LIVING MILLSTONES 
THE mill-like action of their own upper and lower molar- 
teeth upon one another may have been quite sufficient to 
suggest to our prehistoric parents the idea of opposing a 
pair of corrugated stones in such a manner that by mutual 
rotation or revolution they should be capable of reducing 
to powder hard substances placed between them. Indeed, 
the idea of millstones is such a simple and natural one 
that it is quite probable it may have occurred to the human 
mind without any reference to any prototype in nature ; and 
in any case, if such a natural prototype is to be sought, it 
is not necessary to go farther in search of it than our own 
dental organs. Excellent, however, for their special purpose 
as are these organs (when not subject to premature decay), 
there are other types of tooth-structure to be met with in 
the animal kingdom which present a much closer approxima- 
tion to millstones, and might well have foreshadowed these 
instruments, had they only been accessible to the primeval 
savage. But since these natural millstones occur only in 
marine fishes, some of which inhabit distant seas, while 
others are met with only as fossils deeply buried in the 
rocks, it is evident that the idea of artificial millstones 
is not derived from these natural prototypes. In other 
words, to use an expression now fashionable in natural 
science, the development of artificial and natural millstones 
is a case of parallelism, 
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