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.

now these teeth are developed before the flat teeth, in the first place because their function is earlier (for dividing comes before crushing, and the flat teeth are for crushing, the others for dividing), in the second place because the smaller is naturally developed quicker than the larger, even if both start together, and these teeth are smaller in size than the grinders, because the bone of the jaw is flat in that part but narrow towards the mouth. from the greater part, therefore, must flow more nutriment to form the teeth, and from the narrower part less.

the act of sucking in itself contributes nothing to the formation of the teeth, but the heat of the milk makes them appear more quickly. a proof of this is that even in suckling animals those young which enjoy hotter milk grow their teeth quicker, heat being conducive to growth.

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.