330 DISTRIBUTION AND .ETIOLOGY OF THE CRAYFISHES. 



brackish water lagoons of the Gulf of Mexico, but I 

 am not aware that any of them have yet been met with 

 in the sea itself. The Palcemon lacustris (Anchistia 

 migratoria, Heller) abounds in fresh-water ditches and 

 canals between Padua and Venice, and in the Lago di 

 Garda, as well as in the brooks of Dalmatia ; but its 

 occurrence in the Adriatic or the Mediterranean, which 

 has been asserted, appears to be doubtful. So the Nile 

 prawn, though very similar to some Mediterranean 

 prawns, does not seem to be identical with any at 

 present known.* 



In all these cases, it appears reasonable to apply the 

 analogy of the My sis r dicta, and to suppose that the 

 fluviatile prawns are simply the result of the adaptive 

 modification of species which, like their congeners, were 

 primitively marine. 



But if the existing sea prawns were to die out, or to 

 be beaten in the struggle for existence, we should have' 

 scattered over the world in isolated river basins, more 

 or less distinct species of freshwater prawns, \ the areas 

 inhabited by which might hereafter be indefinitely en- 

 larged or diminished, by alteration in the elevation of the 



* Heller, "Die Crustaceen des siidlichen Europas," p. 259. Klunzinger, 

 " Ueber eine Siisswasser-crustacee im Nil," with the notes by Ton Mar- 

 tens and von Siebold: Zeitschrift fur Wissenschaftliohe Zoologie, 1866. 



\ This seems actually to have happened in the case of the widely- 

 spread allies and companions of the fluviatile prawns, Atya and Cari- 

 ditw,. I am not aware that truly marine species of these genera are 

 known. 



