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On The Generation Of Animals   

the material contributed by the mother is not controlled by them, at
last there remains the most general substratum, that is to say the
animal. Then people say that the child has the head of a ram or a
bull, and so on with other animals, as that a calf has the head of a
child or a sheep that of an ox. All these monsters result from the
causes stated above, but they are none of the things they are said
to be; there is only some similarity, such as may arise even where
there is no defect of growth. Hence often jesters compare some one who
is not beautiful to a 'goat breathing fire', or again to a 'ram
butting', and a certain physiognomist reduced all faces to those of
two or three animals, and his arguments often prevailed on people.

That, however, it is impossible for such a monstrosity to come
into existence- I mean one animal in another- is shown by the great
difference in the period of gestation between man, sheep, dog, and ox,
it being impossible for each to be developed except in its proper

This is the description of some of the monsters talked about; others
are such because certain parts of their form are multiplied so that
they are born with many feet or many heads.

The account of the cause of monstrosities is very close and
similar in a way to that of the cause of animals being born
defective in any part, for monstrosity is also a kind of deficiency.


Democritus said that monstrosities arose because two emissions of
seminal fluid met together, the one succeeding the other at an
interval of time; that the later entering into the uterus reinforced
the earlier so that the parts of the embryo grow together and get
confused with one another. But in birds, he says, since copulation
takes place quickly, both the eggs and their colour always cross one
another. But if it is the fact, as it manifestly is, that several
young are produced from one emission of semen and a single act of
intercourse, it is better not to desert the short road to go a long
way about, for in such cases it is absolutely necessary that this
should occur when the semen is not separated but all enters the female
at once.

If, then, we must attribute the cause to the semen of the male, this
will be the way we shall have to state it, but we must rather by all
means suppose that the cause lies in the material contributed by the
female and in the embryo as it is forming. Hence also such
monstrosities appear very rarely in animals producing only one young
one, more frequently in those producing many, most of all in birds and
among birds in the common fowl. For this bird produces many young, not
only because it lays often like the pigeon family, but also because it
has many embryos at once and copulates all the year round. Therefore
it produces many double eggs, for the embryos grow together because
they are near one another, as often happens with many fruits. In
such double eggs, when the yolks are separated by the membrane, two
separate chickens are produced with nothing abnormal about them;
when the yolks are continuous, with no division between them, the
chickens produced are monstrous, having one body and head but four
legs and four wings; this is because the upper parts are formed
earlier from the white, their nourishment being drawn from the yolk,
whereas the lower part comes into being later and its nourishment is
one and indivisible.

A snake has also been observed with two heads for the same reason,
this class also being oviparous and producing many young.
Monstrosities, however, are rarer among them owing to the shape of the

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