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.
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