128 THE TRANSFORMATIONS OF THE ANIMAL WORLD 



If, returning to our example of the Unios of 

 France, we examine more closely one of the groups 

 of the species described by Locard, such as the 

 Unio rhomboideus of Linnaeus, we shall note that 

 of the nine species of this group, three are found 

 indiscriminately throughout all the rivers in France, 

 and should be considered as real ubiquitous varieties 

 of the typical species, Unio rhomboideus, itself 

 enjoying a very extended geographical dispersion. 

 The other six, on the contrary, are strictly localized 

 from a geographical standpoint : the U. moulin- 

 sianus in the Cher and in the Creuse ; the U. bigorri- 

 ensis in the Western Pyrenees ; the U. astierianus in 

 the Lake of Meyranne in Provence ; the U. circulus 

 and sphcericus in the rivers of Saone-and-Loire ; 

 and the U. Pacomei in the Rhone and the Saone, 

 near Lyons. The like remarks could be made re- 

 garding the majority of other genera of Molluscs 

 of the fauna of France. We thus arrive at an 

 interesting distinction between the variation pro- 

 duced on a given spot, and in all places round it of 

 a given type, and that which is only observed in 

 geographically distinct regions often entirely iso- 

 lated as regards intercommunication. If we give 

 to the different sub-species of a Linnsean type the 

 general name of forms, we may designate the forms 

 produced on the spot by the name of varieties, 

 and reserve for variations of a geographical order 

 the name of local or regional races. While the 

 causes of the creation of varieties is hidden from us, 

 it may be said that we shall be completely correct 

 in attributing to climatic and surrounding causes, 

 which, together, are designated by the name of 



