272 HEY : FRESH WATER MUSSELS IN THE OUSE AND FOSS. 



clean, Anodontas, only small in size. They are clean, doubtless, 

 because of the absence of drainage, and they are small and thin 

 because, not only is drainage absent, which often affords rich 

 food, but the river is very clear of vegetable matter. Near the 

 Union, there occur in the Foss, shells of Unio tumidus much 

 curved in form. They resemble Unio viargaritifer in shape. It 

 is curious that all our British Unios and Anodontas have a 

 tendency to assume this form under certain circumstances. 

 These investigations, made over a space of a dozen years or more 

 suggest a few general reflections. The first reflection bears on 

 distribution; that subject which the genius of Wallace has 

 rendered so deeply interesting. For the fact of so temporary 

 and incomplete an obstacle as a lock forming a boundary line 

 between varieties, and even kinds of shells, gives us some limit 

 about the apparently small lines of demarcation which may 

 determine the complete range of species and even genera upon 

 the surface of the globe. Our second reflection is upon the 

 instability of species, and the impossibility of any cut-and-dried 

 definition of the term. Of course a natural history which ignored 

 species would be as absurd as a thermometer ungraduated — only 

 species are best regarded in the same light, merely degrees marked 

 upon the unbroken flow of life. The third reflection is how 

 species are aff'ected by a change of circumstances, and that, though 

 the change is often far from being very obvious. No one would 

 suppose that the difference in the quality of the water in the 

 Ouse and the Foss is so great as the difference between the two 

 forms oi Anodojita living within a few yards of one another shows 

 it to be. A superficial and special view of any department of 

 natural history leads to the conclusion that Nature has, as it 

 were, a number of moulds from which she is never tired of 

 producing the same forms. A wider and more general view, 

 extending to past geological epochs, reveals the great fact, that as 



J.C, iii., January, 18S2 



