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On The Generation Of Animals   

one. Cases are known, too, of the organs changing places, the liver
being on the left, the spleen on the right. These phenomena have
been observed, as stated above, in animals whose growth is
perfected; at the time of birth great confusion of every kind has been
found. Those deficiency which only depart a little from Nature
commonly live; not so those which depart further, when the unnatural
condition is in the parts which are sovereign over life.

The question then about all these cases is this. Are we to suppose
that a single cause is responsible for the production of a single
young one and for the deficiency of the parts, and another but still a
single cause for the production of many young and the multiplication
of parts, or not?

In the first place it seems only reasonable to wonder why some
animals produce many young, others only one. For it is the largest
animals that produce one, e.g. the elephant, camel, horse, and the
other solid-hoofed ungulates; of these some are larger than all
other animals, while the others are of a remarkable size. But the dog,
the wolf, and practically all the fissipeds, produce many, even the
small members of the class, as the mouse family. The cloven-footed
animals again produce few, except the pig, which belongs to those that
produce many. This certainly seems surprising, for we should expect
the large animals to be able to generate more young and to secrete
more semen. But precisely what we wonder at is the reason for not
wondering; it is just because of their size that they do not produce
many young, for the nutriment is expended in such animals upon
increasing the body. But in the smaller animals Nature takes away from
the size and adds the excess so gained to the seminal secretion.
Moreover, more semen must needs be used in generation by the larger
animal, and little by the smaller. Therefore many small ones may be
produced together, but it is hard for many large ones to be so, and to
those intermediate in size Nature has assigned the intermediate
number. We have formerly given the reason why some animals are
large, some smaller, and some between the two, and speaking generally,
with regard to the number of young produced, the solid-hoofed
produce one, the cloven-footed few, the many-toed many. (The reason
of this is that, generally speaking, their sizes correspond to this
difference.) It is not so, however, in all cases; for it is the
largeness and smallness of the body that is cause of few or many young
being born, not the fact that the kind of animal has one, two, or many
toes. A proof of this is that the elephant is the largest of animals
and yet is many-toed, and the camel, the next largest, is
cloven-footed. And not only in animals that walk but also in those
that fly or swim the large ones produce few, the small many, for the
same reason. In like manner also it is not the largest plants that
bear most fruit.

We have explained then why some animals naturally produce many
young, some but few, and some only one; in the difficulty now stated
we may rather be surprised with reason at those which produce many,
since such animals are often seen to conceive from a single
copulation. Whether the semen of the male contributes to the
material of the embryo by itself becoming a part of it and mixing with
the semen of the female, or whether, as we say, it does not act in
this way but brings together and fashions the material within the
female and the generative secretion as the fig-juice does the liquid
substance of milk, what is the reason why it does not form a single
animal of considerable size? For certainly in the parallel case the
fig-juice is not separated if it has to curdle a large quantity of
milk, but the more the milk and the more the fig-juice put into it, so
much the greater is the curdled mass. Now it is no use to say that the
several regions of the uterus attract the semen and therefore more
young than one are formed, because the regions are many and the

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