With regard to the teeth it has been stated previously that they do not exist for a single purpose nor for the same purpose in all animals, but in some for nutrition only, in others also for fighting and for vocal speech.We must, however, consider it not alien to the discussion of generation and development to inquire into the reason why the front teeth are formed first and the grinders later, and why the latter are not shed but the former are shed and grow again.
Democritus has spoken of these questions but not well, for he assigns the cause too generally without investigating the facts in all cases.He says that the early teeth are shed because they are formed in animals too early, for it is when animals are practically in their prime that they grow according to Nature, and suckling is the cause he assigns for their being found too early.Yet the pig also suckles but does not shed its teeth, and, further, all the animals with carnivorous dentition suckle, but some of them do not shed any teeth except the canines, e.g.lions.This mistake, then, was due to his speaking generally without examining what happens in all cases;but this is what we to do, for any one who makes any general statement must speak of all the particular cases.
Now we assume, basing our assumption upon what we see, that Nature never fails nor does anything in vain so far as is possible in each case.And it is necessary, if an animal is to obtain food after the time of taking milk is over, that it should have instruments for the treatment of the food.If, then, as Democritus says, this happened about the time of reaching maturity, Nature would fail in something possible for her to do.And, besides, the operation of Nature would be contrary to Nature, for what is done by violence is contrary to Nature, and it is by violence that he says the formation of the first teeth is brought about.That this view then is not true is plain from these and other similar considerations.