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On The Generation Of Animals   

is plain when there is any moisture about the trachea or when it is
roughened by any affection, for then the voice also becomes uneven.

Flexibility depends on the softness or hardness of the organ, for
what is soft can be regulated and assume any form, while what is
hard cannot; thus the soft organ can utter a loud or a small note, and
accordingly a high or a deep one, since it easily regulates the
breath, becoming itself easily great or small. But hardness cannot
be regulated.

Let this be enough on all those points concerning the voice which
have not been previously discussed in the treatise on sensation and in
that on the soul.


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

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