" DIE STAMME DES THIERREICHS " 69 



similar variations appear among the existing Palu- 

 dines which are found in the province of the Yunnan : 

 the Paludina Margeriana of Lake Tali-Fu presents 

 smooth and keel-shaped forms quite comparable 

 to the group Paludina Neumayri-Hornesi of the 

 Danubian Pliocene. 



An important distinction should, however, be 

 made between the variations which are produced 

 together in any one epoch whether it be the 

 existing epoch or the early periods and the 

 variations which a similar type undergoes in a 

 series of successive epochs, connected, in this latter 

 case, with active causes spread over a length 

 of time. Waagen has suggested reserving the 

 name of varieties for the first, while the variations 

 of chronological order would receive the name of 

 mutations. Each mutation may be accompanied 

 by a train of varieties, but these last have a lesser 

 value than the first, and the extreme forms which 

 result from them are less different from each other 

 than those proceeding from chronological variations. 

 This can be ascertained by comparing the varieties 

 of Paludines found in any one stratum of the Levan- 

 tine stage with the extreme mutations which re- 

 present the general evolution of the group. This 

 is also shown to us in researches on the line of suture 

 of the Ammonites of the genus Phylloceras. 



The mutations have further this special charac- 

 teristic, that they are always produced in the same 

 direction, without oscillations or retrograde steps. 

 There is no example of any single series of forms 

 which has reproduced at the termination of its 

 evolution, its original, type ; if we sometimes find 



