, while 

 this change is going on, and while both sets of organs are 

 too-other executing the function of aeration, all farther de- 

 velopment were prevented, 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 permanent state 

 of the Siren and the Pro/ew.v, 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 concealed 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 

 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 im- 

 proved constructions. Another, 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 development. 



Birds, though destined to a very different sphere of ac- 

 tion 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 tu- 

 bercles, placed longitudinally, like those of fishes, and only 

 assuming its proper character at a later period. The res- 

 piratory 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 lamprey and the shark; and 

 the heart and great vessels are constructed like those of the 

 tadpole, with reference to a branchial circulation. In their 

 conversion to the purposes of aerial respiration, they under- 



