sters, says, that "without the supposition that some kind of affinity or
attraction is exerted between corresponding parts, unions of this kind are
inexplicable." On the other hand, Vrolik, and he is followed by others,
disputes this conclusion, and argues from the existence of a whole series
of monstrosities, graduating from a perfectly double monster to a mere
rudiment of an additional digit, that "an excess of formative power" is the
cause and origin of every monstrous duplicity. That there are two distinct
classes of cases, and that parts may be doubled independently of the
existence of two embryos, is certain; for a single embryo, or even a single
adult animal, may produce doubled organs. Thus Valentin, as quoted by
Vrolik, injured the caudal extremity of an embryo, and three days
afterwards it produced rudiments of a double pelvis and of double hind
limbs. {341} Hunter and others have observed lizards with their tails
reproduced and doubled. When Bonnet divided longitudinally the foot of the
salamander, several additional digits were occasionally formed. But neither
these cases, nor the perfect series from a double monster to an additional
digit, seem to me opposed to the belief that corresponding parts have a
mutual affinity, and consequently tend to fuse together. A part may be
doubled and remain in this state, or the two parts thus formed may
afterwards through the law of affinity become blended; or two homologous
parts in two separate embryos may, through the same principle, unite and
form a single part.

The law of the affinity and fusion of similar parts applies to the
homologous organs of the same individual animal, as well as to double
monsters. Isidore Geoffroy gives a number of instances of two or more
digits, of two whole legs, of two kidneys, and of several teeth becoming
symmetrically fused together in a more or less perfect manner. Even the two
eyes have been known to unite into a single eye, forming a cyclopean
monster, as have the two ears, though naturally standing so far apart. As
Geoffroy remarks, these facts illustrate in an admirable manner the normal
fusion of various organs which during an early embryonic period are double,
but which afterwards always unite into a single median organ. Organs of
this nature are generally found in a permanently double condition in other
members of the same class. These cases of normal fusion appear to me to
afford the strongest support in favour of the present l