No animal, indeed, has ever been born without a heart, but they are born without a spleen or with two spleens or with one kidney; there is no case again of total absence of the liver, but there are cases of its being incomplete.And all these phenomena have been seen in animals perfect and alive.Animals also which naturally have a gall-bladder are found without one; others are found to have more than 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.