wo 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.

In many cases of excess of parts the extra part or member is manifestly
derived from the same ovum, and even the same part of the ovum, being
merely the effect of a redundancy and vagary of growth. Such cases include
most instances of extra digits or other organs, and even of double
monsters, as manifested by the fact that such extra organs grow from the
normal identical organs. Hence the extra digit is attached to the normal
digit, the extra head to the one neck, the extra tail to the croup, extra
teeth to the existing teeth, and even two similarly formed bodies are
attached by some point common to both, as the navels, breastbones, backs,
etc. (Pl. XIX, figs. 1, 2, 3.) This shows that both have been derived from
the same primitive layer of the embryo, which possessed the plastic power
of building up a given structure or set of organs. An inclosed ovum, on the
other hand, has no such identity or similarity of structure to the part
with which it is connected, showing an evident primary independence of both
life and the power of building tissues and organs. The power of determining
extra gr