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On The Generation Of Animals   

surrounded by fine membranes separating them from the uterus and
from the fluids which are formed in it; but neither in these
themselves is there anything of the kind, nor is it possible for the
embryo to take nourishment by means of any of them. Thirdly, it is
plain that all creatures developed in eggs grow when separated from
the uterus.

Natural intercourse takes place between animals of the same kind.
However, those also unite whose nature is near akin and whose form
is not very different, if their size is much the same and if the
periods of gestation are equal. In other animals such cases are
rare, but they occur with dogs and foxes and wolves; the Indian dogs
also spring from the union of a dog with some wild dog-like animal.
A similar thing has been seen to take place in those birds that are
amative, as partridges and hens. Among birds of prey hawks of
different form are thought to unite, and the same applies to some
other birds. Nothing worth mentioning has been observed in the
inhabitants of the sea, but the so-called 'rhinobates' especially is
thought to spring from the union of the 'rhini' and 'batus'. And the
proverb about Libya, that 'Libya is always producing something new',
is said to have originated from animals of different species uniting
with one another in that country, for it is said that because of the
want of water all meet at the few places where springs are to be
found, and that even different kinds unite in consequence.

Of the animals that arise from such union all except mules are found
to copulate again with each other and to be able to produce young of
both sexes, but mules alone are sterile, for they do not generate by
union with one another or with other animals. The problem why any
individual, whether male or female, is sterile is a general one, for
some men and women are sterile, and so are other animals in their
several kinds, as horses and sheep. But this kind, of mules, is
universally so. The causes of sterility in other animals are
several. Both men and women are sterile from birth when the parts
useful for union are imperfect, so that men never grow a beard but
remain like eunuchs, and women do not attain puberty; the same thing
may befall others as their years advance, sometimes on account of
the body being too well nourished (for men who are in too good
condition and women who are too fat the seminal secretion is taken
up into the body, and the former have no semen, the latter no
catamenia); at other times by reason of sickness men emit the semen
in a cold and liquid state, and the discharges of women are bad and
full of morbid secretions. Often, too, in both sexes this state is
caused by injuries in the parts and regions contributory to
copulation. Some such cases are curable, others incurable, but the
subjects especially remain sterile if anything of the sort has
happened in the first formation of the parts in the embryo, for then
are produced women of a masculine and men of a feminine appearance,
and in the former the catamenia do not occur, in the latter the
semen is thin and cold. Hence it is with good reason that the semen of
men is tested in water to find out if it is infertile, for that
which is thin and cold is quickly spread out on the surface, but the
fertile sinks to the bottom, for that which is well concocted is hot
indeed, but that which is firm and thick is well concocted. They
test women by pessaries to see if the smells thereof permeate from
below upwards to the breath from the mouth and by colours smeared upon
the eyes to see if they colour the saliva. If these results do not
follow it is a sign that the passages of the body, through which the
catamenia are secreted, are clogged and closed. For the region about
the eyes is, of all the head, that most nearly connected with the
generative secretions; a proof of this is that it alone is visibly
changed in sexual intercourse, and those who indulge too much in
this are seen to have their eyes sunken in. The reason is that the
nature of the semen is similar to that of the brain, for the

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