GRADUATED SERIES OF FOSSIL SPECIES. 31 
even now they cause the greatest perplexity and occasion 
endless disputes among systematic palzeontologists about the 
arbitrary limits of species. 
_ An excellent example of this is furnished by the celebrated 
and very variable fresh-water snail from the Stuben Valley, 
near Steinheim, in Wiirtemburg, which has been described 
sometimes as Paludina, sometimes as Valvata,and sometimes 
as Planorbis multiformis. The snow-white shells of these 
small snails constitute more than half of the mass of the 
tertiary limestone hills, and in this one locality show such an 
astonishing variety of forms,that the most divergent extremes 
might be referred to at least twenty entirely different species. 
But all these extreme forms are united by such innumerable 
intermediate forms, and they lie so regularly above and 
beside one another, that Hilgendorf was able, in the clearest 
manner, to unravel the pedigree of the whole group of 
forms. In Jike manner, among very many other fossil 
species (for example, many ammonites, terebratulz, sea 
urchins, lily encrinites, etc.) there are such masses of con- 
necting intermediate forms, that they reduce the “ dealers 
in fossil species” to despair. 
When we weigh all the circumstances here mentioned, 
the number of which might easily be increased, it does 
not appear astonishing that the natural accounts or 
records of creation formed by petrifactions are extremely 
defective and incomplete. But nevertheless, the petrifactions 
actually discovered are of the greatest value. Their signifi- 
cance is of no less importance to the natural history of 
creation than the celebrated inscription on the Rosetta 
stone, and the decree of Canopus, are to the history of 
nations—to archzology and philology. Just as it has 
