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On The Generation Of Animals   

They are shed, after they have been formed, partly because it is
better so (for what is sharp is soon blunted, so that a fresh relay
is needed for the work, whereas the flat teeth cannot be blunted but
are only smoothed in time by wearing down), partly from necessity
because, while the roots of the grinders are fixed where the jaw is
flat and the bone strong, those of the front teeth are in a thin part,
so that they are weak and easily moved. They grow again because they
are shed while the bone is still growing and the animal is still young
enough to grow teeth. A proof of this is that even the flat teeth grow
for a long time, the last of them cutting the gum at about twenty
years of age; indeed in some cases the last teeth have been grown in
quite old age. This is because there is much nutriment in the broad
part of the bones, whereas the front part being thin soon reaches
perfection and no residual matter is found in it, the nutriment
being consumed in its own growth.

Democritus, however, neglecting the final cause, reduces to
necessity all the operations of Nature. Now they are necessary, it
is true, but yet they are for a final cause and for the sake of what
is best in each case. Thus nothing prevents the teeth from being
formed and being shed in this way; but it is not on account of these
causes but on account of the end (or final cause); these are
causes only in the sense of being the moving and efficient instruments
and the material. So it is reasonable that Nature should perform
most of her operations using breath as an instrument, for as some
instruments serve many uses in the arts, e.g. the hammer and anvil
in the smith's art, so does breath in the living things formed by
Nature. But to say that necessity is the only cause is much as if we
should think that the water has been drawn off from a dropsical
patient on account of the lancet, not on account of health, for the
sake of which the lancet made the incision.

We have thus spoken of the teeth, saying why some are shed and
grow again, and others not, and generally for what cause they are
formed. And we have spoken of the other affections of the parts
which are found to occur not for any final end but of necessity and on
account of the motive or efficient cause.


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