_Causes._--The causes of monstrosities are varied. Some, like extra digits, lack of horns, etc., run in families, which produce them with absolute certainty when bred in the direct line, although they were originally acquired peculiarities which have merely been fixed by long habit in successive generations. The earliest horse had five toes, and even the most recent fossil horse had three toes, of which the two lateral ones are still represented in the modern animal by the two splint bones. Yet if our horse develops an extra toe it is pronounced a monstrosity. A more genuine monstrosity is the solid-hoofed pig, in which two toes have been merged into one. Another of the same kind is the solid shank bone of the ox, which consists of two bones united into one, but which are still found apart in the early fetus. Though originally acquired peculiarities, they now breed as invariably as color or form. Other monstrosities seem to have begun in too close breeding, by which the powers of symmetrical development are impaired, just as the procreative power weakens under continuous breeding from the closest blood relations. A monstrosity consisting in the absence of an organ often depends on a simple lack of development, the result of disease or injury, as a young bone is permanently shortened by being broken across the soft part between the shaft and the end, the only part where increase in length can take place. As the result of the injury the soft, growing layer becomes prematurely hard and all increase in length at that end of the bone ceases. This will account for some cases of absence of eye, limb, or other organ. Sometimes a monstrosity is owing to the inclosure of one ovum in another while the latter is still but a soft mass of cells and can easily close around the first. Here each ovum has an independent life; they develop simultaneously, only the outer one having direct connection with the womb and being furnished with abundant nourishment advances most rapidly and perfectly, while the inclosed and starved ovum is dwarfed and imperfect often to the last degree.