DO INSTITUTIONS OR ORGANS REAPPEAR ? 227 



of jaws and the two posterior pairs of ambulatory 

 limbs are reformed ; the posterior two pairs of jaws 

 reacquire exopodites, and gills are formed on the 

 ambulatory limbs. 



As a matter of fact, in most Crustacea, the 

 ambulatory appendages appear when they become 

 necessary, that is to say at the end of larval life, but 

 in the Stomatopoda and in the Decapoda Macroura, 

 owing to inheritance, they appear much sooner. 

 But when these appendages are useless during the 

 larval life they disappear again to reappear at the 

 end of larval life as in most Crustacea. This 

 adaptation of the larva to special conditions is of 

 great importance, as the larval life is most im- 

 portant from the point of view of the species. 



SECTION II. 

 Disappeared Institutions. 



The apparent revival of bygone institutions. It 

 seems, at first sight, as if there were many instances 

 of the subsequent revival of bygone institutions. 



Those of ancient Eome and Greece, for instance, 

 appear from time to time to have been recon- 

 structed. In feudal Kome of the fourteenth century, 

 Cola di Kienzi, by turns tribune and senator of the 

 people, re-established the old republican constitu- 

 tion. During the Renaissance period the ancient 



