UNITY OF DESIGN. 441 



cular circulation, and multiple organs of digestion; and the 

 central filaments of the nervous system in both being stud- 

 ded with numerous pairs of equidistant ganglia. In the 

 worm all these features remain as permanent characters of 

 the order: in the insect they are subsequently modified and 

 altered during its progressive metamorphoses. The em- 

 bryo of a crab resembles in appearance the permanent forms 

 of the Myriapodd) and of the lower animals of its own class, 

 but acquires, in the progress of its growth, new parts; while 

 those already evolved become more and more concentrated, 

 passing, in their progress, through all the forms of transition 

 which characterize the intermediate tribes of Crustacea; till 

 the animal attains its last state, and then exhibits the most 

 developed condition of that particular type.* 



However different the conformations of the Fish, the Rep- 

 tile, the Bird, and the warm-blooded quadruped, may be at 

 the period of their maturity, they are scarcely distinguisha- 

 ble from one another in their embryonic state; and their ear- 

 ly development proceeds for some time in the same man- 

 ner. They all possess at first the characters of aquatic ani- 

 mals; 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 progressive 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 transitions all these aquatic cha- 

 racters are changed for those of a terrestrial quadruped, fur- 

 nished with limbs for moving on the ground, and with lungs 

 for breathing atmospheric air; and how the plan of circula- 

 tion is altered from branchial to pulmonary, in proportion 



* This curious analogy is particularly observable in the successive forms as- 

 sumed by the nervous system, which exhibits a gradual passage from that of 

 the Talitrus, to its ultimate greatest concentration in the Maia. (See Fi- 

 gures 439 and 441, p. 3S2 and 383.) Milne Edwards has lately traced a si- 

 milar progression of development in the organs of locomotion of the Crusta- 

 cea. ( Annales des Sciences Naturelles, xxx. 354. ) 



VOL. II. 56 



