the Pearl Oyster and on the Production of Pearls. 95 



covered with oysters, ready to receive any quantity of dew-drops 

 that might fall from the leaves above them. But the oysters 

 unfortunately were of the wrong sort. ^Elian's theory was, that 

 pearls were produced by lightning flashing into the open shells ; 

 and it was not until many years later that a less poetical race of 

 naturalists adopted more material hypotheses, and attributed to 

 pearls a similar origin to that of the gall-stones, urinary calculi, 

 Bezoar-stones, and other concretions found in the higher ani- 

 mals. According to another view, pearls were the eggs of the 

 mollusks ; and Dr. Mobius quotes from Valentini a statement 

 that, in the year 1700, "a Swedish major and a Livonian noble- 

 man saw a shell-fish creep out of a pearl which a fisherman had 

 laid before them on the table." In 1717 Reaumur showed that 

 the structure of pearls was identical with that of the shells pro- 

 ducing them, and that those of the Pinna of the harbour of 

 Toulon consisted either of nacre, or of a system of columns, 

 according to the place in which they were formed. 



Under any circumstances, however, the production of pearls 

 can hardly occur in the natural course of the secretion of the 

 materials of the shells, and they must be regarded as abnormal 

 deposits of this material, the impulse to which must be given by 

 some peculiar cause. Reaumur ascribes the formation of pearls 

 to a morbid eifusion of the coagulating shell material, and Dr. 

 Mobius considers that this may sometimes be the case with those 

 pearls which have a crystalline calcareous nucleus. But these 

 appear to be few in number, and it seems now to be a settled 

 point that, at all events in most cases, the formation of pearls is 

 caused by the intrusion of foreign bodies between the mantle of 

 the animal and its shell. By the majority of writers grains 

 of sand are described as the most frequent irritants, and Dr. 

 Kelaart appears to admit their intrusion as one of the causes 

 of pearl formation* ; but, on the other hand, Dr. Mobius states, 

 that of 44 sea-pearls (from America and the East Indies) and 

 15 freshwater pearls, of which he has prepared sections, not 

 one presented a sand- grain as its nucleus. A few, as already 

 stated, contained a " radiately fibrous, crystalline calcareous 

 nucleus," but in the majority the nuclei were of an organic 

 nature. 



With regard to the origin of these organic nuclei, two theo- 

 ries have been put forward. According to the earliest of these 

 hypotheses, set up by Sir Everard Home in 1826t, the ova of 

 the mollusks form the nuclei of pearls ; and in support of this 

 view the author stated that he had found pearls in the ovary, 

 and adduces two letters from C. Sandius, dated in 1673 and 

 1674, and published in the ^Philosophical Transactions' for the 

 * See p. 83. t Phil. Trans. 1826, p. 338. 



