UNITY OF DESIGN, .5(Jo 



in the insect they are subsequently modified and 

 altered during its progressive metamorphoses. The 

 embryo of a crab resembles in appearance the 

 permanent forms of the Myriaj)oda, and of the 

 lower animals of its own class, but acquires, in the 

 progress of its growth, new parts; while those al- 

 ready evolved become more and more concen- 

 trated ; passing, in their progress, through all the 

 forms of transition which characterize the inter- 

 mediate 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 Reptile, the Bird, and the Warm blooded 

 Quadruped, may be at the period of their maturity, 

 they are scarcely distinguishable from one another 

 in their embryonic state ; and their early develope- 

 ment proceeds for some time in the same manner. 

 They all possess at first the characters of aqnatic 

 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 oro-ans of its nervous system. We have 

 seen by what gradual and curious transitions all 

 these aquatic characters are changed for those of a 



* This cuiions analogy is particularly observable in tlie successive 

 forms assumed by the nervous system, which exhibits a gradual 

 passage from that of the Talitrus, to its ultimate greatest concen- 

 tration in the Maia. (See Figures 439 and 441, p. 488 and 490.) 

 Milne Edwards has lately traced a similar progression of devclope- 

 ment in the organs of locomotion of the Crustacea. (Annales des 

 Sciences Naturellcs ; xxx, 354.) 



