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On The Generation Of Animals   

from mules, it is impossible that anything should be produced from
mules. For (1) another kind cannot be, because the product of male and
female of the same species is also of the same species, and (2) a mule
cannot be, because that is the product of horse and ass which are
different in form, [and it was laid down that from parents
different in form is born a different animal]. Now this theory is too
general and empty. For all theories not based on the special
principles involved are empty; they only appear to be connected with
the facts without being so really. As geometrical arguments must start
from geometrical principles, so it is with the others; that which is
empty may seem to be something, but is really nothing. Now the basis
of this particular theory is not true, for many animals of different
species are fertile with one another, as was said before. So we must
not inquire into questions of natural science in this fashion any more
than any other questions; we shall be more likely to find the reason
by considering the facts peculiar to the two kinds concerned, horse
and ass. In the first place, each of them, if mated with its own kind,
bears only one young one; secondly, the females are not always able to
conceive from the male (wherefore breeders put the horse to the
mare again at intervals). Indeed, both the mare is deficient in
catamenia, discharging less than any other quadruped, and the
she-ass does not admit the impregnation, but ejects the semen with her
urine, wherefore men follow flogging her after intercourse. Again
the ass is an animal of cold nature, and so is not wont to be produced
in wintry regions because it cannot bear cold, as in Scythia and the
neighbouring country and among the Celts beyond Iberia, for this
country also is cold. For this cause they do not put the jackasses
to the females at the equinox, as they do with horses, but about the
summer solstice, in order that the ass-foals may be born in a warm
season, for the mothers bear at the same season as that in which
they are impregnated, the period of gestation in both horse and ass
being one year. The animal, then, being, as has been said of such a
cold nature, its semen also must be cold. A proof of this is that if a
horse mount a female already impregnated by an ass he does not destroy
the impregnation of the ass, but if the ass be the second to mount her
he does destroy that of the horse because of the coldness of his own
semen. When, therefore, they unite with each other, the generative
elements are preserved by the heat of the one of them, that
contributed by the horse being the hotter; for in the ass both the
semen of the male and the material contributed by the female are cold,
and those of the horse, in both sexes, are hotter. Now when either hot
is added to cold or cold to hot so as to mix, the result is that the
embryo itself arising from these is preserved and thus these animals
are fertile when crossed with one another, but the animal produced
by them is no longer fertile but unable to produce perfect offspring.

And in general each of these animals naturally tends towards
sterility. The ass has all the disadvantages already mentioned, and if
it should not begin to generate after the first shedding of teeth,
it no longer generates at all; so near is the constitution of the
ass to being sterile. The horse is much the same; it tends naturally
towards sterility, and to make it entirely so it is only necessary
that its generative secretion should become colder; now this is what
happens to it when mixed with the corresponding secretion of the
ass. The ass in like manner comes very near generating a sterile
animal when mated with its own species. Thus when the difficulty of
a cross contrary to nature is added, (when too even in the other case
when united with their own species they with difficulty produce a
single young one), the result of the cross, being still more
sterile and contrary to nature, will need nothing further to make it
sterile, but will be so of necessity.

We find also that the bodies of female mules grow large because
the matter which is secreted in other animals to form the catamenia is

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