632 UNITY OF DESIGN. 



in the same manner. They all possess at first 

 the characters of aquatic animals ; and the 

 Frog even retains this form for a considerable 

 period after it has left the egg. The young 

 tadpole is in truth a fish, whether we regard 

 the form and actions of its instruments of pro- 

 gressive motion, the arrangement of its organs 

 of circulation and of respiration, or the condition 

 of the central organs of its nervous system. We 

 have seen by what gradual and curious transi- 

 tions all these aquatic characters are changed 

 for those of a 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 proportion 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 dev elopement 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 permanent 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 

 concealed underneath the skin, and to be use- 



