ed by his son,
Isidore Geoffroy, with respect to monsters in the animal kingdom,[843] and
by Moquin-Tandon, with respect to monstrous plants. When similar or
homologous parts, whether belonging to the same embryo or to two distinct
embryos, are brought during an early stage of development into contact,
they often blend into a single part or organ; and this complete fusion
indicates some mutual affinity between the parts, otherwise they would
simply cohere. Whether any power exists which tends to bring homologous
parts into contact seems more doubtful. The tendency to complete fusion is
not a rare or exceptional fact. It is exhibited in the most striking manner
by double monsters. Nothing can be more extraordinary than the manner, as
shown in various published plates, in which the corresponding parts of two
embryos become intimately fused together. This is perhaps best seen in
monsters with two heads, which are united, summit to summit, or face to
face, or, Janus-like, back to back, or obliquely side to side. In one
instance of two heads united almost face to face, but a little obliquely,
four ears were developed, and on one side a perfect face, which was
manifestly formed by the union of two {340} half-faces. Whenever two bodies
or two heads are united, each bone, muscle, vessel, and nerve on the line
of junction seems to seek out its fellow, and becomes completely fused with
it. Lereboullet,[844] who carefully studied the development of double
monsters in fishes, observed in fifteen instances the steps by which two
heads gradually became fused into one. In this and other such cases, no
one, I presume, supposes that the two already formed heads actually blend
together, but that the corresponding parts of each head grow into one
during the further progress of development, accompanied as it always is
with incessant absorption and renovation. Double monsters were formerly
thought to be formed by the union of two originally distinct embryos
developed upon distinct vitelli; but now it is admitted that "their
production is due to the spontaneous divarication of the embryonic mass
into two halves;"[845] this, however, is effected by different methods. But
the belief that double monsters originate from the division of one germ,
does not necessarily affect the question of subsequent fusion, or render
less true the law of the affinity of homologous parts.

The cautious and sagacious J. Mueller,[846] when speaking of Janus-like
mon