The falling off in the yield of pearls in some streams is credited to a certain extent to the building of bridges and the consequent abandonment of fords. This is based on the theory that injury to the mollusk has something to do with the production of pearls, and that they are to be found more plentiful about fords and places where cattle drink. The theory is beautifully stated by the lamented Hugh Miller: “I found occasion to conclude that the Unio of our river-fords secretes pearls so much more frequently than the Unionidae and Anadonta of our still pools and lakes, not from any specific peculiarity in the constitution of the creature, but from the effects of the habitat which it chooses. It receives in the fords and shallows of a rapid river many a rough blow from the sticks and pebbles carried down in time of flood, and occasionally from the feet of men and animals that cross the stream during droughts, and the blows induce the morbid secretions, of which pearls are the result. There seems to exist no inherent cause why _Anadon cygnea_, with its beautiful silvery nacre—as bright often, and always more delicate, than that of _Unio margaritiferus_—should not be equally productive of pearls; but secure from violence in its still pools and lakes, and unexposed to the circumstances that provoke abnormal secretions, it does not produce a single pearl for every hundred that are ripened into value and beauty by the exposed, current-tossed Unionidae of our rapid mountain rivers. Would that hardship and suffering bore always in a creature of a greatly higher family similar results, and that the hard buffets dealt him by fortune in the rough stream of life could be transmitted, by some blessed internal pre-disposition of his nature, into pearls of great price.”[201] The small blue mussel (_Mytilus edulis_) of the British seas yields opaque pearls of a deep blue color, but most of them are more or less white in some part. Sometimes a shell is found in which a blue pearl will be adhering to the blue lip of the shell while a dull white one adheres to the white portion of the shell. These pearls are commonly flattened on one side, doubtless where they have been adjacent to the shell. None of them is of more than very slight value. Probably the principal fishery for the salt-water mussel pearls is that in the estuary of the Conway in Wales. These are mostly quite small and well answer the designation of seed-pearls, although a few are of fair size. In color most of them range from dirty white to the dusky or brownish tint noted by Tacitus eighteen centuries ago, but a few are of a pure silvery tint. In some seasons London dealers have agents at Conway for purchasing these pearls. The price is usually from eight to thirty shillings per ounce. THE CONTINENT OF EUROPE Après l’esprit de discernement, ce qu’il y a au monde de plus rare, ce sont les diamants et les perles.