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On The Generation Of Animals   

uterus, for by reason of its length the numerous eggs are set in a

Nothing of the kind occurs with bees and wasps, because their
brood is in separate cells. But in the fowl the opposite is the
case, whereby it is plain that we must hold the cause of such
phenomena to lie in the material. So, too, monstrosities are
commoner in other animals if they produce many young. Hence they are
less common in man, for he produces for the most part only one young
one and that perfect; even in man monstrosities occur more often in
regions where the women give birth to more than one at a time, as in
Egypt. And they are commoner in sheep and goats, since they produce
more young. Still more does this apply to the fissipeds, for such
animals produce many young and imperfect, as the dog, the young of
these creatures being generally blind. Why this happens and why they
produce many young must be stated later, but in them Nature has made
an advance towards the production of monstrosities in that what they
generate, being imperfect, is so far unlike the parent; now
monstrosities also belong to the class of things unlike the parent.
Therefore this accident also often invades animals of such a nature.
So, too, it is in these that the so-called 'metachoera' are most
frequent, and the condition of these also is in a way monstrous, since
both deficiency and excess are monstrous. For the monstrosity
belongs to the class of things contrary to Nature, not any and every
kind of Nature, but Nature in her usual operations; nothing can happen
contrary to Nature considered as eternal and necessary, but we speak
of things being contrary to her in those cases where things
generally happen in a certain way but may also happen in another
way. In fact, even in the case of monstrosities, whenever things occur
contrary indeed to the established order but still always in a certain
way and not at random, the result seems to be less of a monstrosity
because even that which is contrary to Nature is in a certain sense
according to Nature, whenever, that is, the formal nature has not
mastered the material nature. Therefore they do not call such things
monstrosities any more than in the other cases where a phenomenon
occurs habitually, as in fruits; for instance, there is a vine which
some call 'capneos'; if this bear black grapes they do not judge it
a monstrosity because it is in the habit of doing this very often. The
reason is that it is in its nature intermediate between white and
black; thus the change is not a violent one nor, so to say, contrary
to Nature; at least, is it not a change into another nature. But in
animals producing many young not only do the same phenomena occur, but
also the numerous embryos hinder one another from becoming perfect and
interfere with the generative motions imparted by the semen.

A difficulty may be raised concerning (1) the production of many
young and the multiplication of the parts in a single young one, and
(2) the production of few young or only one and the deficiency of
the parts. Sometimes animals are born with too many toes, sometimes
with one alone, and so on with the other parts, for they may be
multiplied or they may be absent. Again, they may have the
generative parts doubled, the one being male, the other female; this
is known in men and especially in goats. For what are called
'tragaenae' are such because they have both male and female generative
parts; there is a case also of a goat being born with a horn upon
its leg. Changes and deficiencies are found also in the internal
parts, animals either not possessing some at all, or possessing them
in a rudimentary condition, or too numerous or in the wrong place.
No animal, indeed, has ever been born without a heart, but they are
born without a spleen or with two spleens or with one kidney; there is
no case again of total absence of the liver, but there are cases of
its being incomplete. And all these phenomena have been seen in
animals perfect and alive. Animals also which naturally have a
gall-bladder are found without one; others are found to have more than

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