94 PEARLS [CH. 



see how often the theories of sand grain and the 

 parasite causation have been brought forward in 

 the past. The grain of sand theory dates back 

 much further, too, than Redi, 16/1. 



Whilst this view dominated our older text books, 

 and particularly school books and popular Avorks, 

 until quite recently, tlie idea that a parasite pro- 

 vided the stimulus for pearl formation has been re- 

 peatedly put forward since 1554. The older workers 

 did little more than make guesses, but Filippi as far 

 back as 1852 actually discovered a parasite as cause 

 in the fresh- water mussel. It will be seen, therefore, 

 that all the workers of the last ten years who have 

 pushed the researcli upon pearl formation as far as 

 fresh advances in biological technique would allow, 

 have had at least numerous helpful suggestions left 

 as a legacy by their predecessors. 



We shall only refer in detail to one or two of the 

 causes tabulated. First let us look at the old grain 

 of sand theory, which modern writers have tended to 

 throw over completely, notwithstanding the fact that 

 Herdman and other workers have recorded sand 

 grains as occasional nuclei in pearls. 



This theory assumed that grains of sand found 

 their way between the sliell and the mantle of the 

 mollusc and irritated the latter so that it responded 

 by coating the invading object Avith layers of pearly 

 substance. This certainly does take place sometimes, 



