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On The Generation Of Animals   

material of it is watery (the heat being acquired later). And the
seminal purgations are from the region of the diaphragm, for the first
principle of nature is there, so that the movements from the pudenda
are communicated to the chest, and the smells from the chest are
perceived through the respiration.


In men, then, and in other kinds, as said before, such deficiency
occurs sporadically, but the whole of the mule kind is sterile. The
reason has not been rightly given by Empedocles and Democritus, of
whom the former expresses himself obscurely, the latter more
intelligibly. For they offer their demonstration in the case of all
these animals alike which unite against their affinities. Democritus
says that the genital passages of mules are spoilt in the mother's
uterus because the animals from the first are not produced from
parents of the same kind. But we find that though this is so with
other animals they are none the less able to generate; yet, if this
were the reason, all others that unite in this manner ought to be
barren. Empedocles assigns as his reason that the mixture of the
'seeds' becomes dense, each of the two seminal fluids out of which
it is made being soft, for the hollows in each fit into the
densities of the other, and in such cases a hard substance is formed
out of soft ones, like bronze mingled with tin. Now he does not give
the correct reason in the case of bronze and tin- (we have spoken of
them in the Problems)- nor, to take general ground, does he take his
principles from the intelligible. How do the 'hollows' and 'solids'
fit into one another to make the mixing, e.g. in the case of wine
and water? This saying is quite beyond us; for how we are to
understand the 'hollows' of the wine and water is too far beyond our
perception. Again, when, as a matter of fact, horse is born of
horse, ass of ass, and mule of horse and ass in two ways according
as the parents are stallion and she-ass or jackass and mare, why in
the last case does there result something so 'dense' that the
offspring is sterile, whereas the offspring of male and female
horse, male and female ass, is not sterile? And yet the generative
fluid of the male and female horse is soft. But both sexes of the
horse cross with both sexes of the ass, and the offspring of both
crosses are barren, according to Empedocles, because from both is
produced something 'dense', the 'seeds' being 'soft'. If so, the
offspring of stallion and mare ought also to be sterile. If one of
them alone united with the ass, it might be said that the cause of the
mule's being unable to generate was the unlikeness of that one to
the generative fluid of the ass; but, as it is, whatever be the
character of that generative fluid with which it unites in the ass,
such it is also in the animal of its own kind. Then, again, the
argument is intended to apply to both male and female mules alike, but
the male does generate at seven years of age, it is said; it is the
female alone that is entirely sterile, and even she is so only because
she does not complete the development of the embryo, for a female mule
has been known to conceive.

Perhaps an abstract proof might appear to be more plausible than
those already given; I call it abstract because the more general it is
the further is it removed from the special principles involved. It
runs somewhat as follows. From male and female of the same species
there are born in course of nature male and female of the same species
as the parents, e.g. male and female puppies from male and female dog.
From parents of different species is born a young one different in
species; thus if a dog is different from a lion, the offspring of male
dog and lioness or of lion and bitch will be different from both
parents. If this is so, then since (1) mules are produced of both
sexes and are not different in species from one another, and (2) a
mule is born of horse and ass and these are different in species

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